Jean Escande

Christophe Proust
email: christophe.proust@ineris.fr

Jean Christophe Le Coze

Limitations of current risk assessment methods to foresee emerging risks: towards a new methodology ?

Keywords: Decision engineering, Hazards identification, Management and planning, Safety Highlights

The objective of this work-in-progress is to investigate the potentialities but also the limitations of traditional risks analysis tools especially in the context of emerging technologies and develop a method facilitating the early detection of scenarios of accidents. This is certainly a challenge particularly for new industrial fields since, in this case, very little or no lesson from past accidents is available. It is believed that such situations cannot be conveniently treated using traditional risk assessment methods (HAZOP, FMEA,...) and typical examples are given. The reason is that those methods rely heavily on past accidents and are therefore "trapped" in them so that they are largely "inductive". In terms of foreseeing the future, the shortcomings of inductive methods are recalled. The possibility to imagine the future with very little clues is then discussed on the ground of theoretical consideration and a way to do so is proposed (abduction, serendipity). Then on the basis of the observation of how the experts work and how discoveries are made, a potential new methodology is outlined.

Introduction

To identify, rank and control industrial risks a significant number of tools is nowadays available which seems to satisfy the users. Among these tools, ready to use packaged methods such that FMEA, HAZOP, Bow Tie diagrams,… are largely taught in universities and may even be available in laptop softwares (Vinnem, 2014).

Being largely used, those methods and their results have also been, for long, criticized [START_REF] Mannan | Lees' loss prevention in the process industries[END_REF]. In particular, they might be ill adapted to emerging technologies because primarily of the lack of feedback from accidents/incidents and secondly because, as they stand

M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT

(or are currently used) today (in standards, in computer programs, spreadsheet,...) they would lack flexibility to accommodate for totally new situations.

The difficulty is certainly very real since, despite an extensive use of such methods, the Challenger accident of NASA in 1986 or the Fukushima disaster in 2011 did happen. If in very well resourced industrial domains, such as the space and the nuclear industry, where a high level of expertise is available, extreme events of this kind occur, there is certainly still much more to explore about the limits of risk assessment exercises.

So what can go wrong with the risk analysis methods ?

Investigating the limitations of engineering knowledge and the practice of risk analysis is not particularly new because the question of "the unexpected occurrence" is a central issue (Weick and Sutcliffe, 2007;[START_REF] Le Coze | Managing the unexpected[END_REF]. Early examples exists (Turner, 1978;Perrow, 1984) but more recent ones too [START_REF] Tierney | Risk analysis and their social dimensions (in french : « L'analyse des risques et leurs dimensions sociales[END_REF][START_REF] A N U S C R I P T A C C E P T E D Accepted Manuscript | 737-Cabriolet: The Limits of Knowledge and the Sociology of Inevitable Failure[END_REF], suggesting this concern has been lasting for long. The subjective part of the risk assessment is particularly stressed out which is far away from the rational aspects of the relevant methods which are traditionally taught in universities. But risk or safety can also be understood as social constructions (Aven, 2012;[START_REF] Le Coze | Towards a constructivist program is safety[END_REF].

Note first that HAZOP or FMEA, two major risk analysis methods, were developed in the sixties to help the "safe" development of emerging technologies of that period of time ! New chemical plants, nuclear power plants, nuclear weapons, aviation, space industry… within sometimes (especially nuclear developments) a context of very limited feedback from experience…Second, despite some severe pitfalls, the related systems (nuclear plant, planes, rockets,…) have over time achieved an acceptable level of safety, suggesting that a number of potential problems were identified and cured through the use of these risk analysis methods.

So, before thinking about developing alternative risk analysis methods, it is necessary to look how the "traditional methods" were initially developed and how they are currently being used. The idea is to better understand what works in order to understand why might not especially when facing new contexts.

The current practice of traditional risk analysis methods

The "traditional methods" like FMEA, HAZOP, Bow Tie diagrams have been abundantly documented in the scientific and technical literature [START_REF] Mannan | Lees' loss prevention in the process industries[END_REF]. In some cases the historical context of the development of the method is invoked, but they mainly describe the procedural aspects. In the examples given, idealized situations are used smearing out many practical difficulties as illustrated hereafter. The bias of this "pedagogical" (communication ?) method is that a superficial know-how is transmitted together with a preformatted method. Following, resulting safety studies might be highly standardized containing only little new information.

Nevertheless, many long experienced experts know that important and fully relevant scenarios may emerge first from a deep analysis of data (including simulating work situations in order to approach closer to the real operating conditions), second from a detailed investigation of the physical phenomena associated to the process and third from keeping the eyes opened on the general context. Illustrations of some of these issues are given in the follow up, partly coming from the experience of the authors.

M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT

Gathering data: the importance of real life situations

This example is about the safe manipulation of a missile to be attached below the wing of a jet (Figure 1). In this particular context, the kind of risk is known (unwanted ignition due to a shock for instance), the relevant information was available, at least on the paper. And all looked fine : for instance the missile had to be approached on a trailer and the wheels of the latter were mechanically blocked as soon as the operator removed his hands from the trailer (when for instance manipulating other parts of the missile). During a demonstration, because of the limited space below the wing, the operator had, without any other option, to bypass the mechanical safety brake so as to be able to position correctly the trailer. The conditions for an unwanted hazardous situation were met.

Figure 1 : plane and missile example

This is not due to insufficient knowledge but to incomplete information of real life working constraints. The description of the technical lock was given on paper but another piece of information, without words or figures, but accessible only through the observation of the real task performed by the operator, was given out only by the real life simulation.

Expertise of physical (or chemical) phenomena

This aspect pertains to the interpretation of the information and is linked to the level-breath of expertise the risk analysis team incorporates.

To illustrate that point, the example of BP Texas City refinery big explosion is first given. In this accident, the overflooding of a distillation column was not detected and resulted in a massive flammable mixture leading to a large scale explosion. One of the key reasons why the team did not succeed in preventing the overfilling of the distillation column was that they did not know that the level indicator would tell that all is OK when the highest measurable level would be significantly surpassed. Note that many maintenance, integrity, management issues were associated with this disaster as root causes [START_REF] Hopkins | Disastrous Decisions: The Human and Organisational Causes of the Gulf of Mexico Blowout[END_REF], but this example is given to show that a risk analysis team working on the distillation column would have failed in identifying this accident scenario if not aware of the functional details of the level detectors. This is certainly not an isolated example. The following tells more about the kind of spectrum of physical knowledge which should sometimes be engaged. In this LNG harbor (Figure 2),, the LNG filling line is provide with several automatic isolation valves placed in series in particular to prevent a massive leakage. To investigate further the reliability of the device, an FMEA was performed. Immediately it came out that if the energy supply went down, all valves closed jointly because they were chosen « failed closed ». A priori, no more massive leakage : safe! would claim the process control engineer, but…the physicist would comment further that because the lines are never perfectly insulated, the LNG contained between the valves will vaporize and the internal pressure would theoretically rise by tens of atmosphere, breaking the equipments potentially leading to the undesired event, which might, in the present case, even be the worst with the largest consequences.

M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT Figure 2 : LNG lines in a harbour

In those examples, the expertise does exist somewhere but may easily not be implemented if not made available. Turner (Turner, 1978) commented that earlier, talking about disasters which he described as information problems.

General context: humans, organizations & societies

A striking example of ignoring part of the general context in which humans and societies evolve is now given.

Planes are equipped with safety doors separating the pilot cockpit from the passenger cabin.

The motivation is to avoid highjacking by terrorists. This reinforced door needs to be locked all the time and can only be opened from the cockpit side. But in the particular situation of the German Wings disaster (Figure 3), the secondary pilot suffered from mental troubles and wanted to commit a suicide. The captain went in the passenger cabin for a while, but could not come back in the cockpit. Note that it could have been the same if the copilot had lost consciousness for some other reasons. The outcome was that the worst scenario, the crash of the plane, happened. Again this is not an isolated case, and discovering the worse beyond what designers think to be under control happens regularly. Major banks know that they could be in serious troubles if, because of a major event affecting their commodities, a disruption of their activity happens.

It may even produce a major economical crisis. As a consequence, they perform regularly a risk ranking exercise. For instance, a fire is always possible, and this would destroy the bank's headquarters. This situation is foreseen and backup buildings are designed for this. These buildings are shared and maintained by several banks. This strategy is generalized to cover other catastrophies like, for the specific case of Paris, a major flooding from the river La Seine (figure 4). But, because of the geographical configuration, many banks would be affected by the same natural event and would need to use the same backup building. In this situation, the available space and resources in the backup building would not be sufficient (by far…) to maintain the banks' activities. The unwanted scenario would in this case happen and derives from the fact that banks are part of a wider society of organizations with other banks sharing a common mode of failure.

Improving / modifying the traditional risks analysis methods?

At the time of writing these lines, the historical details of the foundation of HAZOP, FMEA,… are still unclear to the mind of the authors and it is not really known if the inventors of such techniques conceived them in such a way that they were to identify any scenario even the most tricky ones. In the meantime, these original methodologies were continuously extended and sometimes incorporated in computer code suite. For instance, the HAZID method [START_REF] Mccoy | HAZID, a computer air for hazard identification[END_REF] is a computer application of the HAZOP methodology

M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT
incorporating from the beginning a sort of PI&D description and MFM-HAZOP [START_REF] Rossing | A functional HAZOP methodology[END_REF] incorporates a knowledge/rule management system to generate the HAZOP tables.

The original FMEA method, best suited to the failure of single component was extended to other entities like computer based control systems or event human procedures [START_REF] Ishimatsu | Modeling and Hazard Analysis Using Stpa[END_REF]. The authors do not overlook these progresses but think they do not change significantly the landscape as far as identifying emerging risks is concerned. Mc Coy acknowledges this when saying HAZID should only be seen as a potential aid since HAZOP is fundamentally a creative process. Paltrinieri (Paltrinieri et al., 2013) recognized implicitly that, even when incorporating systematically the incident/accident databases in the HAZID process (as proposed in their code DyPASI), the improvement expected is merely to try and incorporate "weak signals" but not to extract events that never occurred.

Nevertheless, in a number of situations, it can be noted that unexpected hazardous configurations do emerge while implementing the traditional risk analysis methods : an FMEA analysis was performed about the LNG line when the dangerous situation appeared and a risk ranking exercise was in progress in the case of the banks.

So what can be said however is that these methods certainly trigger a process of "framed imagination". By relying on a series of principles including decomposition of systems into manageable parts combined with systematic exploration of the possibilities of failure according to parameters (e.g., too much pressure, too much temperature, etc in a HAZOP study), a number of scenarios can be identified which may not come out spontaneously.

But something more was required.

The "singular point", the clue ?

So, at least, the risk analysis exercise can be considered as a substrate for discovering unexpected situations but again is not enough. In the examples given above, a "singular point" during the course of the study was first met: a mismatch between the information laid in the documents and the reality in the case of the missile, the knowledge that insulated line are never perfect and that LNG vaporization will produce large overpressures in the second case, the information given that the backup building is shared and should shelter many banks in case of a major flooding,… Note that capturing such a "saddle point" requires a certain culture which in turn comes from curiosity. Second, a desire to go deeper into the analysis is needed which sometimes requires going out of the frame of the ongoing risk analysis method.

For instance, in the case of the LNG line, an FMEA was in progress but the rupture scenario could not be produced by such technique. It is the curiosity, imaginative mind certainly relying on the science of phenomena to perform different kind of analogies and perhaps the quality of the interactions between experts which were behind.

So there is certainly more than applying a technique. There seems to be something added on like observing-investigating-searching-imagining-discovering. Coming to that point, it might be useful to investigate how scientific discoveries have been described and understood in the literature. This literature includes history, sociology and philosophy of science. Authors such as Turner were pioneers of the strategy consisting in finding inspirations and theoretical support outside their domain to conceptualize the limits of risk analysis exercises and the genesis of disasters [START_REF] Le Coze | Managing the unexpected[END_REF]. Relying on this background, Francois Jacob, a French Nobel price biologist [START_REF] Jacob | The mouse, the man and the fly (in french : « La souris, l'homme et la mouche[END_REF], once wrote « On peut presque mesurer l'importance d'un travail scientifique à l'intensité de la surprise qu'il provoque » (the importance of a scientific work may nearly be measured by the degree of surprise it provokes), a notion of surprise that

M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT
Turner applied to disasters. "It becomes clear that the answer to the question 'who is surprised when the large-scale accident does occur?' is an important one for the present analysis" (Turner, Pidgeon, 1997, 72).

Scientific discoveries

The principles of what called the 'scientific method' is associated with the historical period of "Les lumières", and slowly emerged during the Renaissance in Europe (Toulmin, 1991[START_REF] Toulmin | The Return to Reason[END_REF]. What constitutes the scientific mode of investigation drew the attention of many disciplines. It involves issues of causality, determinism, law, induction, realism,… The present authors selected only specific aspects which they judged relevant to the present debate.

Based on Kepler, Galileo's work and on a number of practices of his time, Francis Bacon [START_REF] Conner | A people's history of science: miners, midwives and "low mechanics[END_REF] codified the methodological principles of how laws and theories could be established. This is obvious to us now, but it was surely not at that time. His idea was to first establish facts (and not impressions/interpretations) from observations, then, infer laws from which predictions can be made. The first step is called induction (infer laws from facts) and the second one deduction which consists in "deducing" a set of consequences (or potential causes) from facts using laws. But induction was criticized by Hume then later by Russel and Chalmers [START_REF] Russel | The problems of philosophy[END_REF][START_REF] Chalmers | What is the thing called science ? an assessment of the nature and status of science and its methods[END_REF]. This philosopher used a story to make his point about induction's limit. A turkey, a highly inductive animal, noticed repeatedly that it was fed every morning at the same time and, therefore, induced that it would remains so, not inducing that the day before Xmas, it would be killed (Figure 5) …

Figure 5 : inductive reasoning

The problem of induction is that, on purely logical ground, it is impossible to justify by repeated observations that phenomena will reproduce as they did so far. [START_REF] Popper | The Logic of Scientific Discovery[END_REF] made another radical proposition in order to provide a normative structure for the practice of science despite the limits of induction. His idea is that there is no observation without a background theory, but that this theory is formulated in such a way that it can be refuted by experience. More precisely, by formulating predictions with the help of laws derived from a theoretical background, scientific propositions are set in a position to be refuted. To some extent, looking for a contradicting fact is a way for the research to progress. Science progresses "negatively", by refuting previous theories. On such grounds, Anders [START_REF] Anders | The battle of cherries : dialogue with Hannah Arendt (in german : "Die Kirschenschlacht. Dialoge mit Hannah Arendt[END_REF] considers that the inductive approach fails because it tends to ignore the differences between the various facts/observations in such a way that a general law can be obtained.

Pierce [START_REF] Pierce | Deduction, Induction, and Hypothesis[END_REF] suggested another kind of cognitive step in the process of science called "abduction". It is described as follows. Surprises, disturbing anomalies, annoying elements are experienced when observing and sensing something that does not evolves "as expected".

In order to make sense of that surprise, an explanatory assumption (often based on analogies) is produced (figure 6). This assumption generates the type of information (data) that is needed for its validation [START_REF] Catellin | Abduction : a practise in science and in literature[END_REF]. The paramount importance of this contradicting or annoying fact is remarkably illustrated by Ginzburg [START_REF] Ginzburg | Signs, traces and tracks : roots of a paradigm of the clue (in french : « Signes, traces, pistes. Racines d'un paradigme de l'indice[END_REF]. This author investigates the way three different professionals work : G. Morelli, a renowned expert in paintings, S. Holmes (and behind the author of the novels : A. Conan Doyle), the famous detective and S. Freud, the psychiatrist. The first one claims that, in order to distinguish the original painting from a copy, tiny details are the clues on which new assumptions can be

M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT
built (the shape of a nail for instance), the second unravels puzzling investigations from small illogical details and the last one look for lapses such as a small mismatch of the language to penetrate the psyche of individuals.

Note that all of them were physicians or studied medicine. They were accustomed to look for symptoms to deduce deeper problems. Looking deeper in the history, Ginsburg analyses the way hunters do to trace the animals since millenaries. In particular, a famous oriental tale is quoted (Princes of Serendip) in which the princes were able to describe in details a camel without having ever seen it, just on the basis of the traces left by the animal and establishing the link between them. This technique is sometimes called "serendipity" (Walpole, 1764) and was illustrated by Voltaire in Zadig (Voltaire, 1748) who knew the oriental tale. Abduction and Serendipity use the same ingredients, the former insisting more on the establishment of a plausible "working assumption" and the latter on the organization of the facts in a coherent way [START_REF] Catellin | Abduction : a practise in science and in literature[END_REF]Andel and al., 2013).

Improving the practice ?

It seems now clear that "the surprise" matters. It can be used to refute a theory [START_REF] Popper | The Logic of Scientific Discovery[END_REF] to propose a new working assumption [START_REF] Pierce | Deduction, Induction, and Hypothesis[END_REF], to enlarge a perspective [START_REF] Ginzburg | Signs, traces and tracks : roots of a paradigm of the clue (in french : « Signes, traces, pistes. Racines d'un paradigme de l'indice[END_REF]…

The difficulty is certainly to trap, isolate, and stimulate surprises or anomalies. Note that in the practice of risk analysis described above in examples 1 and 2, a detail was trapped and combined to other data to imagine what could be an unwanted scenario. These are clearly cases of abductions and serendipity. But for this to be possible, imagination, supported by simulation of work or the presence of experts in cases 1 and 2 above, is part of how the risk analysis actually should work. Anders [START_REF] Anders | Obsolescence of manhood 2 ( in german : « Die Antiquiertheit des Menschen 2. Über die Zerstörung des Lebens im Zeitalter der dritten industriellen Revolution[END_REF] acknowledges this saying " So paradoxical it may look like, imagination is what we perceive from the present. We can know the present situation only if we can imagine what can happen".

In this respect, excess of formalism when applying methods may be detrimental. The risk analysis methods tend to be formalized, sometimes standardized, and even mandatory. For instance, in France, the bow-tie diagram to work on the safety barriers and some sort of quantified risk analysis are heavily « advised » by the authorities. As a result of this, much of the effort is devoted into implementing the method, in calculating the risks (probabilities and consequences). Imagination, the qualitative and core dimension of trying to foresee what could go wrong, is very likely to be undermined by the numbers.

Conclusion and perspectives

When considering emerging technologies, some consider that traditional risk analysis methods are inappropriate because they have failed in a number of situations even for standard or well known engineering systems. However, it is recalled here that these same risk analysis methods were developed within the context of earlier emerging technologies (space industry, nuclear power plants and weapons,..) and have been as such very useful for the development of solutions in these areas.

It is the opinion of the present authors that failures of those methods would pertain to the way they were implemented. This question is certainly not limited to some spectacular failures,

Figure 3 :

 3 Figure 3 : Safety door in a plane and the German Wings crash (flight n° 9525) in the south ofFrance(Mars, 25, 2015)

Figure 4 :

 4 Figure 4 : flooding in Paris(1910)

Figure

  Figure 6 : abduction

M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT

which could be isolated cases, but seems much deeper : a recent investigation of the global outcome of the safety studies tells that the frequency of occurrence of major technological accidents is at least tenfold higher in reality than foreseen [START_REF] Lannoy | Limits, shortcomings and benefits from present probabilitic approaches (in french : « Limites, insuffisances et apports des approches probabilistes actuelles : quelles leçons tirer ?[END_REF]. A similar finding is reported by Mannan [START_REF] Mannan | Lees' loss prevention in the process industries[END_REF].

The present work suggests that there might be a qualitative difficulty to identify the root causes of the accidents but also to foresee the whole extent of each scenario. In this perspective, there is certainly a need to :

• Investigate how risk analysis methods are implemented (observing engineers, experts, the dynamics of the working teams,..) to better ground our understanding of the conditions under which these methods produce really what they are expected to produce ; • Work on the way learning from past accidents is used. An analogy with the medical approach could be favored: identifying symptoms and interpreting them. Qualitative reasoning using analogies should be used in fields where too little is known to stimulate imagination; • Enrich our traditional risk analysis method with some additional creative method. On this point, the TRIZ method appeared to the present authors as a possibility [START_REF] Alshuller | Creativity as an Exact Science: the Theory of the Solution of Inventive Problems[END_REF][START_REF] Regazzoni | TRIZ tools to enhance risk management[END_REF] although the real potential (and practicality) needs to be assessed (in the TRIZ method the user is invited to formulate his problem so as to extract the internal contradictions. The resolution of the contradiction provides innovative solutions).

M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT

Highlights :

• illustrated examples of various situations where standard risk analysis failed and explanations;

• illustrated examples of the successful identification of a unknown risk within the context of a formatted risk analysis exercise; • investigation of the way discoveries are made and potential link with the risk analysis exercise.


Proust Christophe

Turbulent Flame Propagation in large Dust Clouds

teaching and research institutions in France or abroad, or from public or private research centers.

Turbulent flame propagation in large dust clouds

Introduction

Since some decades, efforts have been made to develop numerical codes to try and help with the safe design of industrial facilities against explosions. A few are dedicated to dust explosions (Skjöld, 2007;Proust, 2005). One of the main challenges is to be able to correlate the local aerodynamics of the dust air mixture to the flame propagation. In particular, the incidence of the flow is targeted, since turbulence is always present in dust clouds. There is a need to find a correlation establishing the relationship between the combustion velocity (rate at which the mixture is burnt per unit flame area) and the characteristics of the turbulence. Today, data are still severely lacking. Experiments are very difficult to do especially the measurement of the turbulence parameters. One of the pioneering works was performed 30 years ago, (Tezok et al., 1985). Experiments were done in a closed vessel and the turbulence intensity was measured during pretests without dusts with LDA or hot wire techniques, assuming it will be the same with the presence of dust particles and during flame propagation. Very often the flame speed is deduced and extrapolated from the pressure signals. Some progresses very made since that time and new data published (Krause and Kash, 2000;Sattar et al., 2014) but mostly at lab scale and with the same limitations. During the last years however, evolutions were presented both in the way the measure the turbulence in the dust cloud in test conditions and to estimate the turbulent burning velocities both in closed bombs (Snoeys et al., 2006) and in the tube method (Schneider andProust, 2007, Hamberger et al., 2007). But this is still the laboratory scale. In this paper, some estimations of the turbulent burning velocities of dust-air mixtures at larger scales are presented and compared to the small scale results. Some basic turbulence and turbulent combustion are firstly recalled to point out the possible links between the main parameters. The experimental setups are presented and then the results and interpretation.

Turbulence and turbulent combustion

It was shown some time ago (Proust, 2006a & b, Gao and al., 2015, Eckhoff, 1993) that the main lines of the explosion process may be very similar in gas and dust atmospheres. This is especially true for carbonaceous dust like flour, starch,… may be not for metal dusts.

In particular, for those dusts burning in air, laminar, cellular and turbulent flame regimes were identified. The laminar flame regime obeys the same mechanisms: the reactants ahead of the combustion zone are heated by conduction up to being pyrolysed so that the combustion occurs in gaseous phase. Not surprisingly, the relevant parameters of this basic combustion regime are the consumption rate of the flame front, the "laminar burning velocity" Sl, and the flame thickness, η0, deduced from Sl using the thermal diffusivity of the medium.

A flow becomes turbulent as soon as, inside a boundary layer (velocity gradient), the low speed layers of the flow are rolling up with the higher speed layers to produce eddies which appear and dissipate rapidly. Such structures are "chaotic" and can be studied using statistics which has been done for nearly one century : these structures constitute the "turbulence" of the flow (Hinze, 1975). The relevant theories introduce the notion of "turbulent cascade" according to which the initial eddies are destroyed in smaller and smaller structures until dissipation by molecular diffusion so that there is a mechanical link between all the structures of the turbulence. In practice, this "cascading" process is seen as an intrinsic process, independent from the mean flow. Because of this, it is then sufficient to know the characteristics of the largest eddies, those directly issued from the average flowfield, to fully characterize the turbulence. These characteristics are the scale of the largest eddies (L = "integral scale of turbulence") and their peripheral velocity (u'= "rms of the velocity fluctuations"). The parameter u' is in principle a space averaged variable and L is the area under the curve giving the evolution of the correlation coefficient of the velocity signals around a reference point.

It is implicitly assumed that the situation may be comparable with dust clouds (Tezok et al., 1985). This is not fully obvious however, since for instance, direct observation show that the particles are pushed around the turbulent eddies (Proust, 2006b, Bozier, 2004). The mixture does not remain locally homogeneous, and as a consequence, the burning may occur mostly at the periphery of the eddies and potentially in between (figure 1). This makes a difference with turbulent burning in homogeneous gaseous flames. To our knowledge, this issue has not been addressed yet.

Turbulent cloud (laser tomo.) Turbulent flame

Fig. 1.

So the turbulent flame propagation is premixed gaseous mixtures remains the model to refer to. Unfortunately, the propagation mechanisms are still a matter of passionate debate and are the subject of active research. For the present purpose, it is sufficient to recall that the parameter of interest is the "turbulent burning velocity", St, defined in a somewhat similar way as the laminar burning velocity. It can be viewed as the local flame consumption rate over the averaged flame front and should be represents of the speed at which the flame progresses against the mean flow.

For those situations where the turbulent flame can be seen as a disturbed version of a laminar flame, which should be the case when L>> η0 (Borghi and Destriaux, 1998), the characteristics of the turbulence should be coupled to those of the laminar flame front in the propagation process. Following St should be a function of Sl, η0, u' and L. A number of approaches, sometimes conceptually diverging, were used to derived such relationship over the time (Bray, 1990;Gülder, 1990;Yakhot, Peters, 1986) but a simple engineering dimensional analysis says that the relationship should be :

0 ' a b h ae ö ae ö = × × ç ÷ ç ÷ è ø è ø
Where K, α and β are constants. A tentative use of the Gülder model (K=0.6, α=0.75, β=0.25), resembling also that proposed by Peters, was proposed (Schneider andProust, 2005, Hamberger andal., 2007) and seems to correlate well with lab scale experiments (Figure 2 : L=0.03 m). Noteworthy the characteristics of the turbulence were measured in the dust cloud using a technique very similar to that explained hereafter.

Fig. 2.

The agreement seems very reasonable but there is a need to verify it further especially in view of capturing the scale effect. To this purpose large scale experiments were performed.

Setups and metrology

Experiments were done using a 1 m 3 closed vessel, a 10 m 3 vessel and a 100 m 3 chamber (figure 3). In the 1 m 3 vessel the dispersion system is that described in the international ISO 6184/1 standard. The dust is contained in a 5 l reservoir pressurized up to 20 bar (air). An electrical of pyrotechnical valve releases the dust in the chamber via a perforated ring. The overall release area is about 300 mm 2 . The ignition source is normally activated about 600 ms after the start of the dust dispersion. But in the frame of the present testing a larger ignition delay was used (1500 ms) in some instances to obtain a lower level of turbulence. Also, an additional bottle of pressurized air was added for some tests to increase the turbulence level (keeping the 600 ms ignition delay which is required to empty the dust reservoir : Snoeys et al., 2006).

For the other chambers a powerful pneumatic dust disperser was systematically used. It is made of a circular 32 mm orifice (1200 mm 2 release area) connected to a curved reservoir (150 mm internal diameter; 700 mm long). To trigger the dispersion, pressurized air is injected at the top of the reservoir so that the pressure jumps at about at about 7 bar (gauge) at the early beginning of the discharge process and drop to zero in about 2 seconds.

Only one disperser was used in the 10 m 3 vessel and was located on an end flange opposite to the ignition source (on the other end flange). The ignition source is located on the axis of the chamber at 0.8 m from the flange and the ignition delay (elapsed since the pressurization of the disperser) is about 800 ms.

Up to 4 dust dispersers were installed in the 100 m 3 vessels on the frame middle in the chamber (at 5 m from the blind end). The ignition source was located in the center of the rear wall, opposite to the vented section and the ignition delay (elapsed since the pressurization of the disperser) is about 1700 ms.

The overpressure are measured classically Kistler Piezoresistive (0-10 bar) transducers. One is mounted flush on the outer wall of the chamber (in the middle between both extremities) and is thermally insulated. The second one is mounted on the dust reservoir to control the discharge.

In some cases, K thermocouples (1 mm) were used (with the 1 m 3 vessel exclusively).

The detection of a propagating flame can be a challenge especially when short distances are concerned (Snoeys et al., 2006). Usually optical detectors can be employed when separations distances are a meter at least. Amplified photodiodes were used with the 100 m 3 chamber. With shorter run up distances, ionization gages can be used. These were employed with the smallest devices. The electrodes (4 to 6) were aligned on a rod of 0.5 m long close to the ignition source, where the flame is expected to grow isotropically so that the flame velocity is the expansion velocity (expression (2)).

One of the greatest difficulties is to be capable of measuring the turbulence in a dust air mixture. , 2006). It was used to do the measurement of figure 2. The sensor head is a short tube (length 2 cm , diameter 1 cm) with a solid wall in the middle and the differential pressure is measured on both sides with a precise and fast pressure transducer. The aeraulics of the system had to be refined to have sufficient dynamics.

Extensive testing in a reference jet proved that such transducers are able to detect eddies as small as 2 cm with a peripheral velocity of 0.2 m/s. Note in particular that the devices can do measurements even in very dusty mixtures (up to 500 g/m 3 at least).

Fig. 3.

Most turbulence measurements were obtained without dust being dispersed in the chambers but some were performed with dust and gave similar results (Snoeys et al., 2006).

The results presented hereafter were obtained with starch based dusts because the laminar burning velocities and burnt products temperatures are known (Proust, 2006a;Schneider and Proust, 2007). Some published data are recalled below about the fundamental burning properties (measurements using the tube method -10 cm diameter and 25 µm diameter K thermocouples for the temperature measurements).

Table 2:

Reference There is no discernable influence of the nature of the starch and of the particle size. It may then be estimated that the above parameters are representative of the agricultural dusts used for the present testing (table 3). A limited number of comparative tests were done using stoichiometric methane-air mixtures (1 m 3 vessel).

Several of the turbulence probes have to be used simultaneously to do a measurement because first the turbulence may not be homogeneous and second because the signals have to be cross correlated to derive the integral scale of the turbulence (see Schneider and Proust, 2007 for further details). A typical example of such a display is shown on figure 5 for the specific case of the 1 m 3 vessel. Apart in the jet region, there is no average flow inside the vessel but mostly turbulence at least in the combustion time slot. In this particular case, the turbulence probes were too close not to disturb the largest structures of the turbulence and an alternative method was employed to estimate the integral scale of the turbulence. The probes were separated and their signal was correlated in time during the period were a measurable average velocity was detected. The autocorrelation time multiplied by this velocity gives an image of the space correlation (Taylor assumption). The results area presented in figure 4 showing the space correlation obtained this way and the cross correlation of the probes. Clearly, the probes tend to be strongly correlated suggesting there are in the same region of the flow but also that they interact and influence each of them. So the integral scale of the turbulence is about 3 to 4 cm whereas the intensities of the turbulence are respectively 1.7 m/s in the standard conditions, 0.7 m/s when the ignition time is 1500 ms and 4 m/s in the reinforced turbulence configuration.

0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6 1,8 2 -0,6
Time since the opening of the dispersion bottle (s) Distance (m)

Fig. 5.

A set of vertically aligned turbulence probes was installed in the geometrical centre of the 10 m 3 chamber. The results are presented on figure 6. As for the "elongated" 1 m3 vessel, there is an average axial velocity of about 2 m/s but is much less. The turbulence intensity is about 3 m/s and the integral scale of turbulence is about 5 to 10 cm with a reasonable agreement between the time and space correlations.

Fig. 6.

Similarly, a set of vertically aligned turbulence probes was installed on the axis of the 100 m 3 chamber and moved between the blind end towards the vented end. The results are presented on figure 7. The average flow velocity is about 0 m/s and the rms of the fluctuations (u') is not far from 1 m/s at least in the ignition zone. From the area, under the curve of the spatial cross correlation coefficient the integral scale of the turbulence can be deduced and is about 20 cm. (5)

The measurements of Tvmax and Pvmax were measured in the 1 m 3 vessel (figure 8). Knowing that Tvmax/Tinit is about 5.7 at maximum, it can easily be shown, using expression (3), that Tpmax/Tinit should be on the order of 4.7 so that Tpmax = 1100°C which is fully in line with the data from figure 4.

The expansion ratio, deduced from equation (2) and the relationship (5) are represented on figure 9.

Tvmax/Tinit Fig. 9.

The expansion ration is clearly greater that the ratio Tpmax/Tinit (maximum 5) so that expression [4] suggests Nproduct/Ninit should be significantly greater than 1. On average this ratio is about 1.7. The reason for this could be that, in most experimental situations, the concentration of dust is well above the stiochiometric conditions (250 g/m 3 ) so that the excess dust can be pyrolysed in the very hot burnt products (starch gasify at 400°C in flames : Proust 2006b) producing a significant amount of gases. This point was suggested earlier(Lemos, xxx).

  Fig. 3.

  Standard laboratory equipments and especially LDA and hot wire anemometry are not applicable. INERIS has been working for years on the development of alternative techniques (Proust, 2004). A Pitot tube technique based on a very refined concept of Mc Caffrey gauges (Mc Caffrey, 1976) was implemented (Fig. 4). The device provides results fully in line with traditional techniques (in figure 4 the turbulence measured in the 1 m 3 ISO vessel is shown and compared to LDA measurements : from Snoeys et al.

  Fig. 4.

Fig. 7 .

 7 Fig. 7. Turbulence results (100 m 3 vessel)

  close to Tpmax/Tinit only if Nproduct/Ninit. There is a possibility to derive Nproducts/Ninit from the measurement of Tvmax and Pmax using the perfect gas law to rewrite the right hand side of expression (1):

  Fig. 8.

Fig. 10 .Fig. 11 .Fig. 12 .Fig. 13 .

 10111213 Fig. 10. Flame trajectory and pressure trace in the "cubic" 1 m 3 chamber (500 g/m 3cornstarch, standard dust dispersion procedure).

Fig. 14 .

 14 Fig. 14. S t -u' relationship as function of the scale of the experiments4. Conclusion and perspectives

Table 3 :

 3

	Reference	Sauter dia. (µm)	Concentration (g/m 3 ) Max. expl. Overp. (b)	Kst (b.m/s)
	Wheat flour (bakery) 47	250 and 500	7.3	70
	Corn starch	13	250 and 500	7.3	100

Table 4 :

 4 turbulence characteristics of the various setups.

	Nature	Turb. Intensity (m/s)	Int. lengthscale (m)
	3.3.3 Explosion overpressures and burnt product temperatures
	! "#" $ %	& ' (	! "#" )	( )
	* + , * + -.	&	, * + -.
	/ '0 ! 1) %	2 3456 2 7879	! 1	( )
					( )
	/ : ; +456 ; 7879	! 1< %	; 3456


EA Rémi Chambon
email: remi.chambon.1@gmail.com

Sébastien Dugravot

Jean-Marc Paillisson

Jean-Christophe Lemesle

Frédéric Ysnel

Guillaume Gélinaud

Partial migration in inexperienced Pied avocets (Recurvirostra avosetta): distribution pattern and correlates

Birds exhibit a range of wintering behaviour from strictly migrant to strictly resident species. In partially migrating ones, some birds overwinter within their breeding region (resident birds) while others, although breeding in the same area, winter far away (migrant birds). Accordingly, choosing a wintering region is a key stage in the annual life cycle of birds, notably for inexperienced first-year individuals. The present study aimed to investigate this issue, and more specifically to study the distribution pattern during winter and factors influencing the wintering behaviour of first-year Pied avocets, Recurvirostra avosetta. Based on a 10-year ringing study carried out on five of the major breeding colonies distributed along the Atlantic coast, we showed the coexistence of different wintering tactics. The resident tactic was predominant (approximately 85% of the 575 birds re-sighted), while the other birds adopted migration. Among resident individuals, two different tactics occurred: 43% of birds overwintered within their natal colony, whereas the others wintered in another site located at relatively close proximity along the French Atlantic coast. Hatching date was a consistent predictor of all wintering tactics. More specifically, the probability of migrating was the highest for early-hatched birds, and for resident ones, the probability of wintering within their natal colony rather than in another French site was the highest for both median-and latehatched individuals. In addition, a colony effect was demonstrated for resident birds.

Several biological interpretations, including social system, variations in both individual body condition and habitat quality, were put forward to explain these correlates.

Introduction

Winter quarter selection is a major event in the annual life cycle of birds. Environmental conditions experienced within wintering grounds (e.g. habitat quality and predation risk) and travel costs (notably energetic expense) to reach distant sites are known to strongly impact individual fitness (i.e. survival and upcoming reproductive success, [START_REF] Marra | Linking winter and summer events in a migratory bird by using stable-carbon isotopes[END_REF][START_REF] Alcock | Animal behavior: an evolutionary approach -Sunderland[END_REF][START_REF] Norris | Tropical winter habitat limits reproductive success on the temperate breeding grounds in a migratory bird[END_REF][START_REF] Lok | The paradox of spoonbill migration: most birds travel where survival rates are lowest[END_REF][START_REF] Alves | Costs, benefits, and fitness consequences of different migratory strategies[END_REF] and ultimately, population dynamics [START_REF] Adriaensen | Differences in migratory behaviour between early and late brood nestlings of the robin Erithacus rubecula[END_REF]Dhondt 1990, Norris and[START_REF] Norris | Seasonal interactions, habitat quality, and population dynamics in migratory birds[END_REF]. Partial migration is a behavioural dimorphism commonly displayed by bird species [START_REF] Lack | The problem of partial migration[END_REF][START_REF] Newton | The migration ecology of birds[END_REF][START_REF] Dingle | Migration: the biology of life on the move[END_REF]. It implies that some individuals overwinter in their breeding region (called resident or sedentary birds) while others leave it to reach geographically disjunct and distant quarters during the winter (migrant birds).

Several hypotheses have been explored to explain this pattern, suggesting a continuum in the control of the migratory tendency from obligate to facultative partial migration among bird species [START_REF] Lundberg | The evolution of partial migration in birds[END_REF][START_REF] Newton | The migration ecology of birds[END_REF][START_REF] Chapman | The ecology and evolution of partial migration[END_REF] for a recent review, [START_REF] Hegemann | Causes and consequences of partial migration in a passerine bird[END_REF]. In the case of obligate partial migration, individual differences in wintering behaviour are supported by genetic determinism and result in two different phenotypes (i.e. resident and migratory strategies) which are fixed over the lifetime of individuals [START_REF] Biebach | Genetic determination of partial migration in the European robin (Erithacus rubecula)[END_REF][START_REF] Lundberg | Partial bird migration and evolutionary stable strategies[END_REF][START_REF] Lundberg | The evolution of partial migration in birds[END_REF][START_REF] Berthold | Bird migration: a general survey[END_REF]). Alternatively, facultative partial migration is considered as a conditional strategy providing two ecophenotypes (i.e. resident and migratory tactics) which can alternate during the lifetime of individuals [START_REF] Ketterson | The evolution of differential bird migration[END_REF][START_REF] Lundberg | Partial bird migration and evolutionary stable strategies[END_REF][START_REF] Lundberg | The evolution of partial migration in birds[END_REF][START_REF] Chapman | The ecology and evolution of partial migration[END_REF][START_REF] Alcock | Animal behavior: an evolutionary approach -Sunderland[END_REF]. More precisely, any selected tactic is expected to mainly result from the combined influences of environmental conditions (e.g. density-dependence, climatic conditions, [START_REF] Newton | The migration ecology of birds[END_REF], Accepted Ar ticle .' [START_REF] Vélez-Espino | Ecological advantages of partial migration as conditional strategy[END_REF]) and individual differences in some traits for example body size/condition and dominance status [START_REF] Ketterson | Geographic variation and its climatic correlates in the sex ratio of eastern-wintering Dark-eyed Juncos (Junco hyemalis hyemalis)[END_REF][START_REF] Gauthreaux | The ecology and evolution of avian migration systems[END_REF][START_REF] Lundberg | Dominance behaviour, body weight and fat variations, and partial migration in European blackbirds Turdus merula[END_REF], Pérez-Tris and Telleria 2002[START_REF] Hegemann | Causes and consequences of partial migration in a passerine bird[END_REF].

Overall, factors driving facultative partial migration are thought to vary according to species ecology (e.g. social system) and to the status of individuals (e.g. age/experience). For inexperienced birds, leaving breeding grounds or not when winter approaches, represents one of their first crucial decisions (Sans-Aguilar et al. 2012), and hatching date and body condition at fledging are two of the main determinants of the wintering behaviour. In the Blue tit, Cyanistes caeruleus, [START_REF] Smith | Intraspecific variation in migratory pattern of a partial migrant, the blue tit (Parus caeruleus): an evaluation of different hypotheses[END_REF] showed that late-hatched males were more inclined to migrate than early-hatched ones.

However, [START_REF] Adriaensen | Differences in migratory behaviour between early and late brood nestlings of the robin Erithacus rubecula[END_REF] observed the opposite pattern in the European robin, Erithacus rubecula. Since many passerine species display a strong territorial behaviour throughout the annual cycle (e.g. [START_REF] Schwabl | Winter and breeding territorial behaviour and levels of reproductive hormones of migratory european robins[END_REF][START_REF] Stutchbury | Competition for winter territories in a neotropical migrant: the role of age, sex and color[END_REF][START_REF] Cuadrado | Winter territoriality in migrant black redstarts Phoenicurus ochruros in the Mediterranean area[END_REF]), competition for food resources was assumed to be involved in both species [START_REF] Schwabl | Control of partial migration and autumnal behaviour[END_REF]. The basis of the difference reported here on the hatching date effect between these two species is probably linked to variations in their respective ecology and in the strength of agonistic interactions between individuals and age classes [START_REF] Schwabl | Control of partial migration and autumnal behaviour[END_REF]. [START_REF] Colwell | Philopatry and natal dispersal of the western snowy plover[END_REF] found that the probability of migration during the first winter in the Snowy plover, Charadrius alexandrines nivosus, a gregarious species, was higher for early-hatched birds. The authors suggested that this tendency was related to social determinants. In addition, [START_REF] Barbraud | Phenotypic correlates of postfledging dispersal in a population of greater flamingos: the importance of body condition[END_REF] highlighted that young Greater flamingos, Phoenicopterus ruber, in good condition at fledging had a higher probability of leaving their natal region during their first winter and travelled over long distances to reach a wintering area.

Accepted Ar ticle

.'

The Pied avocet, Recurvirostra avosetta, is a gregarious shorebird species (Recurvirostridae, [START_REF] Cramp | The birds of the western palearctic[END_REF], with a relatively wide range (from western Eurasia to Africa, Hötker and Dodman 2009). Birds of the western European breeding population mainly winter in a few typical coastal grounds (mostly inter-tidal mudflats and lagoons) distributed along the Atlantic coast of western Europe and also the western coast of north Africa (Hötker and Dodman 2009). However, different migratory tendencies occur among its constituent subpopulations. There is evidence of partial migration in the British subpopulation, although neither a quantitative assessment of the proportion of resident and migrant birds nor the associated factors determining wintering behaviour were provided [START_REF] West | Pied Avocets wintering in Britain, 1947-2001[END_REF]. Conversely, Danish and German subpopulations are strictly migrant [START_REF] Salvig | Migratory movements and mortality of Danish Avocets Recurvirostra avosetta[END_REF][START_REF] Hötker | Choice of winter quarters in Avocets (Recurvirostra avosetta)[END_REF][START_REF] Hötker | Arrival of Pied Avocets Recurvirostra avosetta at the breeding site: effects of winter quarters and consequences for reproductive success[END_REF].

Furthermore, first-year individuals originating from the Wadden Sea were found wintering in different regions according to the severity of winter [START_REF] Hötker | Choice of winter quarters in Avocets (Recurvirostra avosetta)[END_REF]. The latter result clearly suggests that environmental conditions could affect wintering ground selection and probably, more generally, the choice of the migratory behaviour in this species.

Concerning the French Atlantic subpopulation, first-year Pied avocets are assumed to be partial migrants (Hötker and Dodman 2009) but this has previously never been documented. Here, we specifically addressed this issue using data from a 10-year ringing program carried out in five of the major French Atlantic breeding colonies. Our data provide a unique opportunity to examine wintering destinations on both large and fine spatial scales and to test for a facultative partial migration pattern. Our main objectives were thus twofold: (i) to analyse the distribution pattern of birds during their first winterthis notably consisted of quantifying the proportion of individuals adopting each wintering tactic and also testing for the occurrence of a spatial structure

Accepted Ar ticle

.' between the French natal colonies and the wintering grounds, and (ii) to explore the influence of factors that might account for the choice of a wintering tactic, including context-dependent and intrinsic variables.

Methods

Study sites and fieldwork

Breeding colonies

From 2006 to 2015, a joint ringing study was conducted within five major breeding colonies of Pied avocets located along the French Atlantic coast (Fig. 1): (i) the gulf of Morbihan (thereafter called Morbihan), (ii) the marshes of Mesquer (Mesquer), (iii) the marshes of Guérande (Guérande), (iv) the bay of Bourgneuf including the island of Noirmoutier (Bourgneuf), and (v) the island of Ré (Ré). Each colony included a wetland area composed of habitats actively used by Pied avocets during the breeding periodman-made habitats such as salt pans. Colonies were separated by unsuitable habitats and distances between them ranged from 12 to 207 km (orthodromic distances).

Capture and ringing

Every year (except in 2007 and 2012, respectively for Bourgneuf and Morbihan), chicks were captured by hand at an estimated age of 20 to 30 days (i.e. just before fledging) between April and August. Once captured, each bird was ringed with a unique colour combination fixed onto the two tibiotarsi, plus a metal ring from the French Ringing Scheme (C.R.B.P.O.) on one of the tarsometatarsi. Chicks were then weighed, using a spring balance (in g, ± 0.1 g). Both the bill length (from the tip of the upper mandible to the base of the first feathers of the skull) and the tarsus length (from the notch of the intertarsal joint to the base of the 90° bent foot) were measured, using a Vernier calliper (in mm, ± 0.1 mm). The wing length (from the bend of the wing to the tip of the longest primary feather) was also taken, using a metallic stop ruler (in mm, ± 0.5 mm). Given

Accepted Ar ticle

.' the lack of sexual dimorphism, the gender of chicks could not be determined. Finally, spatial coordinates of the capture location of all ringed birds were recorded. For Morbihan, Mesquer and Guérande, ringed chicks represented more than 50% of the fledglings over the study period (we have no such estimates for the other two colonies).

Winter re-sightings

Thanks to a large network of professional and amateur ornithologists, French-ringed Pied avocets have been observed across the wintering range of the western European breeding population (Hötker and Dodman 2009). During winter, birds are mainly observed foraging in flocks on intertidal mudflats, coastal lagoons, salt pans and, to a lesser extent, in fishponds and shallow lakes, depending on the locations (Hötker and Dodman 2009). Concerning the French Atlantic coast, birds were observed in relatively few sites that are regularly used by the Pied avocet during winter. Nine of these sites usually support more than 90% of birds wintering in France [START_REF] Mahéo | L'Avocette élégante Recurvirostra avosetta hivernant en France (Littoral Manche-Atlantique), 1977-2005[END_REF]).

Among them are notably the five wetlands also used as breeding grounds (Morbihan, Mesquer, Guérande, Bourgneuf and Ré). Birds were also observed in northwest Europe, across the Iberian Peninsula and along the west coast of northern Africa. Since most of the French wintering sites are included in protected areas, birds were routinely observed by both the local staff of managers and our research team. Consequently, the re-sighting effort during winter was considered to be relatively more intensive in France when compared to the other regions, and was comparable between years. Also, fieldwork was very regular outside France, notably in the Iberian Peninsula where the major wintering sites were visited for a period of several weeks every year. In Africa, the observation effort was much less intensive and less regular from year to year.

Data selection

Accepted Ar ticle

.'

We restricted investigations to the re-sightings collected during the first winter of ringed birds (from winter 2006/2007 up to winter 2015/2016). Since only three birds were recorded in Africa, re-sightings from this region were excluded from the dataset. Based on the distances recorded between capture and wintering locations, first-year individuals were categorized into three groups corresponding to three different wintering tactics: (i) birds wintering within their natal colony (called strictly resident birds), (ii) individuals wintering in a site located on the French Atlantic littoral, but not in their natal colony (called locally resident birds, orthodromic distance < 320 km), and (iii) birds observed farther away (called migrant birds, see Supplementary material Appendix 1, Fig. A1) either in the south of the Iberian Peninsula (> 1030 km) or in northwest Europemore exactly in the United-Kingdom, after crossing the British Channel (> 350 km) or along the North Sea coast (> 690 km). Distances travelled by migrant birds (orthodromic distances) were probably underestimated if one considers that individuals mainly follow the littoral for their journey. Additionally, natal and wintering grounds reached by migrant birds were separated by wide areas of unsuitable habitats. The first two categories (strictly and locally resident birds) have also been defined in this way elsewhere (see for instance [START_REF] Adriaensen | Population dynamics and partial migration of the European robin (Erithacus rubecula) in different habitats[END_REF]. Re-sightings of migrant Pied avocets occurred from 1 st November to 31 th March. Based on the range of migratory timing in our database (irrespective of bird age), individuals re-sighted in France from 20 th December to 25 th February were defined as resident birds with high certainty.

The dataset analyzed was composed of 575 first-year Pied avocets (i.e. 28.9% of the total number of chicks ringed from 2006 to 2015) observed during their first winter.

The number of individuals originating from the five breeding colonies ranged from 55 to 179 (see details in Fig. 1). Based on survival probabilities calculated between the capture period and the beginning of the subsequent winter (unpublished data), we

Accepted Ar ticle

.' estimated that our sample size represented approximately 45% of ringed birds likely to be alive in their first winter. In cases of multiple re-sightings (55% of birds were observed from two to a maximum of 21 times during winter), one location was randomly selected to assign spatial coordinates to each individual for spatial analyses.

Each migrant bird was observed in a unique wintering region (i.e. there was no movement between northwest Europe and the Iberian Peninsula regions during a given winter). A small fraction of resident birds (5.3%) was observed in two different wintering sites. In this case, we selected the location whose re-sighting date was the closest to 15 th January (the middle date of the wintering period). In the statistical analyses, no distinction was made between years (i.e. all years were pooled): it could have been appropriate to explore a year effect if for example, severe winter conditions had occurred during the study period, but that was not the case.

Statistical analyses

Distribution pattern of wintering birds

We firstly carried out a set of comparisons, using Binomial tests of equality: the proportions of migrant and resident birds (strictly and locally resident individuals pooled), the proportions of migrant birds according to their wintering region (northwest Europe or the Iberian Peninsula), and the proportions of strictly and locally resident birds.

Then, we explored whether a spatial structure occurred for different groups of birds, by looking for a preferential winter distribution with respect to their natal origin.

For migrant birds, we investigated the relationship between their origin (five natal colonies) and their wintering regions (northwest Europe or the Iberian Peninsula) using a Fisher's exact test of independence. For locally resident birds, we used a Mantel test which is particularly relevant in the case of mapped point data as it occurred here. We Accepted Ar ticle .' followed the method used by [START_REF] Ambrosini | A quantitative measure of migratory connectivity[END_REF] to study migratory connectivity.

For this, two matrices of orthodromic distances were built: one composed of interindividual distances calculated from the location of birds at the time of capture and a similar one based on wintering locations. A Mantel test (with 9999 random permutations) was then performed; it consisted of regressing wintering distances against natal distances. When significantly positive, a correlation (Mantel coefficient: r m ranging from -1 to +1) means that birds captured at close distances overwinter at close distances too. Additionally, we tested whether distances travelled by birds between their capture and wintering locations varied according to the natal colony, using a Mood test (a non-parametric median test) for the global test, and a Mood test with a false discovery rate correction for post-hoc pair-wise comparisons.

Correlates to wintering tactics

We investigated the influence of several variables on the probability of a bird (i) displaying migrant behaviour (migrant vs resident birds, thereafter called ʽMigrant model'), or (ii) being strictly resident when wintering in France (strictly resident vs locally resident birds, thereafter called ʽStrictly resident model'), using Generalized Linear Models (GLMs) with Binomial error family and logit link-function. In the ʽMigrant model', we did not differentiate wintering regions (northwest Europe or the Iberian Peninsula). We explored the effects of two context-dependent and one intrinsic candidate explanatory variables. As context-dependent variables, we used natal colony (n = 5 colonies) and hatching date within season (n = 3 categories). Hatching date of chicks was derived from the estimation of chick age when captured using a reversed logistic growth regression (see Supplementary material Appendix 2, Fig. A2) applied to bill length (n = 557 individuals). Finally, estimated hatching dates were classified into three groups: early-(33.9% of the total number of chicks), median-(44.7%) and late-Accepted Ar ticle .' hatched (21.4%) birds (see Supplementary material Appendix 2, Fig. A3). This categorization was consistent according to years and colonies. The body condition of chicks at capture was the only intrinsic variable tested. For this, a Principal Component Analysis (PCA) was performed on body size variables (bill, wing and tarsus lengths). Because of a few missing values for the hatching date variable, the ʽMigrant model' and ʽStrictly resident model' were respectively performed on a total of 557 and 475 birds. For each GLM, the effect of natal colony and hatching date, and their interaction were tested. Tests of Type II were carried out and non-significant terms were removed from each model. Based on the least squares means (LSMeans) we performed pair-wise post-hoc Tukey comparisons of significant variables in both models. The LSMeans were used to provide adjusted probabilities by taking the effect of each significant variable into account.

Accepted Ar ticle

.' All statistical analyses were carried out using R version 3.3.1 (R Core Team 2016) and the ʽade4', ʽcar', ʽlsmeans', ʽmixtools', ʽRVAideMemoire' and ʽsp' libraries.

The significant level was fixed at α = 0.05.

Results

Distribution pattern of wintering birds

From a total of 575 Pied avocets re-sighted during their first winter, most of them were resident (85.6%) while the others (14.4%) were migrant (Binomial test of equality, p < 0.001). Migrant individuals wintered predominantly across the Iberian Peninsula (80.7%, Binomial test of equality, p < 0.001), and more precisely in southern Portugal and along the south-western coast of Spain (see Supplementary material Appendix 1, Fig. A1). The other migrant birds wintered across northwest Europe (19.3%), mainly along the English Channel in the south of the United Kingdom and to a minor extent in the Netherlands and Germany. Concerning resident birds, the proportion of individuals wintering in their natal site (43.1%) was significantly lower than that of locally resident birds (56.9%, Binomial test of equality, p = 0.002).

Natal colony did not influence the distribution of migrant birds between the two

European regions reached (northwest Europe and the Iberian Peninsula, Fisher's exact test of independence, p = 0.19). Median travelled orthodromic distances (± MAD -Median Absolute Deviation) were 512.32 ± 125.12 km and 1102.07 ± 72.88 km for birds wintering in the northwest Europe and in the Iberian Peninsula, respectively.

Resident birds were recorded in 11 distinct French sites (Fig. 2). At first sight, there was evidence for a preferential distribution of birds in the vicinity of their natal colony during winter (Fig. 2). In other words, most birds originating from a given colony were recorded during winter either in the same place or within nearby sites. Indeed, for locally resident birds, individuals captured at close distance to each other tended to

Accepted Ar ticle

.'

winter at relatively short distance to each other (Mantel test, r m = 0.28, p < 0.001).

Additionally, locally resident birds were observed at a distance of 31.96 ± 25.42 km (median ± MAD, orthodromic) from capture locations (2.81 ± 2.25 km for strictly resident birds), with differences depending on the natal colony (Mood test, 4 DF, χ 2 = 82.91, p < 0.001). More precisely, the median distance travelled by locally resident birds was higher (from post-hoc comparisons) for birds originating from Bourgneuf and Ré (74.86 ± 34.76 km and 103.09 ± 86.35 km,respectively) than for birds originating from Morbihan and Guérande (36.16 ± 14.86 km and 36.12 ± 13.18 km,respectively).

The lowest median distance was found for birds originating from Mesquer (15 ± 6.53 km).

Correlates to wintering tactics

For first-year Pied avocets, the probability of migrating rather than being resident (ʽMigrant model') was not influenced by the colony of origin, but it was significantly associated with hatching date. More specifically, the probability of migrating was higher for early-hatched birds (probability ± SE = 0.23 ± 0.03) than for median-and late-hatched birds (0.12 ± 0.02 and 0.08 ± 0.02, respectively, Table 1 and Fig. 3).

Among first-year birds wintering in France, the probability of being strictly resident rather than locally resident (ʽStrictly resident model', Table 1 and Fig. 4A) was significantly lower for birds originating from Mesquer and Ré (0.22 ± 0.03 and 0.30 ± 0.06, respectively) than for birds originating from the other three colonies for which estimates were similar (ranging from 0.57 ± 0.04 to 0.62 ± 0.06). Furthermore, earlyhatched birds exhibited a significantly lower probability (0.32 ± 0.04) than median-and late-hatched individuals (0.45 ± 0.04 and 0.59 ± 0.05, respectively) of being strictly resident rather than locally resident (Table 1 and Fig. 4B).

Accepted Ar ticle

As already mentioned, body condition index was significantly influenced by natal colony (permutation test: n = 556, F = 26.69, DF = 4, p = 0.001): body condition index of birds hatched in Ré (mean ± SE = -16.93 ± 2.04) was significantly lower than for birds hatched among the four other colonies for which condition was statistically similar (mean values ranging from 0.20 ± 1.88 to 5.50 ± 1.32). Furthermore, analyses showed that body condition index varied significantly according to the hatching date of birds (permutation test: n = 555, F = 22.92, DF = 2, p = 0.001). The three two-by-two post-hoc tests were significant: body condition index was the highest for early-hatched birds (mean = 6.46 ± 1.24), the lowest for median-hatched individuals (mean = -5.11 ± 1.12), and intermediate for late-hatched ones (mean = 1.34 ± 1.77).

Discussion

The present study clearly demonstrates that first-year Pied avocets originating from the French Atlantic subpopulation display a partial migration pattern. More precisely, we showed that most of the birds were either strictly or locally resident, while only approximately 14% were migrant, mainly joining wintering grounds in the Iberian Peninsula or, to a lesser extent, in northwest Europe. To our knowledge, this is the first time that quantitative data has been provided on partial migration in a Pied avocet subpopulation of western Europe. Conversely, German and Danish Pied avocets were found to be strictly migrant [START_REF] Salvig | Migratory movements and mortality of Danish Avocets Recurvirostra avosetta[END_REF][START_REF] Hötker | Choice of winter quarters in Avocets (Recurvirostra avosetta)[END_REF][START_REF] Hötker | Arrival of Pied Avocets Recurvirostra avosetta at the breeding site: effects of winter quarters and consequences for reproductive success[END_REF]. A possible explanation for this variation in the migratory tendency between Pied avocet subpopulations might arise from differences in weather conditions during winter. Indeed, severe winter conditions are known to trigger a decrease in habitat suitability (notably a decrease in food resources) and would promote migration in birds [START_REF] Lundberg | The evolution of partial migration in birds[END_REF][START_REF] Newton | The migration ecology of birds[END_REF].

Overall, weather conditions experienced in France during winter are less severe than in regions located at higher latitudes as in Germany and Denmark. In addition, since Accepted Ar ticle .' relatively clement winters were noted during the study period, this could explain why most of the French first-year Pied avocets stayed in France.

A second major result was that early-hatched Pied avocets had a higher probability of migrating (migrant tactic) compared to other birds even if only a small proportion of them migrated. A biological interpretation could be that early-hatched birds are in better condition (i.e. they have more fat reserves, see Merilä and Svensson 1997) at the time of migration departure and thus are able to bear the cost of travelling long distances to reach wintering quarters. Two main reasons, potentially acting in synergy, can explain this fact. Firstly, we showed that early-hatched birds were in better condition at fledging, a result which could be explained by the higher availability of high-quality rearing grounds (notably regarding food resources, Adret 1983) for early broods. If the inter-individual differences in body condition are maintained from fledging until migration departure, as demonstrated in another avian species (Merilä and Svensson 1997), it could explain the relationship found between hatching date and migration propensity. This is consistent with the role of body and local environmental conditions on post-fledging movements suggested in the Greater flamingo [START_REF] Barbraud | Phenotypic correlates of postfledging dispersal in a population of greater flamingos: the importance of body condition[END_REF]. Secondly, early-hatched birds had more time to build up reserves compared to later-hatched individuals, making the former more inclined to undertake a costly migratory journey [START_REF] Pierce | Breeding and social patterns of banded Dotterels (Charadrius bicinctus) at Cass river[END_REF]. [START_REF] Colwell | Philopatry and natal dispersal of the western snowy plover[END_REF] suggested that early-hatched Snowy plovers, another gregarious shorebird species, developed a migrant behaviour by wandering to join conspecific postnuptial flocks outside their own colony in which most birds were still raising their chicks or attempting to breed. In contrast, later-hatched birds would be directly incorporated at fledging into wintering flocks within their natal colony, promoting their resident behaviour [START_REF] Colwell | Philopatry and natal dispersal of the western snowy plover[END_REF]. Pied avocets also display post-nuptial flocks (Cramp and Simmons 1983, personal obs.). However, given Accepted Ar ticle .' the distribution (i.e. close proximity) of sites used by early post-nuptial flocks in the neighbourhood of the five colonies studied, it is unlikely that the wandering of earlyhatched birds to join groups would promote migration as suggested in the hypothesis put forward by [START_REF] Colwell | Philopatry and natal dispersal of the western snowy plover[END_REF].

Complementarily, we distinguished two wintering tactics among resident individuals: strictly resident birds (43%) wintering within their natal colony, and locally resident birds (57%) wintering in other French sites. For locally resident birds, we found that the distance travelled between capture and wintering locations fluctuated according to the natal colony. These variations are probably due to the heterogeneous distribution of suitable wintering sites in the vicinity of the natal colonies. For instance, these distances are greater for birds hatched in Bourgneuf and Ré which are the most isolated grounds from other suitable wintering sites (see Figure 2). While we did not provide evidence of any preferential wintering destination for migrant birds according to their natal colony, we found that locally resident birds wintered preferentially in a few sites located in very close vicinity to their natal colony. Additionally, we showed that the probability of resident birds wintering within their colony of origin varied according to the natal colony: it was lower for birds from Mesquer and Ré (about 25% vs 60% for the three other colonies). Differences in carrying capacity between these five wintering sites might explain this result, due to a density-dependent process known to influence facultative partial migration [START_REF] Vélez-Espino | Ecological advantages of partial migration as conditional strategy[END_REF]. Mesquer and Ré support a lower proportion of the almost 20 000 Pied avocets wintering annually in France when compared to the three other sites (2-3% vs 6-14%, Wetland International databases).

That might mean that Mesquer and Ré are less suitable habitats at that time when competition for food resources is likely to be exacerbated, and maybe also during the breeding period. Indeed, birds originating from Ré had a lower body condition.

Accepted Ar ticle

Furthermore, we showed that early-hatched birds had a lower probability of being strictly resident as compared to median and late-hatched ones. As already discussed, body condition is correlated with hatching dates and might thus affect the choice between the two resident tactics. Nevertheless, its influence on wintering decisions is probably less important for resident birds than for migrant ones (i.e. very short vs long distances to reach wintering sites). Moreover, resident birds hatched in Ré displayed a low body condition and a low probability of being strictly resident. Therefore, we suggest that the effect of hatching date might be related to the specific social behaviour already mentioned [START_REF] Colwell | Philopatry and natal dispersal of the western snowy plover[END_REF]. By wandering in the vicinity of their natal colony searching to join post-nuptial flocks, early-hatched Pied avocets would have more opportunities to reach high-quality sites to overwinter, whereas later-hatched individuals would stay with conspecifics within their natal colony, increasing their probability of becoming strictly resident.

Conclusion

We demonstrated that first-year French Pied avocets display a partial migration pattern in contrast with what has been found among some of the other western European subpopulations for both inexperienced birds [START_REF] Hötker | Choice of winter quarters in Avocets (Recurvirostra avosetta)[END_REF]) and adults [START_REF] Salvig | Migratory movements and mortality of Danish Avocets Recurvirostra avosetta[END_REF][START_REF] Hötker | Choice of winter quarters in Avocets (Recurvirostra avosetta)[END_REF][START_REF] Hötker | Arrival of Pied Avocets Recurvirostra avosetta at the breeding site: effects of winter quarters and consequences for reproductive success[END_REF]. More precisely, we showed that most of the birds were resident, overwintering either in their natal colony or in a relatively close wintering site. In addition, we showed the influence of two context-dependent variables, hatching date being a consistent predictor of all wintering tactics. For this latter, underlying associated factors explaining wintering behaviours might differ according to tactics. For instance, the effect of body condition of birds on the decision to migrate is strongly suspected, and social behaviours would be predominant for the choice Accepted Ar ticle between the two resident tactics. Based on our findings, we can hypothesize that inexperienced first-year French Pied avocets display a facultative partial migration. Nevertheless, additional relevant factors necessary to clarify the control of the wintering behaviour effectively may be missing (e.g. genetic factors, sex, immune function and personality: [START_REF] Biebach | Genetic determination of partial migration in the European robin (Erithacus rubecula)[END_REF][START_REF] Chapman | The ecology and evolution of partial migration[END_REF][START_REF] Hegemann | Causes and consequences of partial migration in a passerine bird[END_REF][START_REF] Nilsson | Energy reserves, information need and a pinch of personality determine decision-making on route in partially migratory blue tits[END_REF]). An exploration of the fitness consequences in terms of survival and reproductive success would be very helpful to evaluate the differences between the wintering tactics. In the case of facultative partial migration, the fitness of the different wintering categories would not necessarily be equal and may notably vary according to environmental conditions [START_REF] Lundberg | The evolution of partial migration in birds[END_REF][START_REF] Gillis | Life history correlates of alternative migratory strategies in American dippers[END_REF]. Therefore, the study of the degree of individual lifetime fidelity (or flexibility) to wintering tactics would also be a perspective of major interest. Additionally, further investigation is needed to determine whether resident tactics promote breeding philopatry. All these issues would contribute to our understanding of the demographic mechanisms involved in the growth of the French breeding subpopulation since its settlement during the middle of the 20 th century (Hötker andWest 2005, BirdLife International 2015).

  Then, a second step consisted of regressing body mass of chicks against individual component scores from the first axis of the multivariate analysis (as recommended bySchulte-Hostedde et al. 2005). The first axis explained 76.5% of the variance in chick body size. Lastly, residuals of the regression (R² = 0.56, 1 and 554 DF, F = 696.7, p < 0.001; estimated slope ± SE = 14.06 ± 0.53, t = 26.4, p < 0.001, estimated intercept = 190.08 ± 0.81, t = 235.3, p < 0.001, n = 556 chicks) were used as a body condition index of chicks at the time of capture (values ranging from -105.3 to 83.8). However, body condition index was dependent on both natal colony and hatching date (two permutation tests, see the Results for details) and was thus removed from the set of explanatory variables finally tested in the GLMs in order to minimize statistical biases in the variable selection process. Post-hoc permutation tests (using the false discovery rate method) allowed us to determine how the body condition index varied according to each level of both natal colony and hatching date variables (see the Results).

FIGURE LEGENDS Figure 1 :

 LEGENDS1 FIGURE LEGENDS Figure 1: Geographical location of (A) the breeding region of the Pied avocet along the French Atlantic littoral, and (B) the five French colonies studied. Numbers of first-year birds considered in this study are reported for each colony.Accepted Ar ticle

Figure 2 :

 2 Figure 2: Distribution of resident Pied avocets (n = 492) among 11 wintering sites situated along the French Atlantic littoral (with decreasing latitudes: Morbihan, Vilaine-Pénerf, Mesquer, Guérande, Loire, Bourgneuf, Olonne, Aiguillon, Ré, Moëze-Oléron and Teich-Arcachon) according to their natal colony (shades from white to black). The numbers of birds re-sighted are reported for each wintering site.Accepted Ar ticle

Figure 3 :

 3 Figure 3: Probability of first-year Pied avocets migrating (n = 557) as a function of hatching date (expressed in three classes: early, median and late). Post-hoc significant differences were represented by different letters. Vertical lines represent 95% confidence intervals.Accepted Ar ticle

Figure 4 :

 4 Figure 4: Probability of being strictly resident for first-year Pied avocets wintering in France (n = 475) as a function of (A) natal colony (five colonies), and (B) hatching date (expressed in three categories: early, median and late). Post-hoc significant differences were represented by different letters. Vertical lines represent 95% confidence intervals.Accepted Ar ticle

TABLE LEGEND Table 1 :

 LEGEND1 Generalized Linear Models testing the simple and interaction effects of two explanatory variables on the probability of migrating for first-year Pied avocets (n = 557, ʽMigrant model'), and being strictly resident when wintering in France (n = 475, ʽStrictly resident model').

	Accepted Ar ticle	Effect Natal colony Hatching date Interaction term	Migrant model DF p 7.46 χ 2 4 0.11 18.62 2 < 0.001 5.92 8 0.66	Strictly resident model χ 2 DF p 57.03 4 < 0.001 16.24 2 < 0.001 7.15 8 0.52

Acknowledgements

We thank the Avocet field teams of the 2006-2015 period for their invaluable assistance in the field, both in France and abroad, and the many more birdwatchers for colour-ring re-sightings across the flyway. We particularly thank Frédéric Touzalin, Alain Gentric, Julien Gernigon, François Hémery, Loïc Jomat, Fabien Mercier, Jean-Guy Robin, Franck Salmon and Matthieu Vaslin for their ringing skills and the following nature reserves involved in the Pied avocet program: Réserve naturelle des marais de Séné, Accepted Ar ticle

. ' Réserve Naturelle du marais de Müllembourg, Réserve naturelle de Lilleau des Niges, and Réserve naturelle de Moëze-Oléron. José Alves, Claudine de le Court and Alfonso Rocha helped with logistics and access to the staging sites in Portugal and Spain. We

Accepted Ar ticle