Geetanjali Das
email: geetanjali.das@univ-eiffel.fr

Andry Razakamanantsoa

Dimitri Deneele

Gontran Herrier

Keywords: lime-treated soil, pore solution, hydraulic conductivity, leaching, macropores

Hydraulic performance and microstructure evolution of kneading compacted lime-treated silty soil permeated with low-ionic strength solution.

INTRODUCTION

Lime treatment of fine-grained soil is a widely implemented technique to improve the engineering properties of soil available in the worksites. Two significant mechanisms occur on adding lime with soil: (a) immediate cation exchange reactions and soil agglomeration, resulting in decreased soil plasticity and improved workability, and (b) pozzolanic reactions, mainly between calcium from lime and dissolved silica and alumina from the soil, resulting in the development of cementitious bonding and hence improving the hydromechanical performances of the soil in the long-term [START_REF] Baldovino | Optimizing the evolution of strength for lime-stabilized rammed soil[END_REF], 2018[START_REF] Bell | Lime stabilization of clay minerals and soils[END_REF], Das et al. 2020,2021[START_REF] Dhar | The strength and microstructural behavior of lime stabilized subgrade soil in road construction[END_REF][START_REF] Diamond | Mechanisms of soil-lime stabilization[END_REF][START_REF] Little | Stabilization of pavement subgrades and base courses with lime[END_REF], Rogers & Glendinning 199, Rosone et al. 2020).

The durability of a lime-treated earth structure is inter-linked with the hydromechanical performance of the structure during its service life. The hydromechanical behaviors of lime-treated soil were shown to be influenced by compaction conditions [START_REF] Cuisinier | Microstructure and hydraulic conductivity of a compacted limetreated soil[END_REF], Das et al. 2021[START_REF] Runigo | Performance of lime-treated silty soil under long-term hydraulic conditions[END_REF][START_REF] Runigo | Impact of initial state on the fabric and permeability of a limetreated silt under long-term leaching[END_REF], lime content [START_REF] Baldovino | Effects of lime addition on geotechnical properties of sedimentary soil in Curitiba, Brazil[END_REF][START_REF] Rosone | Analysis of the chemical and microstructural modifications effects on the hydromechanical behaviour of a lime-treated clay[END_REF], and curing conditions [START_REF] Lemaire | Effects of lime and cement treatment on the physicochemical, microstructural and mechanical characteristics of a plastic silt[END_REF][START_REF] Verbrugge | Strength and micro observations on a lime treated silty soil[END_REF]. Specimens compacted at different compaction moisture contents, energies were found to have a different magnitude of hydraulic conductivity (Le [START_REF] Runigo | Impact of initial state on the fabric and permeability of a limetreated silt under long-term leaching[END_REF][START_REF] Mitchell | Permeability of compacted clay[END_REF]. Le [START_REF] Runigo | Impact of initial state on the fabric and permeability of a limetreated silt under long-term leaching[END_REF] stated that lime-treated soil compacted on the wet side of optimum moisture content shows lower hydraulic conductivity than the one compacted on the dry side of optimum moisture content. Besides, the hydraulic conductivity of soil was demonstrated to be a function of macropore amount, which is influenced by the implemented compaction conditions [START_REF] Ranaivomanana | Effects of cement treatment on microstructural, hydraulic, and mechanical properties of compacted soils: Characterization and modeling[END_REF][START_REF] Ranaivomanana | Permeability prediction of soils including degree of compaction and microstructure[END_REF]. Moreover, hydraulic conductivity and leaching of lime-treated soil were shown to increase with increased lime content [START_REF] Mccallister | Leach tests on lime-treated clays[END_REF]Petry 1992, Tran et al. 2014). On the other hand, De Bel et al. 2005 stated that the leaching of lime could be minimized by increasing the curing time of treated soil after compaction, which promoted the formation of cementitious compounds, thus minimizes the hydraulic conductivity.

Though studies were reported regarding several parameters that influence the hydraulic conductivity and leaching performances of lime-treated soil, whether the chemistry of pore solution can also influence these behaviors is a question. The influence of pore fluid chemistry on the hydraulic behavior of natural soil, particularly bentonites, is widely investigated. Bentonites, because of their favorable character as a low permeable material, are extensively used as a liner material in the disposal of the nuclear waste repository [START_REF] Gapak | Laboratory determination of volumetric shrinkage behavior of bentonites: A critical appraisal[END_REF][START_REF] Razakamanantsoa | Hydraulic performance of activated calcium bentonite treated by polyionic charged polymer[END_REF][START_REF] Razakamanantsoa | Long term chemohydro-mechanical behavior of compacted soil bentonite polymer complex submitted to synthetic leachate[END_REF][START_REF] Sato | Comparison of prehydration an d polymer addings effects on Na activated Ca Bentonite by free swell index test[END_REF]. [START_REF] Razakamanantsoa | Long term chemohydro-mechanical behavior of compacted soil bentonite polymer complex submitted to synthetic leachate[END_REF] demonstrated the aggressive effect of leachates in the hydraulic behavior of natural bentonites in the long term. Similarly, [START_REF] Demdoum | Effect of landfill leachate on the hydromechanical behavior of bentonite-geomaterials mixture[END_REF] confirmed the negative effect of landfill leachates on the hydraulic behavior of natural bentonites. Such studies confirm the necessity of considering the impact of pore fluid chemistry during the evaluation of the hydraulic and leaching performances of even lime-treated soil.

In most of the existing laboratory studies, DW was used as a conventional permeant solution to investigate the hydraulic and leaching behaviors of lime-treated soil, whereas in the field, water from a natural source influences such behaviors.

In this context, this study investigates the comparative hydraulic and leaching evolution in lime-treated soil subjected to DW and a LW to evaluate the chemistry of the pore fluid effect. The study was made on lime-treated soil, kneading compacted, at a laboratory scale. The first part of the study shows the differences observed in hydraulic conductivity, followed by leaching. The second part explains the preceding observation based on pore structure evolution.

MATERIALS AND SAMPLE PREPARATIONS

Silty soil, obtained from Marche-Les-Dames, Belgium, was treated with quicklime (calcium oxide, CaO). The Lime Modification Optimum (LMO) of the soil was 1 %, which was determined by [START_REF] Eades | A quick test to determine lime requirements for lime stabilization[END_REF] test [START_REF] Astm D | Standard Test Method for Using pH to Estimate the Soil-Lime Proportion Requirement for Soil Stabilization[END_REF]. Lime content higher than the LMO, i.e., 2.5% was used herein since soil treated at LMO was shown to have limited contribution towards long-term strength evolution (Das et al. 2021). Two permeant solutions, i.e., DW and a LW solution composed with DW + 10 -3 M NaCl were used. The use of 10 -3 M NaCl was made to clarify the role of low ionic strength solution on the hydraulic performance and the microstructure of lime-treated soil. 10 -3 M NaCl has been used previously [START_REF] Razakamanantsoa | Hydraulic performance of activated calcium bentonite treated by polyionic charged polymer[END_REF][START_REF] Razakamanantsoa | Long term chemohydro-mechanical behavior of compacted soil bentonite polymer complex submitted to synthetic leachate[END_REF][START_REF] Sato | Comparison of prehydration an d polymer addings effects on Na activated Ca Bentonite by free swell index test[END_REF] to highlight the negative effect of leachates on bentonites in the context of landfill.

The pH and Electric Conductivity (EC) of the permeant solutions are provided in Table 1. The compaction characteristics of the soil obtained by Standard Proctor compaction as per ASTM D 698-12e2 (ASTM 2012) are presented in Table 2. The soil was first air-dried and then mixed with water to obtain soil paste. This soil paste was then mixed with 2.5% of lime, rested for 1 hour, and then compacted at the optimum moisture content (OMC). 6 specimens, of which 3 untreated and 3 lime-treated of dimensions 0.05m in height and 0.05m in diameter were prepared by kneading (K) compaction. The motive behind conducting kneading compaction is to develop the kneading action the finegrained soil experience under the pad-foot roller in the field (Das et al. 2021[START_REF] Kouassi | A new technique of kneading compaction in the laboratory[END_REF][START_REF] Williams | Compaction of Soils[END_REF]). The process of kneading compaction was conducted, as demonstrated by Das et al. 2021.

Compacted specimens were then wrapped in plastic films and were cured at ambient laboratory temperature for 28 days.

METHODOLOGIES

Hydraulic conductivity test

At the end of the curing time, 4 specimens were subjected to hydraulic conductivity test. Of the 4 specimens, 1 untreated and lime-treated specimens were subjected to LW and the remaining to DW. The test was conducted using a constant head flexible wall permeameter. The setup of the hydraulic conductivity test was similar to the one used by [START_REF] Ranaivomanana | Permeability prediction of soils including degree of compaction and microstructure[END_REF], which includes the injection of permeant solution from the base of the specimen and collection of supernatants from the top. This is done to maintain a uniform flow in the specimen by removing any entrapped air if present. A confining pressure of 88 ± 2 kPa was applied for at least 24-48 hours before the application of hydraulic head pressure to ensure the homogeneity of the stress distribution. The hydraulic head applied was 170 ± 5 cm.

The hydraulic conductivity test was carried out in accordance with the flow conditions laid by Darcy's law. The test involves two phases: saturation and percolation. In the course of the saturation phase, each specimen was allowed to saturate until an inflow volume of permeant solution corresponding to 1 Pore Volume Flow (PVF) enter into the specimen. The volume of pore water required to renew the total water initially present in the soil is called 1 PVF [START_REF] Katsumi | Long-term barrier performance of modified bentonite materials against sodium and calcium permeant solutions[END_REF]. This is done to ensure the full saturation of the specimens. The concept of PVF was implemented herein to consider the duration of contact of the lime-treated soil with the pore solution, as such duration might contribute to the mechanism of dissolution or precipitation inside the soil. However, such a concept was used in several studies related to natural soil [START_REF] Katsumi | Long-term barrier performance of modified bentonite materials against sodium and calcium permeant solutions[END_REF][START_REF] Shackelford | Evaluating the hydraulic conductivity of GCLs permeated with non-standard liquids[END_REF].

At the end of the saturation phase, the percolation phase was initiated, and the permeability coefficient, k was measured as a function of PVF. The hydraulic conductivity test was ended: (a) after 40 PVF of influent percolates through the specimen, and (b) the least 5 values of Electric Conductivity become almost constant.

The pH and ECs of the effluents collected during the percolation phase were measured. A part of the effluents was subjected to chemical analysis by Inductively Coupled Plasma Optical Emission Spectrometry (ICP OES) to determine the concentrations of Calcium (Ca), Silica (Si), and Alumina (Al), as these three elements are the most reactive during the pozzolanic reaction after lime treatment.

Pore structure determination

The pore structure of samples was analyzed by using Mercury Intrusion Porosimetry (MIP) test. The procedure of MIP test can be obtained from [START_REF] Romero | Microstructure investigation in unsaturated soils: a review with special attention to contribution of mercury intrusion porosimetry and environmental scanning electron microscopy[END_REF]. The analysis was made on specimens sampled from the core of compacted soil obtained at the end of curing and at the end of the hydraulic conductivity test.

The pore classification was made as per the International Union of Pure and Applied Chemistry (IUPAC) [START_REF] Rouquerol | Recommendations for the characterization of porous solids (Technical Report)[END_REF], which classifies pores based on their pore-width as macropores (> 500 Å), mesopores (20-500 Å), and micropores (< 20 Å).

In this study, the nomenclature used to represent the specimens is as follows: type of soil-compaction methodpermeant solution (percolation periods). For example, 2.5 %-K-DW (4 days) refers to 2.5 % lime-treated kneading compacted soil, percolated by DW for 4 days. The k measured in 2.5 % lime-treated kneaded specimen, subjected to LW, was observed to be almost equivalent with the magnitude of k measured from the untreated specimens. Also, the period of percolation to reach 40 PVF was almost equivalent in these specimens, i.e., 80-90 days. On the other hand, the magnitude of k was about more than an order higher for the corresponding specimen, permeated by DW with a percolation time of only 04 days to reach 40 PVF (Figure 1).

RESULTS

Evolution of hydraulic conductivity

Evolution of pH and EC in the effluents

The pH and EC of the effluents measured during the percolation phase are presented as a function of the number of PVF in Figure 2. The pH of the effluents gathered from the untreated specimens was around 8 (Figure 2a). The pH of the effluents obtained from the lime-treated soil remained at about 12, irrespective of the type of permeant solutions the specimens were subjected to.

According to Figure 2b, the trend in the evolution of EC was similar for the untreated and the lime-treated specimens subjected to both types of solutions. The loss in EC was maximum up to about 10 PVF, and then the trend of decrease in EC became almost stable.

The initial EC of DW was 4 µs/cm (as seen in Table 1). The measured EC from the effluents gathered from the untreated soil subjected to DW was significantly higher than the initial EC of the DW (Figure 2b). At the same time, this relative difference in EC was observed to be comparatively lower in the effluents collected from the LW submitted specimens. Besides, though the overall trend of evolution of EC was almost similar for both DW and LW leached lime-treated soil, the attained EC was obtained in only 04 days for the DW subjected soil while it took 80 days for the LW subjected soil to reach the same level.

Evolution of Ca, Si, and Al in the effluents

The concentration of Ca, Si, and Al elements leached from the lime-treated soil was measured from the effluents collected during the hydraulic conductivity test up to 40 PVF. The measured concentration is presented in Figure 3 as a function of PVF. The evolution of these elements was compared with the maximum concentration of the same measured in the effluent of the untreated soil, which was 100 mg/l, < 0.2 mg/l, and 8 mg/l for Ca, Al, and Si, respectively. According to Figure 3a, 2.5% lime-treated kneaded specimen, leached by DW, showed a relatively greater loss in calcium when compared to the corresponding LW leached specimen. In the DW-subjected specimen, the concentration of Ca measured in the effluents were even lower than 100 mg/l, which is the maximum Ca concentration measured in the effluents of the untreated specimen leached by DW. Thus, this indicates that in addition to the loss of Ca coming from lime, particularly available around the flow path, a part of soluble calcium was also lost from the soil. However, the Ca leached from the corresponding LW submitted specimen was relatively lower and remained higher than 100 mg/l.

The trend in the evolution of the concentration of Al measured in the effluents obtained from the lime-treated soil subjected to both types of solutions remained almost the same (Figure 3b). The obtained concentration of Al varied in the range of 4-6 mg/l during the hydraulic conductivity test. However, this concentration was much higher than what was measured in the effluents obtained from the untreated soil (< 0.2 mg/l). Thus, it can be said that the obtained Al in the effluents probably comes from the Al released from the soil due to increased soil pH or from the cementitious compounds formed during curing.

Figure 3c shows a significant loss in Si within the first 5 PVF, and the loss was then stabilized in both types of percolated limetreated soil. However, the maximum concentration of Si measured was 8 mg/l from the effluents of the untreated soil, which was much higher than the one measured in the lime-treated soil's effluent. Thus, the obtained Si probably comes from the soluble Si present in the silty soil or from the cementitious compounds available in the soil.

Besides, although the observed trend in the loss of all the above minerals was almost similar in both types of percolated specimens with respect to PVF, it varies widely in the duration of percolation time. In DW permeated specimen, the above loss in minerals was achieved in only 4 days of percolation. On the other hand, it took 80 days to undergo an equivalent loss of similar minerals from the LW-subjected specimen.

Pore structure modifications

Hydraulic conductivity of compacted soil was demonstrated to be the function of macropores [START_REF] Hunt | Flow, transport, and reaction in porous media: Percolation scaling, critical-path analysis, and effective medium approximation[END_REF]Sahimi 2017, Le Runigo et al. 2009). In this aspect, a comparative evaluation of the percentage of pore volumes greater and lower than 500 Å, obtained by MIP, was made for the unleached (after 28 days of curing) and the DW-and LW-leached specimens in Figure 4. According to Figure 4, the total percentages of macropores volume of diameter greater than 500 Å was maximum in the 28 days cured specimen (unleached). For the DW-and LW-leached specimens, these macropores' volume was lowered by about 8% and 12%, respectively. However, LW-subjected specimens showed the minimum presence of macropores.

Besides, the evolution of mesopores was comparatively maximum for the LW-subjected specimen. These mesopores volume were 5% and 12% higher in the LW-leached soil compared to the DW-leached and untreated soil, respectively.

DISCUSSIONS

The results evidenced that lime-treated kneaded soil subjected to pore fluids exhibiting different chemistry showed different magnitude of hydraulic conductivity and characteristics of leaching.

As mentioned in section 3.1 that lime-treated soil subjected to hydraulic conductivity test underwent two phases, i.e., saturation and percolation phase, respectively. In the present case, during the saturation phase, the development of cementitious bonding was enhanced due to constant contact of soil-lime with the pore solution and also due to the limited flow of the pore solution. The preceding statement was evident by the decrease and increased in macropores and mesopores, respectively, in the leached specimens compared to the unleached (Figure 4). Thus, a rise in hydromechanical performance can be expected.

Once the percolation phase was initiated, the renewal of pore water occurred. During the percolation phase, the EC measured in the effluent obtained from the DW subjected untreated soil was relatively higher than the initial EC present in the DW (Figure 2b). On the other hand, this feature was less significant in the corresponding LW-subjected soil. In the lime-treated soil, the time required to renew 40 PVF was 20 times lower in the DW-subjected soil compared to the corresponding LW-subjected soil (Figure 1). Thus, a quick renewal of the pore solution occurred in the DW-subjected, which created accelerated leaching of Ca, Si, and Al, as evident from Figure 3. The aboveobserved difference in the chemical characteristic of effluent obtained between DW-and LW-subjected soil is attributed to the electronically unstable nature of DW. DW being devoid of any ions, leached more minerals in a short time in the process of attaining its required ionic equilibrium. Thus, enhanced leaching of lime components under the influence of DW, particularly available around the flow path, triggered an increase in macropores of diameter greater than 500 Å (Figure 4). As a result, the hydraulic conductivity of LW-subjected soil was similar to the untreated soil, whereas it was about more than an order higher in the DW-subjected soil (Figure 1).

Based on the above discussion, it was confirmed that the analysis of the hydraulic performances of the lime-treated soil against pore solution is not only a function of permeation time but must include the mechanism of leaching and the rate of contact of the pore fluid within the soil matrices. Thus, the analysis is required to be made in terms of PVF, where the number of times the pore water is renewed can be measured.

Besides, the pozzolanic reactions developed between soillime and pore water depend on the duration of contact between both the components. As a result, due to the longer contact of LW with the soil during the hydraulic test, a greater generation of cementitious compounds occurred. This was evident by the evolution of a greater number of mesopores in the LW-subjected specimens (Figure 4). Such an evolution is beneficial for the long-term durability of lime-treated soil.

CONCLUSIONS

This study investigates the evolution of hydraulic conductivity and leaching mechanism in lime-treated compacted silty soil on being subjected to demineralized water and low-mineralized solution. Based on the investigations, the following conclusion was derived: 1) Demineralized water increased the hydraulic conductivity and accelerated the leaching mechanism in lime-treated soil compared to the low-mineralized solution. Demineralized water, being devoid of any ions, dissolute a relatively greater concentration of lime components, particularly available around the flow path. This increased the macropores of diameter greater than 500 Å resulting in increased hydraulic conductivity.

2) The analysis of the hydraulic performances of the lime-treated soil against pore solution is required to be made as a function of pore volume flow. Measuring pore volume flow considers the number of times the pore water is renewed in the lime-treated soil, which governs the mechanism of leaching influenced by the pore solution chemistry.

3) The duration of contact between the soil components and the permeant solution impact the development of pozzolanic reactions during the hydraulic conductivity test. A longer contact of LW with the soil component enhanced the evolution of cementitious compounds. Such an evolution contributes towards the long-term durability of lime-treated soil.

Thus, the study showed that the 10 -3 M of NaCl concentration added was sufficient enough to modify the hydraulic, leaching performance, and microstructure of lime-treated soil. Based on these findings, it is important to consider the chemistry of the permeant solution chosen at a laboratory scale. The selected permeant solution must be representative of the pore water that the structure is likely to be subjected to in the field. This can help to give an accurate prediction of the long-term hydraulic performance of in-situ hydraulic structures.

FUTURE STUDY

Further studies are going on in the laboratory to understand if such a difference in the permeant solution can also impact mechanical behavior in the long term. Besides, these behaviors will also be compared with lime-treated soil prepared at different lime contents.

Figure 1

 1 Figure 1 presents the hydraulic conductivity evolution in the lime-treated and the untreated specimens, permeated by DW and LW. The presented hydraulic conductivity value, i.e., the permeability coefficient, k represents the average of the last 10 values of k before reaching 40 PVF.The k measured in 2.5 % lime-treated kneaded specimen, subjected to LW, was observed to be almost equivalent with the magnitude of k measured from the untreated specimens. Also, the period of percolation to reach 40 PVF was almost equivalent in these specimens, i.e., 80-90 days. On the other hand, the magnitude of k was about more than an order higher for the corresponding specimen, permeated by DW with a percolation time of only 04 days to reach 40 PVF (Figure1).

Figure 1 .

 1 Figure 1. Comparative evolution of k in untreated and 2.5 % limetreated kneading compacted soil subjected to DW and LW.

Figure 2 .

 2 Figure 2. Evolution of pH and EC in the effluents collected from the DW and LW subjected specimens during the hydraulic conductivity test.

Figure 3 .

 3 Figure 3. Comparative evolution of Ca (a), Al (b), and Si (c) in the effluents obtained from the untreated and 2.5 % lime-treated kneading compacted soil.

Figure 4 .

 4 Figure 4. Comparative evolution of the percentage of pore volume in 2.5 % lime-treated unleached and DW-and LW-leached kneading compacted soil by MIP

Table 1

 1

	. pH and EC of the permeant solutions
	Permeant solutions	pH	Electric Conductivity (µs/cm)
	DW	7.40	4
	LW	6.53	172

Table 2 .

 2 Maximum dry density and OMC of the soil

	Soil	Maximum dry density (kN/m 3 )	OMC) (%)
	Untreated silty soil	18.4	14.3
	Silty soil + 2.5% lime	17.1	18.5

ACKNOWLEDGEMENTS

This work was financially supported by Association Nationale de la Recherche et de la Technologie with grant N˚2018/0219 and Lhoist Southern Europe with grant N˚RP2-E18114. The authors are very thankful to the research team of Université Gustave Eiffel, and Lhoist R&D for their great support in performing laboratory experiments and technical supports.


Geetanjali Das
email: geetanjali.das@univ-eiffel.fr

Andry R Razakamanantsoa
email: andry.razakamanantsoa@univ-eiffel.fr

Gontran Herrier
email: gontran.herrier@lhoist.com

Dimitri Deneele
email: dimitri.deneele@univ-eiffel.fr

Hydromechanical and pore-structure evolution in lime-treated kneading compacted soil

Hydromechanical performances and pore-structure evolution in in-situ lime-treated soil is influenced by the implemented methodology of execution. In-situ lime-treated soil experiences kneading action during soil compaction; however, little is investigated regarding the contribution of the kneading mechanism towards hydromechanical and pore-structure evolution. This study evaluates the evolution of Unconfined Compressive Strength (UCS), hydraulic conductivity, and pores of different categories in laboratory kneaded soil. The evaluation involves results from limetreated soil subjected to different curing times and temperatures. The results obtained from laboratory kneaded and cured soils are interpreted with the one obtained from in-situ sampled soil of the same configuration after 7 years of atmospheric curing. The obtained interpretation provides an acceptable insight towards the expected long-term hydromechanical and pore-structure evolution of lime-treated soil. Thus, the study highlights the importance of reproducing an implementation mechanism in the laboratory which closely represents the field compaction.

INTRODUCTION

Management of natural resources such as soil is essential in any land development project. In this context, improvement of the engineering properties of natural soil by chemical stabilization using lime was shown to be an efficient and effective process [START_REF] Bell | Lime stabilization of clay minerals and soils[END_REF][START_REF] Little | Stabilization of pavement subgrades and base courses with lime[END_REF]. In addition to chemical stabilization, mechanical improvement, which involves the implementation of control mixing and compaction conditions by incorporating appropriate water content, binder amount, compaction energy, and compaction mechanism, was also important [START_REF] Das | Evaluation of the long-term effect of lime treatment on a silty soil embankment after seven years of atmospheric exposure: Mechanical, physicochemical, and microstructural studies[END_REF][START_REF] Das | Compressive strength and microstructure evolution of lime-treated silty soil subjected to kneading action[END_REF][START_REF] Le Runigo | Impact of initial state on the fabric and permeability of a lime-treated silt under long-term leaching[END_REF][START_REF] Le Runigo | Performance of lime-treated silty soil under long-term hydraulic conditions[END_REF][START_REF] Makki-Szymkiewicz | Evolution of the properties of lime-treated silty soil in a small experimental embankment[END_REF].

The present literature provides several laboratories [START_REF] Bell | Lime stabilization of clay minerals and soils[END_REF][START_REF] Das | Compressive strength and microstructure evolution of lime-treated silty soil subjected to kneading action[END_REF]) and few field studies evidencing the improvement brought in the soil through lime treatment. One such field study was recently reported by [START_REF] Das | Evaluation of the long-term effect of lime treatment on a silty soil embankment after seven years of atmospheric exposure: Mechanical, physicochemical, and microstructural studies[END_REF]. [START_REF] Das | Evaluation of the long-term effect of lime treatment on a silty soil embankment after seven years of atmospheric exposure: Mechanical, physicochemical, and microstructural studies[END_REF] reported the long-term effect of lime treatment on the compressive strength, physicochemical and microstructural modification on specimens sampled from an experimental embankment, which has been exposed to the external atmosphere for 7 years. A significant evolution of average UCS of 3.29 ± (0.45) MPa was reported in core-sampled soil. Besides, a uniform distribution of pH and water content was reported throughout the embankment. The study concluded that due to controlled mixing and compaction condition implemented during the construction of the embankment, as reported by [START_REF] Makki-Szymkiewicz | Evolution of the properties of lime-treated silty soil in a small experimental embankment[END_REF], such homogeneity in the physicochemical measurement was attained. A similar conclusion was reported by [START_REF] Makki-Szymkiewicz | Evolution of the properties of lime-treated silty soil in a small experimental embankment[END_REF] regarding the contribution of controlled mixing and compaction condition towards attaining a lower permeability in lime-treated soil. Thus, the above studies highlight the importance of undergoing a proper implementation process during the construction of a lime-treated structure.

Of the several parameters maintained during implementation, the compaction mechanism is an important parameter. However, how well the laboratory implemented compaction mechanism represents the in-situ implementation mechanism is less investigated. Thus, it is essential to bring a comparative interpretation between the evolution of properties between filed and laboratory compacted lime-treated soils. On the field, fine-grained soil, particularly clayey soil, is preferred to be compacted by a vibratory Padfoot roller. Padfoot roller, during compaction, produces a combined rolling and kneading action. The impact of the rolling mechanism on natural soil during compaction was shown by [START_REF] Sedeki | Degradability of unpaved roads submitted to traffic and environmental solicitations: laboratory scale[END_REF]. However, the effect of the kneading mechanism is not well investigated. The importance of implementing kneading action at laboratory scale was urged by [START_REF] Clegg | Kneading compaction[END_REF] and [START_REF] Kouassi | A new technique of kneading compaction in the laboratory[END_REF], who showed a similar generation of soil structure, residual interparticle stresses, and other soil properties between in-situ compacted and laboratory kneaded fine-grained natural soil.

Recently, [START_REF] Das | Compressive strength and microstructure evolution of lime-treated silty soil subjected to kneading action[END_REF] demonstrated the importance of undergoing kneading compaction of lime-treated soil by showing the contribution of kneading action towards long-term strength evolution. Thus, it becomes important to undergo further studies on the lime-treated kneaded soil and to investigate the relevancy between laboratory and field obtained results. In this context, this study focuses on evaluating the evolution of Unconfined Compressive Strength (UCS), hydraulic conductivity, and pore-structure of laboratory kneaded lime-treated soil. The obtained laboratory results are then interpreted with lime-treated specimens sampled from 7 years atmospherically cured experimental embankment built with the same soil configuration to understand the long-term evolution.

MATERIALS

This study involves the use of in-situ sampled soil from a 7 year environmentally cured experimental embankment, which was built with 2.5% quicklime treated Marche-Les-Dames (MLD, Belgium) silty soil. Details regarding the sampling procedure and layout of sampling depths were reported in [START_REF] Das | Evaluation of the long-term effect of lime treatment on a silty soil embankment after seven years of atmospheric exposure: Mechanical, physicochemical, and microstructural studies[END_REF]. Compacted specimens in the form of blocks and loose materials were obtained at a depth of 0.30m and 0.75m from the slope of the experimental embankment, i.e., specimens were sampled from the core of the embankment.

The same silty soil and lime content were used for the preparation of specimens in the laboratory. The quicklime used consists of 90.9% of available CaO and a reactivity (t60) of 3.3 min, as reported by [START_REF] Nguyen | Stabilisation des sols traités à la chaux et leur comportement au gel[END_REF]. The Lime Modification Optimum (LMO) of the silt was 1% by weight of lime, which was determined by Eades and Grim test as per ASTM D 6276-99a (ASTM 2006).

SAMPLE PREPARATIONS

The compaction characteristics of the soil were similar to the one reported by [START_REF] Das | Evaluation of the long-term effect of lime treatment on a silty soil embankment after seven years of atmospheric exposure: Mechanical, physicochemical, and microstructural studies[END_REF], which were obtained as per ASTM D698-91 (ASTM 2012) before the construction of the embankment. The maximum dry density and optimum moisture content (OMC) of untreated soil were 18.4 kN/m 3 and 14.3%, respectively, and for 2.5% lime-treated soil were 17.1 kN/m 3 and 18.5%, respectively.

The experimental embankment was constructed with 2.5% quicklime at a wet of optimum moisture content (WMC), which was 1.1 times OMC [START_REF] Makki-Szymkiewicz | Evolution of the properties of lime-treated silty soil in a small experimental embankment[END_REF]. Hence, the laboratory specimens were also prepared at the same lime content and water content. The prepared soil paste was then placed in sealed plastic bags for 24 hours at ambient laboratory temperature to allow the soil to undergo moisture content homogenization. Then, the soil paste was mixed with 2.5% lime and was set aside for 1 hour before the soil mixture was subjected to compaction. This process of sample preparation employed herein was according to the procedure mentioned in French GTS Technical Guide for soil treatment (GTS -LCPC-Setra [START_REF] Guide | Soil treatment with lime and/or hydraulic binders: Application to the Construction of fills and capping layers[END_REF]. This reference of sample preparation is also followed during the in-situ construction of lime-treated structures.

Three cubic specimens of dimension having a length to diameter ratio (l/d) of 1 were trimmed from the block excavated at the core of the embankment. 2 of these were from the block sampled at depth 0.30m, and the remaining ones from the block sampled at a depth of 0.75m from the slope. The former specimen is represented as S0.30 and the latter as S0.75, respectively, in the rest of the study. Correspondingly, cylindrical specimens of length and diameter, each of 0.05m, were prepared by kneading compaction, i.e., the same l/d ratio was maintained with the field sampled soil. The kneading compaction was performed by a laboratory-developed kneading tool, which was reported in the study of [START_REF] Das | Compressive strength and microstructure evolution of lime-treated silty soil subjected to kneading action[END_REF]. During the application of kneading compaction, specimens were kneaded by rotation of the 3-kneading feet at an angle of 45° between 2 successive loadings. A dynamic load corresponding to the compaction energy as per the ASTM D698-91 (ASTM 2012) was applied. The process of kneading compaction is similar to the one mentioned by [START_REF] Das | Compressive strength and microstructure evolution of lime-treated silty soil subjected to kneading action[END_REF]. Nine specimens were prepared, of which 3 were untreated, and 6 were lime-treated soils. The specimens were then wrapped in plastic film. The untreated and 3 lime-treated specimens were cured for 28 days at laboratory ambient temperature of 20°C and the remaining lime-treated specimens were subjected to accelerated curing at 40°C for 90 days. A similar level relative humidity is maintained in both curing conditions.

LABORATORY EXPERIMENTS

Two cubic fields sampled soil, i.e., S0.30 and S0.75, and 6 laboratory cured specimens were subjected to UCS test using a mechanical press with a load sensor at a constant displacement rate of 1mm/min. The 6 laboratory specimens consist of 2 of each 28 days laboratory cured and 90 days accelerated cured soils.

The remaining one cubic field sampled soil (S0.30), and 3 laboratories cured specimens, i.e., one untreated 28 days cured, and two lime-treated 28 days and 90 days accelerated cured subjected to hydraulic conductivity test. The hydraulic conductivity test was performed using a low ionic strength solution, i.e., 10 -3 NaCl. This is because the in-situ hydraulic structure is in general subjected to water from natural sources, which may consist of few to numerous dissolved minerals. Besides, use of 10 -3 NaCl was commonly made in previous studies [START_REF] Razakamanantsoa | Long term chemo-hydro-mechanical behavior of compacted soil bentonite polymer complex submitted to synthetic leachate[END_REF][START_REF] Sato | Comparison of prehydration an d polymer addings effects on Na activated Ca Bentonite by free swell index test[END_REF]). The hydraulic conductivity test was conducted using a flexible wall permeameter, in which water was allowed to pass through the base of the specimens, and the effluents are collected from the top. This was done to ensure a uniform flow of water through the specimens and to reduce any entrapped air if present within the compacted soil. A confining pressure of 88±2 kPa and a hydraulic head of 170±5 cm were applied to the specimens during the hydraulic conductivity test. The applied hydraulic head was intended to keep almost similar to the in-situ embankment's height which is about 180 cm, as reported in the study of [START_REF] Das | Evaluation of the long-term effect of lime treatment on a silty soil embankment after seven years of atmospheric exposure: Mechanical, physicochemical, and microstructural studies[END_REF]. The measurement of hydraulic conductivity was conducted until the last five values of Electric Conductivity measured from the effluents collected became stable.

Loose specimens obtained from the core of the in-situ sampled block (S0.30) and those obtained from laboratory kneaded soil at the end of curing were subjected to pore-structure analysis. Specimens were freeze-dried and then were subjected to Mercury Intrusion Porosimetry (MIP), and Brunauer-Emmett-Teller (BET) [START_REF] Brunauer | Adsorption of gases in multimolecular layers[END_REF] tests to analyze the porestructure evolution. The procedure of MIP and BET tests can be referred to in studies of [START_REF] Romero | Microstructure investigation in unsaturated soils: a review with special attention to contribution of mercury intrusion porosimetry and environmental scanning electron microscopy[END_REF] and [START_REF] Westermarck | Use of mercury porosimetry and nitrogen adsorption in characterisation of the pore structure of mannitol and microcrystalline cellulose powders, granules and tablets[END_REF], respectively. The discussion made regarding porestructure involves the classification provided in IUPAC [START_REF] Rouquerol | Recommendations for the characterization of porous solids (Technical Report)[END_REF], which categories pores as macropores (>500 Å), mesopores (20-500 Å), and micropores (<20 Å).

RESULTS

Evolution of Unconfined Compressive Strength in lime-treated soil

Figure 1 presents the comparative evolution of UCS in the kneaded silty soil with and without lime treatment after being exposed to different curing conditions (i.e., temperature and duration of curing). The UCS of the untreated laboratory kneaded soil after 28 days of curing at laboratory temperature was 0.21 MPa. The same soil being treated with 2.5% lime and cured under similar conditions presented a UCS of 0.70 MPa.

To attain the maximum strength in an acceptable laboratory duration and to compare the same with the 7 years atmospherically cured soil, 2.5% lime-treated laboratory kneaded soil was subjected to 90 days of accelerated curing at 40˚C. The resulting UCS was 3.80 MPa. The average UCS obtained from the environmentally cured core-sampled specimens, i.e., from S0.30 and S0.75 was 4.40 MPa.

Evolution of hydraulic conductivity in lime-treated soil

The evolution of hydraulic conductivity in the untreated and the lime-treated soil cured under different conditions was measured up to 100 days of percolation time and is compared in Figure 2. The measurement of hydraulic conductivity was expressed in terms of the magnitude of permeability coefficient, k. From Figure 2, it was observed that almost all the lime-treated specimens, despite being subjected to different curing conditions, attained stability of k after about 20 days of percolation time, which is in the same range, i.e., lower than 10 -9 m/s and higher than 10 -10 m/s. However, the hydraulic conductivity of the untreated soil was slightly higher than the lime-treated soils.

Evolution of pore structure in lime-treated soil

The hydraulic conductivity of lime-treated soil was demonstrated to be affected by the interaggregates pore of diameter greater than 10000 Å [START_REF] Le Runigo | Impact of initial state on the fabric and permeability of a lime-treated silt under long-term leaching[END_REF]. In this context, the cumulative macropore volume of pores greater than 10000 Å, which was analyzed by the MIP test, was compared between the untreated and lime-treated soil in Figure 3.

The untreated silty soil was observed to exhibit the maximum cumulative pore volume of diameter greater than 10000 Å. This feature was comparatively lower in the lime-treated soil, and the minimum was observed for the environmentally cured S0.30 soil.

Since lime treatment of soil was shown to enhance the development of pores lower than 3000 Å [START_REF] Das | Evaluation of the long-term effect of lime treatment on a silty soil embankment after seven years of atmospheric exposure: Mechanical, physicochemical, and microstructural studies[END_REF][START_REF] Das | Compressive strength and microstructure evolution of lime-treated silty soil subjected to kneading action[END_REF] thus, the evolution of such pores was expressed by the isotherm plots obtained from the BET test in Figure 4. This is because BET analysis was shown to provide 28 days cured lime-treated soil compared to the untreated soil. Then with increased curing time to 90 days and under accelerated conditions, this feature increased compared to both the untreated and 28 days cured soil. However, the preceding feature was significant in the 7 years cured S0.30 soil.

The hysteresis region in the isotherm was delineated to be associated with the delay in capillary condensation and evaporation that occurs in the mesopores (20-500 Å) by [START_REF] Mcgregor | Conditions affecting the moisture buffering measurement performed on compressed earth blocks[END_REF] and [START_REF] Collet | Porous structure and water vapour sorption of hemp-based materials[END_REF]. In the present case, the hysteresis thickness was minimum for the untreated soil and then increased with an increase in curing time and temperature of the laboratory cured lime-treated soil. However, this thickness is slightly lower for the environmentally S0.30 cured soil compared to the accelerated cured soil.

The peak of the isotherm represents the total nitrogen adsorption capacity of the present soil in the range of pore diameter of about 20-2000 Å, as obtained through Barrett-Joiner-Halenda pore (BJH) analysis [START_REF] Barrett | The determination of pore volume and area distributions in porous substances. I. Computations from nitrogen isotherms[END_REF]. This range involves the total generation of pores lower than 3000 Å due to lime treatment. Thus, an enhanced peak indicates a greater presence of pores of diameter 20-2000 Å. In the present case, the maximum peak was observed for the 7 years S0.30 environmentally cured soil, followed by the laboratory accelerated cured and 28 days cured soil. The minimum was observed for the untreated specimen.

DISCUSSIONS

UCS evolution in lime-treated soil was shown to be contributed by the generation of pores lower than 2000 Å [START_REF] Das | Compressive strength and microstructure evolution of lime-treated silty soil subjected to kneading action[END_REF]. This evolution of pores is influenced by curing time and temperature [START_REF] Little | Stabilization of pavement subgrades and base courses with lime[END_REF][START_REF] Lemaire | Effects of lime and cement treatment on the physicochemical, microstructural and mechanical characteristics of a plastic silt[END_REF]. Again, pores lower than 2000 Å involves the complete range of micropores, mesopores, and a part of macropores as per IUPAC classification. However, little is known regarding the evolution of each pore category in lime-treated soil exposed to different curing conditions and its contribution to UCS. In the present case, the isotherm in Figure 4 provides a distinguishing feature of the above pore categories. As expected, 28 days cured lime-treated soil showed an increase in UCS compared to the untreated kneaded soil (Figure 1). This is attributed to the increased hysteresis peak and hysteresis thickness which indicated the evolution of pores in the pore range 20-2000 Å (Figure 4). However, the slightly lower presence of micropores in the short-term 28 days cured lime-treated soil compared to the untreated was a consequence of flocculation of clay particles [START_REF] Little | Stabilization of pavement subgrades and base courses with lime[END_REF]. On curing the same lime-treated soil at an elevated temperature of 40˚C and for a longer time, the pozzolanic reactions were accelerated, giving to the quick development of smaller pores, particularly in the pore range 20-2000 Å (Figure 4). This phenomenon resulted in increased UCS by about five times than the 28 days cured soil (Figure .  1). However, the evolution of micropores in the accelerated cured soil though higher than 28 days cured soil, was less significant. This indicates that accelerated curing brings modifications, particularly in pores greater than 20 Å. At the same time, the same configured soil, after 7 years of curing, showed the maximum micropores and hysteresis peak (Figure 4). However, the hysteresis thickness in the 7 years cured soil was slightly lower than the accelerated cured soil. The preceding observations explain the increased intensities of micropores with increased curing time due to increased cementitious bonding, which have led to the maximum UCS (Figure 1). The above discussion helps in visualizing the evolution of pores of different categories in soil exposed to different curing conditions and its contribution to UCS. Thus, based on the above interpretation made, the evolution of UCS and pore-structure observed in the in-situ sampled soil can be said to be an expected long-term evolution of the laboratory kneaded soil.

Short-term cured lime-treated soil was shown to have resulted in an increase in permeability due to the formation of pores greater than 10000 Å as a result of aggregation compared to the untreated soil [START_REF] Khattab | Etude multi-échelles d'un sol argileux plastique traité à la chaux[END_REF][START_REF] Nguyen | Stabilisation des sols traités à la chaux et leur comportement au gel[END_REF]. However, [START_REF] Locat | Mechanical and hydraulic behaviour of a soft inorganic clay treated with lime[END_REF] demonstrated that after long-term curing, lime-treated soil shows a decrease in hydraulic conductivity due to increased formation of smaller pores as a result of pozzolanic reactions. Recently, [START_REF] Makki-Szymkiewicz | Evolution of the properties of lime-treated silty soil in a small experimental embankment[END_REF] explained that a controlled mixing and execution process during construction of a lime-treatment embankment could give a level of hydraulic conductivity close to the untreated soil even after 6 months from construction. This embankment was the same embankment from which in-situ sampled soil was reported herein. Figure 2 showed a level of permeability between 10 -9 to 10 -10 m/s in the in-situ sampled soil, which was almost similar to the level reported by [START_REF] Makki-Szymkiewicz | Evolution of the properties of lime-treated silty soil in a small experimental embankment[END_REF]. This indicates that though a significant evolution of pores lower than 2000 Å occurred during the 7 years of curing (Figure 4), the level of permeability remained almost the same as the one obtained after 6 months from construction. The same configured soil, when prepared by laboratory kneading action, gave the same level of permeability despite being subjected to different curing conditions (Figure 2). The preceding observations made in the in-situ sampled and laboratory specimens again confirm the fact that the attainment of lower permeability in lime-treated soil is dependent on the implementation mechanism. However, the hydraulic conductivity of the untreated soil was slightly higher compared to the lime-treated soil (Figure 2). This is attributed to the maximum availability of cumulative pores volume of diameter greater than 10000 Å in the untreated (Figure 3). However, the difference in cumulative pores volume of diameter greater than 10000 Å between lime-treated soil seems to have an insignificant effect on the evolution of hydraulic conductivity.

CONCLUSIONS

This study focuses on interpreting the comparative evolution of hydromechanical behaviors and pore-structure modifications between laboratory prepared and in-situ sampled lime-treated soil for emphasizing the relevancy of laboratory and field implemented kneading mechanisms. Comparison of laboratory kneaded untreated and lime-treated soil subjected to different curing times and temperatures and 7 years atmospherically cured specimens were made.

1) The evolution of UCS and pore-structure observed in the 7 years cured in-situ sampled soil can be said to be an expected long-term evolution of the laboratory kneaded soil. The laboratory kneaded soil showed a rise in mesopores and 70% UCS evolution after 28 days of curing compared to the untreated soil. After 90 days of laboratory accelerated curing at elevated temperature (40˚C), a slight increase in micropores and significant mesopores and 94% increase in UCS resulted. After 7 years of in-situ curing, the same configured soil showed the maximum micropores and 95% increase in UCS.

2) Attainment of lower permeability in lime-treated soil is dependent on the implementation mechanism and is less impacted by curing time and temperature. 28 days laboratory cured, and 90 days accelerated cured laboratory kneaded soil gave a magnitude of hydraulic conductivity almost similar to the one achieved in the 7 years cured in-situ sampled soil. Thus, the above discussion confirms the relevancy between the laboratory and fieldimplemented kneading mechanism by interlinking the hydromechanical and microstructural evolution. Hence, the development of kneading action at a laboratory scale is essential to have a close prediction of the hydromechanical evolution of a lime-treated in-situ structure. Besides, since the compaction mechanism plays an important role in the hydromechanical evolution of limetreated soil, further investigation on the compaction mechanism, which involves kneading and rolling impact together, should be made. This is because the Padfoot roller, in addition to kneading action, produces roller action on the soil during in-situ compaction.

Figure 1 .

 1 Figure 1. Comparative evolution of UCS in untreated to lime-treated kneaded soil subjected to different curing conditions.

Figure 2 .

 2 Figure 2. Comparative evolution of hydraulic conductivity in untreated to lime-treated kneaded soil subjected to different curing conditions.

an elaborative explanation of smaller pores formed due to lime treatment in the study reported by [START_REF] Das | Evaluation of the long-term effect of lime treatment on a silty soil embankment after seven years of atmospheric exposure: Mechanical, physicochemical, and microstructural studies[END_REF]. From the obtained isotherm plots, three distinct features can be observed, i.e., the evolution of (i) micropores, (ii) hysteresis thickness, and (iii) the peak of the isotherm. The first feature, i.e., the total availability of micropores, is usually demonstrated to corresponds to the range of relative pressure of 0.05-0.3 [START_REF] Huang | Improved calculations of pore size distribution for relatively large, irregular slit-shaped mesopore structure[END_REF]). In the present soil, this feature was slightly lower in the


Christophe Amiot
email: christophe.amiot@univ-angers.fr

Clément Harmange

Weihong Ji

Morphological differences along a

Keywords: bill morphology, insectivorous bird, locomotion-related traits, New Zealand fantail, resource-related traits, urbanisation gradient

Introduction

Human impact on natural environments has been considerably increasing around the planet, triggering major changes in many ecosystems [START_REF] Sanderson | The human footprint and the last of the wild: the human footprint is a global map of human influence on the land surface, which suggests that human beings are stewards of nature, whether we like it or not[END_REF]Steffen et al. 2004;[START_REF] Venter | Sixteen years of change in the global terrestrial human footprint and implications for biodiversity conservation[END_REF]. The subsequent human-driven disturbances constitute major driving forces leading to ecological and evolutionary phenotypic changes in wildlife populations, at a rate outpacing the changes observed in natural contexts (Hendry et al. 2008;Darimont et al. 2009;Gaynor et al. 2018). Urbanisation is one of the most evident human-induced rapid environmental changes, developing at the expense of natural or semi-natural (e.g., rural) areas and wildlife habitats [START_REF] Marzluff | Avian ecology and conservation in an urbanizing world[END_REF]Steffen et al. 2004;[START_REF] Sih | Evolution and behavioural responses to human-induced rapid environmental change[END_REF][START_REF] Gil | Avian urban ecology[END_REF]. The urbanisation process contributes to create new environmental conditions (see [START_REF] Kowarik | Novel urban ecosystems, biodiversity, and conservation[END_REF], such as increases of temperatures, human-induced disturbances and alterations, introductions of non-native species, resulting in new patterns of access to resources such as supplementary feeding [START_REF] Fuller | Garden bird feeding predicts the structure of urban avian assemblages[END_REF][START_REF] Galbraith | Supplementary feeding restructures urban bird communities[END_REF] and of predation pressure [START_REF] Marzluff | Avian ecology and conservation in an urbanizing world[END_REF][START_REF] Shochat | From patterns to emerging processes in mechanistic urban ecology[END_REF][START_REF] Gil | Avian urban ecology[END_REF]. Such human-induced alteration of environmental factors and ecological processes put the urban development as one of the main threats to biodiversity [START_REF] Maxwell | Biodiversity: The ravages of guns, nets and bulldozers[END_REF]. Understanding responses of animals to urbanisation and to the subsequent changes in ecological processes is thus crucial for evolutionary ecology and conservation in urban ecosystems [START_REF] Chace | Urban effects on native avifauna: a review[END_REF].

To date, extensive work has explored the effects of urbanisation on the functional composition of communities, and especially for avian assemblages [START_REF] Chace | Urban effects on native avifauna: a review[END_REF][START_REF] Marzluff | A decadal review of urban ornithology and a prospectus for the future[END_REF][START_REF] Oliveira Hagen | Impacts of urban areas and their characteristics on avian functional diversity[END_REF][START_REF] Pithon | Grasslands provide diverse opportunities for bird species along an urban-rural gradient[END_REF]. However, there is less information about the intra-specific response to urbanisation [START_REF] Liker | Lean birds in the city: body size and condition of house sparrows along the urbanization gradient[END_REF][START_REF] Caizergues | Urban versus forest ecotypes are not explained by divergent reproductive selection[END_REF]), which may be critical in predicting species colonisation or their long-term persistence in urban ecosystems [START_REF] Sih | Evolution and behavioural responses to human-induced rapid environmental change[END_REF]. Recently, [START_REF] Caizergues | Urban versus forest ecotypes are not explained by divergent reproductive selection[END_REF] demonstrated the existence of an urban morphotype in the great tit Parus major, with urban individuals being smaller (lower body size, shorter wing, tarsus and tail lengths) than their rural conspecifics (see also [START_REF] Liker | Lean birds in the city: body size and condition of house sparrows along the urbanization gradient[END_REF] for similar patterns in the house sparrow Passer domesticus). However, the strength and direction of such differences appear to vary substantially between areas and/or species [START_REF] Evans | The effect of urbanisation on avian morphology and latitudinal gradients in body size[END_REF][START_REF] Seress | Habitat urbanization and its effects on birds[END_REF][START_REF] Sepp | A review of urban impacts on avian lifehistory evolution: Does city living lead to slower pace of life?[END_REF], likely reflecting variation in the local ecological forces involved [START_REF] Shochat | From patterns to emerging processes in mechanistic urban ecology[END_REF]. Therefore, studies addressing phenotypic changes in the light of mechanisms that underlie the urbanisation process, which have so far often remained disconnected in the literature [START_REF] Chace | Urban effects on native avifauna: a review[END_REF][START_REF] Shochat | From patterns to emerging processes in mechanistic urban ecology[END_REF][START_REF] Sol | Behavioural adjustments for a life in the city[END_REF][START_REF] Alberti | Urban driven phenotypic changes: empirical observations and theoretical implications for eco-evolutionary feedback[END_REF], are needed. The current study thus focuses on morphological traits, particularly suitable to investigate phenotypic responses to urbanisation process [START_REF] Yeh | Rapid Evolution of a Sexually Selected Trait Following Population Establishment in a Novel Habitat[END_REF][START_REF] Shochat | From patterns to emerging processes in mechanistic urban ecology[END_REF][START_REF] Liker | Lean birds in the city: body size and condition of house sparrows along the urbanization gradient[END_REF], because of the rapid adaptive or plastic changes they may imply (e.g., in ten generations in [START_REF] Amiot | Rapid morphological divergence of introduced Red-whiskered Bulbuls Pycnonotus jocosus in contrasting environments[END_REF]; see also [START_REF] Bitton | Change in wing morphology of the European starling during and after colonization of North America[END_REF].

Alterations of food resources and predation pressure are two major forces underlying the urbanisation process based on bottom-up and top-down effects respectively [START_REF] Shochat | Credit or debit? Resource input changes population dynamics of city-slicker birds[END_REF][START_REF] Chace | Urban effects on native avifauna: a review[END_REF][START_REF] Shochat | From patterns to emerging processes in mechanistic urban ecology[END_REF][START_REF] Chamberlain | Avian productivity in urban landscapes: a review and meta-analysis[END_REF][START_REF] Seress | Habitat urbanization and its effects on birds[END_REF]. First, urban areas often provide a higher food availability and predictability through supplementary feeding, communal waste and compost [START_REF] Fuller | Garden bird feeding predicts the structure of urban avian assemblages[END_REF][START_REF] Galbraith | Supplementary feeding restructures urban bird communities[END_REF][START_REF] Seress | Habitat urbanization and its effects on birds[END_REF]. Such increase in anthropogenic food and alteration in food item composition may lead to changes in the diet of birds along rural-urban gradients (e.g., [START_REF] Mennechez | Settlement of breeding European Starlings in urban areas: Importance of lawns vs. anthropogenic wastes[END_REF][START_REF] Scott | Food preference of the Black-headed Gull Chroicocephalus ridibundus differs along a rural-urban gradient[END_REF]. Feeding ecology is closely related with the bill morphology in birds [START_REF] Smith | Reaching new adaptive peaks: evolution of alternative bill forms in an African finch[END_REF]: higher bill depth and width are positively linked to the bill bite force [START_REF] Herrel | Bite performance and morphology in a population of Darwin's finches: implications for the evolution of beak shape[END_REF]) and to the feeding efficiency according to the hardness of food items [START_REF] Durell | Sex-related differences in diet and feeding method in the oystercatcher Haematopus ostralegus[END_REF][START_REF] Smith | Reaching new adaptive peaks: evolution of alternative bill forms in an African finch[END_REF]. Therefore, the urbanisation-induced changes in the diet of birds may constitute a major force expected to trigger changes in bill traits. The higher food availability and predictability in urban areas may also paradoxically contribute to reduce the body mass or condition of urban birds [START_REF] Shochat | Credit or debit? Resource input changes population dynamics of city-slicker birds[END_REF][START_REF] Demeyrier | Disentangling drivers of reproductive performance in urban great tits: a food supplementation experiment[END_REF][START_REF] Caizergues | Urban versus forest ecotypes are not explained by divergent reproductive selection[END_REF]) as a result of the non-necessity to constitute fat reserves [START_REF] Cuthill | Body mass regulation in response to changes in feeding predictability and overnight energy expenditure[END_REF] and of the higher concentration of competitors resulting in a lower per capita amount of food items [START_REF] Shochat | Credit or debit? Resource input changes population dynamics of city-slicker birds[END_REF][START_REF] Seress | Habitat urbanization and its effects on birds[END_REF].

Urban areas also favour the proliferation of predators (increased human-commensal and/or non-native predator species, humans perceived as predators by animals: [START_REF] Frid | Human-caused Disturbance Stimuli as a Form of Predation Risk[END_REF][START_REF] Chace | Urban effects on native avifauna: a review[END_REF][START_REF] Fischer | Urbanization and the Predation Paradox: The Role of Trophic Dynamics in Structuring Vertebrate Communities[END_REF]. Predation pressure may thus induce morphological adjustments [START_REF] Gosler | Predation risk and the cost of being fat[END_REF], adaptations [START_REF] Swaddle | Morphological adaptations to predation risk in passerines[END_REF], or exert a selection in birds based on the morphological traits that are directly related to the aerodynamics or locomotory performance in escaping from predators (see [START_REF] Møller | Viability selection on prey morphology by a generalist predator[END_REF][START_REF] Møller | Risk-taking and the evolution of mechanisms for rapid escape from predators[END_REF][START_REF] Van Den Hout | Predator escape tactics in birds: linking ecology and aerodynamics[END_REF]. For instance, wing shape may affect the take-off angle and would affect the relative risk of predation for birds, while tarsus length would be negatively linked to the relative risk of predation (longer tarsus facilitating the take-off leap) (Swaddle andLockwood 1998, 2003). High body mass or wing loading also impair flight abilities such as manoeuvrability [START_REF] Dietz | Intraspecific variation in avian pectoral muscle mass: constraints on maintaining manoeuvrability with increasing body mass[END_REF][START_REF] Van Den Hout | Predator escape tactics in birds: linking ecology and aerodynamics[END_REF]), take-off velocity and angle [START_REF] Kullberg | Impaired predator evasion in fat blackcaps (Sylvia atricapilla)[END_REF][START_REF] Møller | Birds[END_REF], and reduce the flight speed [START_REF] Burns | The effects of wing loading and gender on the escape flights of least sandpipers (Calidris minutilla) and western sandpipers (Calidris mauri)[END_REF][START_REF] Lindström | Activity and predator escape performance of common greenfinches Carduelis chloris infected with sindbis virus[END_REF], thus altering the antipredator response [START_REF] Gosler | Predation risk and the cost of being fat[END_REF][START_REF] Lindström | Activity and predator escape performance of common greenfinches Carduelis chloris infected with sindbis virus[END_REF][START_REF] Møller | Birds[END_REF]. Body mass may also be affected in urban areas because of changes in predation pressure through the trade-off starvation-predation risk, which should favour a lower investment in antipredator strategies and a higher investment in energy gain when predation risk relaxes [START_REF] Gosler | Predation risk and the cost of being fat[END_REF][START_REF] Macleod | Mass-dependent predation risk as a mechanism for house sparrow declines?[END_REF]).

In the current study, we investigated morphological divergence in the New Zealand (NZ) fantail Rhipidura fuliginosa, a bird species recently subjected to urbanisation. This endemic insectivorous species is one of the most common and best-known passerine species of New Zealand. It seems to survive well the urbanisation and is commonly observed in the remnant forest patches of urbanised areas [START_REF] Robertson | Atlas of bird distribution in New Zealand 1999-2004[END_REF]. Because of its recent colonisation by European societies (early 19 th century, Duncan and Young 2000), New

Zealand offers an interesting context to study the response of birds to rapid novel environmental conditions, such as urbanisation. Particularly, the recent introduction of human-commensal mammalian predators (i.e., mainly ship rats Rattus rattus, domestic and feral cats Felis catus, and common brushtail possums Trichosurus vulpecula, [START_REF] Van Heezik | Nest survival of birds in an urban environment in New Zealand[END_REF][START_REF] Aguilar | Distribution characteristics of unmanaged cat colonies over a 20 year period in Auckland, New Zealand[END_REF] by Europeans, originally absent in New Zealand, offers an interesting context to address our questions (see also [START_REF] Chace | Urban effects on native avifauna: a review[END_REF].

Based on a synchronic comparison of three areas depicting a chronological gradient of urbanisation (old urban, recent urban and rural areas) in the Auckland region of New Zealand, we (i) tested whether birds living under different degree of urban development show morphological differences, and (2) interpreted the results through a functional and mechanism-orientated standpoint. Since urbanisation was shown to affect the composition of invertebrate communities in New Zealand (see [START_REF] Toft | Invertebrates of an urban old growth forest are different from forest restoration and garden communities[END_REF], we may reasonably assume that composition of food items differs along the urbanisation gradient, as well as the predation pressure, as indicated by the daily nest predation rates increasing from rural to urban study areas (Table 1, see also [START_REF] Van Heezik | Do domestic cats impose an unsustainable harvest on urban bird populations?[END_REF]. Therefore, we tested the hypothesis that birds differed in morphological traits that are related to feeding ecology (bill morphology, body mass and condition) and/or to the locomotory performance in escaping from predators (i.e., wing, tarsus and tail morphology, body mass and condition) (see Table 2). Urban birds were expected to have stubbier bills than their rural conspecifics [START_REF] Badyaev | Evolution on a local scale: developmental, functional, and genetic bases of divergence in bill form and associated changes in song structure between adjacent habitats[END_REF][START_REF] Evans | The effect of urbanisation on avian morphology and latitudinal gradients in body size[END_REF]) as a result of a putative urbanisation-induced alteration in food resources, and/or longer wings, tail and tarsus that increase escape flight abilities (see Table 2), as a result of the putative increased predation pressure. Morphological differences were expected to be particularly pronounced between bird populations inhabiting the most contrasted study areas, i.e., the preserved rural area and the old urban area.

Material and methods

Study areas

The study was conducted from 2011 to 2014 in the North Shore district of the Auckland region, New Zealand (36.471°S 174,452°E). The study area is divided in three areas (Fig. 1), depicting a chronological gradient of urbanisation since the early 1900s (old urban, recent urban and rural areas). All these areas have preliminarily undergone a common historical fragmentation of native forest, induced by agricultural development. Thus, they are all composed of isolated remnant patches of mature native broadleaf-podocarp forest. These patches vary from 1.74 to 113.6 ha [START_REF] Myers | North Shore City Ecological Survey-A Survey of Sites of Ecological Significance in Tamaki and Rodney Ecological Districts[END_REF]. Between-area divergences have emerged with their differential history of urban development, through chronological settlements of built-up areas around the pre-existing patches of native forest, without any further habitat destruction [START_REF] Myers | North Shore City Ecological Survey-A Survey of Sites of Ecological Significance in Tamaki and Rodney Ecological Districts[END_REF]. Daily human visitation was assessed in each study area as an index of human disturbance. First, for each patch, we calculated the number of people present during the sampling session, divided by the total number of people reported in this patch over all sampling sessions (i.e., at least 10). Then, an index was calculated for each study area, as a mean of the values calculated for each single-patch within the area. Daily nest predation rate (as a proxy of predation risk) was also assessed during the nesting period between 2011 and 2014. Estimations were made using the Mayfield method [START_REF] Mayfield | Nesting success calculated from exposure[END_REF], providing an unbiased estimate for nests found at different developmental stages.

Nests were located and monitored from September to late January 2011-2014 to optimize predation rate estimation.

Old urban area

The old urban area has been exposed to urban development since the early 1900s (Auckland Regional Council 2010a). This area is predominantly characterized by dense industrial and residential areas (431-1695 dwelling/km 2 , [START_REF] Grimes | Housing supply in the Auckland Region 2000-2005[END_REF]. Patches of native forest, associated with open vegetated spaces of short vegetation cover and low density of mature trees, are part of the urban matrix. In this study area, forest patches were exposed to the higher level of human visitation reported (Table 1). Birds were also exposed to a high predation pressure, as indicated by the high nest predation rate (Table 1). In this area, the main predators were non-native and human-commensal mammals such as domestic and feral cats [START_REF] Aguilar | Distribution characteristics of unmanaged cat colonies over a 20 year period in Auckland, New Zealand[END_REF], ship rats and the common brushtail possum. Native predators (morepork Ninox novaeseelandiae and swamp harriers Circus approximans) were less present in this area.

Recent urban area

The recent urban area differs from the previous one by its recent exposure to urban development (<25 years, Auckland Regional Council 2010a). It is characterized by a mix of recent urban infrastructures (i.e., both residential and industrial) with a lower dwelling density (2-431 dwelling/km 2 , [START_REF] Grimes | Housing supply in the Auckland Region 2000-2005[END_REF]. Vegetation is characterized by the remnant patches of native forest, associated with open vegetated spaces of mature trees and shrubs.

Forest patches were exposed to a lower level of human visitation but have similar nest predation rates compared to the old urban area (Table 1).

Rural area

The rural area is part of the Tawharanui Regional Park established in 1973. It is characterized by pastures, patches of mature native forest and restored wetlands. Agricultural activity is mainly oriented toward extensive farming (sheep and beef) for maintenance of open grassland habitats (Auckland Regional Council 2010b). Management is based on wildlife conservation through the restoration of native habitats and integration of biodiversity in farming practices. However, it is an open recreational park with campground, beach access and walkways in native forest remnants, resulting in intermediate level of human visitation (Table 1). The establishment of predator-exclusion fence in 2004 associated with pest control, has resulted in a lower density of predation from terrestrial mammals (Table 1), though avian predators remain present (e.g., morepork and swamp harrier). This fence has also contributed to reduce the area access for humans to a single road or aquatic access.

Data collection

Birds were captured at multiple sampling sites spread within the forest patches that cover each of the study areas (i.e., old urban, recent urban and rural areas), to ensure that morphological measurements are representative of subpopulations inhabiting each study area (number of sampling sites within the old urban area = 13; recent urban = 10; rural = 9).

Captures were conducted using mist-nets, deployed from 6 am to 12 pm, from September to late February during the period 2011-2014. Once captured, each bird was measured and ringed with an alphanumeric metal ring purchased from the New Zealand Department of Conservation. Seven morphological traits were measured. Bill length (from the bill base to the tip), bill width and height (at the base), and tarsus length (length of the tarsometatarsal bone) were measured with a precision of 0.1 mm. Tail length (from the distal end of the uropygial gland to the tip of the largest rectrix), wing chord length (from the carpal joint to the tip of the longest primary feather or feather sheath) were measured with a precision of 0.5 mm. Bird body mass was measured with a precision of 0.01 g using a high-precision balance. Bird sex was determined using molecular sexing, following the method of [START_REF] Fridolfsson | A simple and universal method for molecular sexing of non-ratite birds[END_REF], from blood samples collected from the brachial vein. A total of 27, 11 and 34 mature individuals were captured and measured respectively in the old urban, recent urban and rural areas.

Index calculation

To determine the body condition of birds, we first calculated a synthetic index of body size using the first axis (PCI) of a principal component analysis performed on the wing, tarsus and tail lengths (see [START_REF] Chastel | Brood size and body condition in the House Sparrow Passer domesticus: the influence of brooding behaviour[END_REF][START_REF] Geslin | Sex-specific patterns in body condition and testosterone level changes in a territorial migratory bird: the Bluethroat Luscinia svecica[END_REF]). The first axis explained 56.27% of the overall variance in body size. Then, a body condition index was calculated as the residual mass from a linear regression performed between body mass and body size index (see [START_REF] Jakob | Estimating fitness: a comparison of body condition indices[END_REF][START_REF] Chastel | Brood size and body condition in the House Sparrow Passer domesticus: the influence of brooding behaviour[END_REF][START_REF] Geslin | Sex-specific patterns in body condition and testosterone level changes in a territorial migratory bird: the Bluethroat Luscinia svecica[END_REF]). Finally, we also calculated ratios of bill sizes (as proxies of bill shape) and a ratio of body mass/wing chord as a proxy of wing loading.

Statistical analyses

To test for between-area (old urban, recent urban and rural) differences in morphological traits, we used a multivariate analysis of variance (MANOVA). A first MANOVA was performed testing for the interacting effects of area and sex on morphological traits. There was no interaction between area and sex on morphological traits (MANOVA; F 2,39 = 0.929, p = 0.565). Sex was thus excluded from the final design of the MANOVA. Statistical significance of the F-statistic was assessed using a Pillai test. Then, ANOVAs were performed to identify the significant morphological metrics contributing to between-area differences. When needed, Tukey's multiple comparison tests were finally performed to reveal significant differences between modalities (i.e., areas). Since we expected differences in body mass along the urbanisation gradient that needed to be tested (see predictions in introduction), and because some dependent variables already integrate body mass in their calculation (e.g., wing loading, body condition index), body mass was first implemented as a dependent variable rather than as a covariate in the analyses. The potential influence of the body mass of the birds on our results was then assessed as a complementary multivariate analysis of covariance, performed on the morphological traits that have previously showed significant between-area differences (see Online Resource 1). All analyses were performed using R software (R Core Team 2019).

Results

The multivariate analysis of variance showed between-area differences in morphological traits (MANOVA; F 2, 57 = 2.651, P =0.001). All bill traits (i.e., bill length, depth and width) differed between areas (Fig. 2, Table 3). In the old urban area, birds had shorter (by 16-19%), but deeper (+ 11-13%) and wider (+ 11-17%) bills, than birds inhabiting the recent urban and rural areas (Fig. 2a-c, Tables 3,4). Bill ratios (as proxy of bill shape) also varied between areas (Fig. 2d-e, Table 3). The significantly greater ratios of bill depth / bill length (+ 40%) and bill width / bill length (+ 43-54%) observed in birds inhabiting the old urban area were suggestive of stubbier bill shape in this population, relatively to recent urban and rural populations (Fig. 2d-e, Tables 3,4). All these differences remained consistent when controlling for the body mass of the birds (Online Resource 1). No between-area difference was detected for the ratio of bill depth / bill width (Fig. 2f, Table 3). Overall, patterns of morphological differences in bill traits remained consistent, with differences observed between the old urban area and the two others (recent urban and rural), while no differences were detected between the recent urban and rural areas (Table 4). Finally, we did not detect any difference in bird condition indices nor in locomotory morphology (i.e., wing chord, tail and tarsus length) (Table 3).

Discussion

We found between-area differences in morphological traits in the insectivorous NZ fantail species, along a gradient of urbanisation. Our results thus support that the anthropogenic process of urbanisation can induce intra-specific phenotypic changes in bird populations.

Particularly, we found empirical evidence of divergence in bill size and bill ratios along the urban-rural gradient, while we did not detect any difference in bird condition or locomotionrelated traits (i.e., wing-, tarsus-and tail-related metrics). We discussed these results through a mechanism-orientated and functional approach.

Morphological divergence and resource-based mechanisms

We found differences in bill size and ratios along the urbanisation gradient, that remained consistent when controlling for the body mass of the birds (see Online Resource 1). In the old urban area, birds had stubbier bills (i.e., shorter, deeper and wider bills, and higher ratios of bill depth/length and bill width/length) than in the two other areas (i.e., recent urban and rural). These results are consistent with [START_REF] Evans | The effect of urbanisation on avian morphology and latitudinal gradients in body size[END_REF] showing that urban blackbirds Turdus merula, tended to have stubbier bills than rural ones. Given that bill size and shape are closely related to feeding ecology in birds [START_REF] Smith | Reaching new adaptive peaks: evolution of alternative bill forms in an African finch[END_REF][START_REF] Corbin | Foraging ecomorphology within North American flycatchers and a test of concordance with southern African species[END_REF]), such changes in bill traits may result from urbanisation-induced changes in food resources [START_REF] Shochat | Credit or debit? Resource input changes population dynamics of city-slicker birds[END_REF][START_REF] Shochat | From patterns to emerging processes in mechanistic urban ecology[END_REF][START_REF] Seress | Habitat urbanization and its effects on birds[END_REF].

Deeper and wider bills are known to favour a greater bite force [START_REF] Herrel | Bite performance and morphology in a population of Darwin's finches: implications for the evolution of beak shape[END_REF][START_REF] Badyaev | Evolution on a local scale: developmental, functional, and genetic bases of divergence in bill form and associated changes in song structure between adjacent habitats[END_REF], and could therefore reflect a shift in the between-area composition of the available food items [START_REF] Durell | Sex-related differences in diet and feeding method in the oystercatcher Haematopus ostralegus[END_REF][START_REF] Badyaev | Evolution on a local scale: developmental, functional, and genetic bases of divergence in bill form and associated changes in song structure between adjacent habitats[END_REF]. [START_REF] Badyaev | Evolution on a local scale: developmental, functional, and genetic bases of divergence in bill form and associated changes in song structure between adjacent habitats[END_REF] showed that urban house finches Carpodacus mexicanus mainly feed on harder and larger seeds and adapted towards deeper and wider bills procuring a greater bite force, compared to individuals living in semi-natural areas [START_REF] Hulscher | Is the bill of the male Oystercatcher a better tool for attacking mussels than the bill of the female?[END_REF][START_REF] Durell | Sex-related differences in diet and feeding method in the oystercatcher Haematopus ostralegus[END_REF]). This hypothesis is also supported by the clear shift reported in the composition of insects communities between old growth forests and suburban areas in New Zealand [START_REF] Toft | Invertebrates of an urban old growth forest are different from forest restoration and garden communities[END_REF], but also by a potential urbanisation-induced increase in invertebrate body size (e.g., spiders in [START_REF] Lowe | Urbanisation at multiple scales is associated with larger size and higher fecundity of an orb-weaving spider[END_REF]) likely favouring stubbier-billed individuals. Stubbier bills could also be more advantageous in urban areas with a hard ground surface [START_REF] Cuthill | The function of bill-wiping[END_REF][START_REF] Evans | The effect of urbanisation on avian morphology and latitudinal gradients in body size[END_REF]. [START_REF] Kleindorfer | Adaptive divergence in contiguous populations of Darwin's small ground finch (Geospiza fuliginosa)[END_REF] demonstrated that lowland Darwin's small ground finches Geospiza fuliginosa feeding mainly seeds on the ground by picking and/or chipping had shorter and deeper bills than highland individuals feeding more on insects in short vegetation by gleaning and/or sliding. Disparity in bill morphology in the NZ fantail could, therefore, result from a shift of substrate use and foraging strategies, from hawking, gleaning and flushing in native environments [START_REF] Gill | Abundance, feeding, and morphology of passerine birds at Kowhai Bush, Kaikoura, New Zealand[END_REF][START_REF] Gill | Laying seasons of three insectivorous song-birds at Kowhai Bush, Kaikoura[END_REF], to sallying and picking in urban areas. This hypothesis is supported by [START_REF] Møller | Successful city dwellers: a comparative study of the ecological characteristics of urban birds in the Western Palearctic[END_REF] showing that birds persisting in urban areas display high rates of feeding or foraging innovations. Bill divergence may also be explained by temperature differences (thermoregulation hypothesis, see LaBarbera et al.

2017), higher in urban than in rural areas (according to the urban heat island effect, see [START_REF] Shochat | From patterns to emerging processes in mechanistic urban ecology[END_REF][START_REF] Kowarik | Novel urban ecosystems, biodiversity, and conservation[END_REF]. However, we are less confident in this hypothesis as an increased urban temperature would have favoured lower bill depth and width (LaBarbera et al. 2017), which contrasts with our results. Since birds were measured over the years 2011-2014, morphological traits may have varied over the study period. However, we believe that this did not alter our results and conclusions, since we did not detect any influence of time for most of the morphological trait measurements (Online Resource 2), except for bill depth (though this is not the case for bill ratios) for which a very slight effect was detected (i.e., coefficient = -0.08 mm, while the precision of bill measurements was 0.1 mm).

Although differences in body mass might also be expected as a result of an alteration of food resources [START_REF] Cuthill | Body mass regulation in response to changes in feeding predictability and overnight energy expenditure[END_REF][START_REF] Dulisz | Differences in biometry and body condition of the House Sparrow (Passer domesticus) in urban and rural population during breeding season[END_REF][START_REF] Salleh Hudin | Predictable food supplies induce plastic shifts in avian scaled body mass[END_REF][START_REF] Caizergues | Urban versus forest ecotypes are not explained by divergent reproductive selection[END_REF], we did not detect any difference in body mass or body condition. This could result from the maintenance of a balanced trade-off in food acquisition-predation risk, consistent with the predicted higher abundance of food resources counterbalanced by the higher level of predation risk observed in urban areas.

Morphology and predation pressure

Although differences in locomotory morphology (i.e., wing-, tarsus-and tail-related traits) might have been expected along the urbanisation gradient as a result of alterations in predation risk [START_REF] Swaddle | Morphological adaptations to predation risk in passerines[END_REF][START_REF] Shochat | From patterns to emerging processes in mechanistic urban ecology[END_REF][START_REF] Møller | Viability selection on prey morphology by a generalist predator[END_REF][START_REF] Møller | Risk-taking and the evolution of mechanisms for rapid escape from predators[END_REF], we did not detect any difference between study areas. It does not imply that birds did not respond to the changes in predation risk, but that they may display other regulatory responses such as adjustments in their internal state (i.e., physiological and psychological states, see [START_REF] Nathan | A movement ecology paradigm for unifying organismal movement research[END_REF]) rather than biomechanical adjustments (see [START_REF] Van Der Veen | Escape flights of yellowhammers and greenfinches: more than just physics[END_REF][START_REF] Møller | Successful city dwellers: a comparative study of the ecological characteristics of urban birds in the Western Palearctic[END_REF][START_REF] Møller | Escape behaviour of birds provides evidence of predation being involved in urbanization[END_REF][START_REF] Møller | Risk-taking and the evolution of mechanisms for rapid escape from predators[END_REF]. For instance, [START_REF] Witter | Experimental investigations of mass-dependent predation risk in the European starling, Sturnus vulgaris[END_REF] suggested that birds would adjust their decision-making in escape behaviour according to their morphology at a given time (see also [START_REF] Sol | Risk-taking behavior, urbanization and the pace of life in birds[END_REF]).

This hypothesis is also supported by the many other behavioural responses reported in New Zealand passerines to predation risk from introduced mammals (e.g., adjustment of parental investment in [START_REF] Massaro | Introduced mammalian predators induce behavioural changes in parental care in an endemic New Zealand bird[END_REF]; adjustment of nest site selection in [START_REF] Lawrence | When the neighbourhood goes bad: can endangered black robins adjust nest-site selection in response to the risk of an invasive predator?[END_REF].

Alternatively, the high predation risk detected in the current study for nests in urban areas does not necessarily relate to high predation risk for adults, as nest and adult predation often imply different predator species [START_REF] Marzluff | Avian ecology and conservation in an urbanizing world[END_REF][START_REF] Seress | Habitat urbanization and its effects on birds[END_REF]. Adults could, therefore, be subjected to a relaxed predation pressure in urban areas, thus reducing top-down effects (see [START_REF] Shochat | Credit or debit? Resource input changes population dynamics of city-slicker birds[END_REF][START_REF] Seress | Habitat urbanization and its effects on birds[END_REF].

Overall, our results are not typical of the urban morphotype described in other bird species, which have generally smaller locomotion-related traits (shorter wing, tarsus and tail lengths) than their rural conspecifics [START_REF] Liker | Lean birds in the city: body size and condition of house sparrows along the urbanization gradient[END_REF][START_REF] Dulisz | Differences in biometry and body condition of the House Sparrow (Passer domesticus) in urban and rural population during breeding season[END_REF][START_REF] Caizergues | Urban versus forest ecotypes are not explained by divergent reproductive selection[END_REF]. This could results from a locality-specific process of urbanisation, influencing ecological processes differentially between Europe and New Zealand (see [START_REF] Evans | The effect of urbanisation on avian morphology and latitudinal gradients in body size[END_REF].

Indeed, New Zealand context differs by the predation pressure which seems to be higher in urban than rural areas (see also [START_REF] Chace | Urban effects on native avifauna: a review[END_REF]. Secondly, native birds are naive about introduced mammalian predators (but also introduced competitors) thus affecting the mechanisms involved in response to predation (see [START_REF] Sih | Predator-prey naïveté, antipredator behavior, and the ecology of predator invasions[END_REF]. Thirdly, the urbanisation process of Auckland has occurred by keeping intact the native forest remnant patches in urban areas (see Ramalho and Hobbs 2012 for a review on such urbanisation modes). These remnants forest patches may play a key role (e.g., conservation in indigenous invertebrates, [START_REF] Toft | Invertebrates of an urban old growth forest are different from forest restoration and garden communities[END_REF], buffering the effect of urbanisation, thus reducing pressures acting on morphological traits [START_REF] Chace | Urban effects on native avifauna: a review[END_REF][START_REF] Kowarik | Novel urban ecosystems, biodiversity, and conservation[END_REF].

Conclusion and perspectives

To conclude, morphological divergence in bill traits between birds inhabiting the old urban area versus the recent urban and rural areas, provides empirical evidence of a morphological response to the urbanisation process, potentially supported by resource-based explanations.

The absence of morphological divergence between recent urban (< 25 years) and rural areas may indicate that morphological response is not linearly related with time since urbanisation (see also [START_REF] Evans | The effect of urbanisation on avian morphology and latitudinal gradients in body size[END_REF]. It therefore suggests that a time-lag of 25-60 years is necessary to enter in the phase of phenotypic adjustment to urbanisation (see [START_REF] Evans | A conceptual framework for the colonisation of urban areas: the blackbird Turdus merula as a case study[END_REF]. Such rapid responses in bill morphology to urbanisation, already suggested in [START_REF] Evans | The effect of urbanisation on avian morphology and latitudinal gradients in body size[END_REF].

They are also consistent with morphological responses of invasive species to novel environments and local conditions in food resources [START_REF] Amiot | Rapid morphological divergence of introduced Red-whiskered Bulbuls Pycnonotus jocosus in contrasting environments[END_REF], and could thus be critical for the ecology and evolution of species subjected to rapid environmental changes [START_REF] Benkman | Adaptation to single resources and the evolution of crossbill (Loxia) diversity[END_REF][START_REF] Møller | Successful city dwellers: a comparative study of the ecological characteristics of urban birds in the Western Palearctic[END_REF][START_REF] Sih | Evolution and behavioural responses to human-induced rapid environmental change[END_REF]. While human-induced alterations of resources may influence interspecific interactions and cause cascading effects on ecological networks [START_REF] Thompson | Rapid evolution as an ecological process[END_REF][START_REF] Auer | Climate change has indirect effects on resource use and overlap among coexisting bird species with negative consequences for their reproductive success[END_REF], the observed morphological responses in bill traits may have major implications on trophic interactions and ecological dynamics of communities. Further mechanistic studies, consisting in well-replicated experimental assessment of the urbanisation-induced changes in resource composition, and subsequent effects on the diet and morphology of urban birds, is critically needed. They should contribute to better understand implications of urbanisation processes for ecological communities.

Morphological traits Mechanisms involved References

Bill morphology -Bill length -Bill width -Bill depth -Bill ratio Bill size and shape are proxy of the bill bite force and related to the bird feeding ecology (e.g., diet) [START_REF] Durell | Sex-related differences in diet and feeding method in the oystercatcher Haematopus ostralegus[END_REF][START_REF] Smith | Reaching new adaptive peaks: evolution of alternative bill forms in an African finch[END_REF][START_REF] Herrel | Bite performance and morphology in a population of Darwin's finches: implications for the evolution of beak shape[END_REF] Bird condition -Body mass -Body condition index Availability and predictability of food resources influence the bird body mass/condition. Body mass depends on the predation risk, mediated by the trade-off starvation-predation risk [START_REF] Cuthill | Body mass regulation in response to changes in feeding predictability and overnight energy expenditure[END_REF][START_REF] Caizergues | Urban versus forest ecotypes are not explained by divergent reproductive selection[END_REF]) [START_REF] Gosler | Predation risk and the cost of being fat[END_REF][START_REF] Macleod | Mass-dependent predation risk as a mechanism for house sparrow declines?[END_REF] Body mass is negatively related to flight ability in response to predation risk [START_REF] Witter | Experimental investigations of mass-dependent predation risk in the European starling, Sturnus vulgaris[END_REF][START_REF] Kullberg | Impaired predator evasion in fat blackcaps (Sylvia atricapilla)[END_REF]

Figure captions : Figure 1

 :1 Figure captions:

Figure 2

 2 Figure2Bill morphology of the New Zealand fantail, along a chronological gradient of urbanisation (old urban, recent urban and rural areas). Bill traits considered are simple morphological traits (i.e., bill length, depth and width) and ratios as proxy of bill shape (D/L: bill depth/length; W/L: bill width/length; D/W: bill depth/width).

Figure 2

 2 Figure 2

Table 1

 1 Estimates of daily human visitation (proxy of anthropogenic disturbance) and daily nest predation rate on the three study areas (old urban, recent urban and rural areas).

	Area	Daily human frequentation Daily nest predation rate
	Old urban	0.436	0.022
	Recent urban	0.181	0.021
	Rural	0.275	0.014

Table 2

 2 Expected divergence in morphological traits as a response to urbanisation, and the underlying mechanisms involved.

Table 4

 4 Tukey post-hoc multicomparisons testing for significant pairwise differences between each modality (i.e., areas). Significant p-values are indicated in bold.

	Group mean