Fabrice Roda
email: roda_fabrice@yahoo.fr

Jean-Marc Roda
email: roda@cirad.fr

Working paper-Foraging traces as an indicator to monitor wild boar impact on ground nesting

Keywords: ungulates, birds, wild boar, ungulates management, over-abundance Graphical Mean ±SE 0,2 0,3 0,4 0,5 0,6 0,7 0,8

birds.

Introduction

In past decades, the decline of temperate bird species in Western Europe and in North America has attracted much concern (Birdlife International, 2004;Julliard et al., 2004a;Fuller et al., 2007a;[START_REF] Gregory | Wild bird indicators: using composite population trends of birds as measures of environmental health[END_REF]EBCC, 2015). The causes of the observed decline of temperate songbirds are complex. Habitat quality loss may be responsible of the observed decline of forest (Fuller et al., 2007a;[START_REF] Gregory | Population trends of widespread woodland birds in Europe[END_REF][START_REF] Quine | Stand management: a threat or opportunity for birds in British woodland Ibis[END_REF][START_REF] Fuller | Avian responses to transitional habitats in temperate cultural landscapes: woodland edges and young-growth. Birds And Habitat: Relationships In Changing Landscapes[END_REF] and farmland songbirds (Chamberlain & Fuller, 2000;Chamberlain et al., 2000;[START_REF] Voříšek | Trends in abundance and biomass of widespread European farmland birds: how much have we lost[END_REF]. In addition, climate change poses a threat to some bird species [START_REF] Julliard | Evidence for the impact of global warming on the long-term population dynamics of common birds[END_REF][START_REF] Jiguet | Climate envelope, life history traits and the resilience of birds facing global change[END_REF][START_REF] Gregory | An indicator of the impact of climatic change on European bird populations[END_REF].

In parallel to the decline of temperate songbirds, wild ungulate populations increases have been spectacular in many regions of North America [START_REF] Côté | Ecological impacts of deer overabundance[END_REF][START_REF] Rawinski | Impacts of white-tailed deer over-abundance in forest ecosystems: an overview[END_REF] and Western Europe [START_REF] Milner | Temporal and spatial development of red deer harvesting in Europe: biological and cultural factors[END_REF][START_REF] Massei | Wild boar populations up, numbers of hunters down? A review of trends and implications for Europe[END_REF]. The main causes proposed for wild ungulates over-abundance (as defined by [START_REF] Côté | Ecological impacts of deer overabundance[END_REF] are climate change, the local extermination of natural predator species, supplementary feeding, and changes in agricultural, sylvicultural and game management practices [START_REF] Flueck | Population regulation in large northern herbivores: evolution, thermodynamics, and large predators[END_REF][START_REF] Milner | Temporal and spatial development of red deer harvesting in Europe: biological and cultural factors[END_REF][START_REF] Servanty | Pulsed resources and climateinduced variation in the reproductive traits of wild boar under high hunting pressure[END_REF][START_REF] Massei | Wild boar populations up, numbers of hunters down? A review of trends and implications for Europe[END_REF]. For example in Europe, wild boar (Sus scrofa) populations reach historical peaks [START_REF] Saint-Andrieux | La progression des grands ongulés en France: bilan et conséquences[END_REF][START_REF] Massei | Wild boar populations up, numbers of hunters down? A review of trends and implications for Europe[END_REF]. The impact of over-abundant wild boar on conservation and economic interests include spread of diseases to livestock and people, vehicle collisions, and damage to crops , as well as reduction in plant and animal abundance and richness [START_REF] Bourcet | Evaluation des risques liés à l'augmentation des densités des sangliers sauvages en France[END_REF][START_REF] Massei | The environmental impact of wild boar[END_REF][START_REF] Brandt | Régime alimentaire du sanglier en milieu forestier de plaine agricole[END_REF][START_REF] Vignon | Collisions entre véhicules et ongulés sauvages: quel coût économique[END_REF][START_REF] Barrios-Garcia | Impact of wild boar (Sus scrofa) in its introduced and native range: a review[END_REF]. As wild boar populations increase in many European countries [START_REF] Massei | Wild boar populations up, numbers of hunters down? A review of trends and implications for Europe[END_REF], mitigating wild boar impact on environment and economic interests will present a significant challenge. What are the known consequences of large ungulates over-abundance on ecosystems? Numerous studies focus on high deer density impacts on habitats. Deer over-abundance has strong effects on plant communities, vegetation structure, and on abundance of many invertebrate taxa [START_REF] Fuller | Ecological impacts of increasing numbers of deer in British woodland[END_REF][START_REF] Gill | The impact of deer on woodlands: the effects of browsing and seed dispersal on vegetation structure and composition[END_REF][START_REF] Côté | Ecological impacts of deer overabundance[END_REF][START_REF] Joys | Influences of deer browsing, coppice history, and standard trees on the growth and development of vegetation structure in coppiced woods in lowland England[END_REF]Allombert et al., 2005a;[START_REF] Stockton | A natural experiment on the effects of high deer densities on the native flora of coastal temperate rain forests[END_REF][START_REF] Dolman | Ecosystem and competition impacts of introduced deer[END_REF][START_REF] Holt | Breeding and post-breeding responses of woodland birds to modification of habitat structure by deer[END_REF][START_REF] Onf | Colloque ONF des 27-28 mai à Velaine[END_REF]. Such changes in the woodland ecosystem affecting food and safe nesting availability are responsible for the negative cascading effects on songbird abundance and biodiversity [START_REF] Mcshea | Managing the Abundance and Diversity of Breeding Bird Populations through Manipulation of Deer Populations[END_REF][START_REF] Allombert | A natural experiment on the impact of overabundant deer on songbird populations[END_REF][START_REF] Hewson | Species-specific responses of woodland birds to stand-level habitat characteristics: The dual importance of forest structure and floristics[END_REF][START_REF] Holt | Breeding and post-breeding responses of woodland birds to modification of habitat structure by deer[END_REF][START_REF] Martin | Browsing down our natural heritage: Deer impacts on vegetation structure and songbird populations across an island archipelago[END_REF]. As omnivorous mammals, wild boars can have a substantial environmental impact and may affect many ecosystem components [START_REF] Massei | The environmental impact of wild boar[END_REF]. Their rooting activity can remove the herbaceous cover [START_REF] Bratton | The effect of the European wild boar, Sus scrofa, on gray beech forest in the Great Smoky Mountains[END_REF][START_REF] Howe | Forage relationships of European wild boar invading northern hardwood forest[END_REF]Carpio et al., 2014a), and cause a decrease of tree regeneration [START_REF] Gomez | Impact of vertebrate acorn-and seedling-predators on a Mediterranean Quercus pyrenaica forest[END_REF]. Wild boars negatively affect rodents by direct predation on juveniles [START_REF] Schley | Diet of wild boar Sus scrofa in Western Europe, with particular reference to consumption of agricultural crops[END_REF] or/and competition for food resource [START_REF] Focardi | Competition for acorns among wild boar (Sus scrofa) and small mammals in a Mediterranean woodland[END_REF]Carpio et al., 2014a). Wild boar over-abundance has cascading effects on threatened predators such as wildcat (Felis silvestris) by reducing rodent prey availability [START_REF] Lozano | Increase of large game species in Mediterranean areas: Is the European wildcat (Felis silvestris) facing a new threat[END_REF]. Previous studies on wild boar-birds interactions in their native range focused mainly on game bird species [START_REF] Massei | The environmental impact of wild boar[END_REF]. Wild boar predated capercaillie (Tetrao urogallus) and hazel grouse (Tetrastes bonasia) nests [START_REF] Saniga | Nest loss and chick mortality in capercaillie (Tetrao urogallus) and hazel grouse (Bonasa bonasia) in West Carpathians[END_REF][START_REF] Saniga | Ecology of the capercaillie (Tetrao urogallus) and forest management in relation to its protection in the West Carpathians[END_REF] and simulated nests of red-legged partridge [START_REF] Carpio | Predation of simulated redlegged partridge nests in big game estates from south central spain[END_REF]. As over-abundant wild boar preys on everything on the ground, their negative effects both in their native and introduced range are thus well established [START_REF] Barrios-Garcia | Impact of wild boar (Sus scrofa) in its introduced and native range: a review[END_REF].

To manage wild ungulates ecological footprint, it is necessary firstly to evaluate population size and secondly to precisely measure the ungulates impact through environmental indices. Monitoring changes in these indices provides a basis for setting management objectives [START_REF] Morellet | Indicators of ecological change: new tools for managing populations of large herbivores[END_REF]. Large herbivores populations are usually controlled through hunting in Europe [START_REF] Milner | Temporal and spatial development of red deer harvesting in Europe: biological and cultural factors[END_REF][START_REF] Massei | Wild boar populations up, numbers of hunters down? A review of trends and implications for Europe[END_REF]. Capture-Mark-Recapture methods (CMR) provide an efficient approach to estimate reliably population size, but are extremely time-consuming and expensive when applied to large herbivores [START_REF] Buckland | Wildlife population assessment: past developments and future directions[END_REF]. Distance sampling methods are less time-consuming but suffer from less accuracy and precision [START_REF] Buckland | Wildlife population assessment: past developments and future directions[END_REF][START_REF] Thomas | Distance software: design and analysis of distance sampling surveys for estimating population size[END_REF]. Both methods do not provide any functional information on the population-habitat system, as population size per se does not provide information on the environment. In addition, the primary concern for ungulates managers is often to prevent over-abundance of herbivore populations [START_REF] Mysterud | The concept of overgrazing and its role in management of large herbivores[END_REF][START_REF] Nichols | Monitoring for conservation[END_REF] rather than predicting accurately how many animals may be harvested or not. Researchers have recently developed tools for managing ungulate populations, which are based on the monitoring of a set of indicators of ecological change ("IEC"; [START_REF] Cederlund | Managing roe deer and their impact on the environment: maximizing the net benefits to society[END_REF][START_REF] Morellet | Indicators of ecological change: new tools for managing populations of large herbivores[END_REF]. An IEC is an easily measured parameter, sensitive to changes in the relationship between the population and its resources and habitat [START_REF] Cederlund | Managing roe deer and their impact on the environment: maximizing the net benefits to society[END_REF][START_REF] Dale | Challenges in the development and use of ecological indicators[END_REF]. [START_REF] Morellet | Indicators of ecological change: new tools for managing populations of large herbivores[END_REF] suggested tracking over time the variations of at least three categories of IECs: one describing animal performance, another describing herbivore impact on environment, and the last describing relative animal abundance. A set of IECs has been first developed for deer management [START_REF] Morellet | Indicators of ecological change: new tools for managing populations of large herbivores[END_REF], but such a set of indicators is currently lacking for wild boar. Recently, [START_REF] Acevedo | Estimation of European wild boar relative abundance and aggregation: a novel method in epidemiological risk assessment[END_REF] developed a method describing wild boar abundance based on the frequency of fecal droppings. However, IECs describing wild boar impact on fauna, flora, biodiversity and habitat are currently lacking. The main objective of this study was thus to find such an IEC to monitor the impact of wild boar on avifauna.

We made the hypothesis that H1) wild boar preying activity should disturb ground nesting birds and thus decrease their abundances in highly foraged areas. Tree nesting birds are out of reach of wild boars, so we hypothesized that H2) tree nesting birds abundances should not negatively correlate with wild boar foraging activity. As wild boars may have an impact on birds by food competition, we investigated H3) the impact of wild boars on birds depending of ground for food resource, but nesting out of reach of wild boars.

Material and methods

Study area

The forest massif of Morières / Siou Blanc is situated in southeastern France (8638-ha forest area; altitude 500-650m, Figure S1), in an area with low human density, within which, there are only a few isolated houses.

2.2

Sampling protocol

Twenty-six stations were randomly selected. They are evenly spread over the whole forest massif, without prior knowledge of the vegetation, structure or bird communities. Ground visits were organized during the winter season and the positions of the stations were recorded using GPS to allow their subsequent localisation. The stations were in the main body of the forest of Morrières / Siou Blanc and were therefore surrounded on all sides by woodlands. Around each station, a 1 km survey transect was set-up to search for wild boar foraging traces. The survey routes were along pre-existing paths or forest roads of 1-5m width (allowing the possibility for cars or people to enter to the different places).

Each survey route was divided into 100m-length segments, yielding 10 segments per transect. In each segment, the presence of wild boar foraging traces was recorded, which allowed the derivation of a simple indicator of kilometric foraging (Ikf, a continuous variable), that is, the number of segments with wild boar foraging traces/10 segments. This methodology gives an index derived on the frequency of occurrence (see a similar methodology based on fecal droppings in [START_REF] Acevedo | Estimation of European wild boar relative abundance and aggregation: a novel method in epidemiological risk assessment[END_REF]. When signs of rooting activity were seen and occupied a contiguous ground area larger than 2m², the segments were considered as positive. Stations with Ikf ranging from 0 to 0,5 were considered as low wild boar traces (Ikf-). Stations with Ikf from 0,6 to 1 were considered as high wild boar traces (Ikf+). These two level factor were used for GLM analysis (see below, statistics section). Two measures of Ikf were made during spring for each station, between 1st April and 30th May 2012. We measured Ikf the same days as bird surveys (see 2.3). We found 25 counts with low densities of wild boar foraging traces (IkF-), and 27 counts with high wild boar foraging traces (IkF+).

Along each survey route, we measured variables related to habitat structure, viz., tree cover, density of shrub cover, average tree and shrub heights. Based on results of previous bird studies (see below "Vegetation data" section ), we expected these variables to play an important role.

Bird survey

Spot-counts were all done by the same observer (FR). Counts were carried out in each plot during two visits in spring 2012 ; point-count times were separated by an interval of at least four weeks to detect both early and late breeding birds (1st-15th April and 15th-30th May), following the recommendations of the French Breeding Bird Survey of the National Museum of Natural History (MNHN, 2012;[START_REF] Jiguet | French citizens monitoring ordinary birds provide tools for conservation and ecological sciences[END_REF]. Counts were performed within 1-4h after sunrise. Point-count stations (n = 26) were positioned a minimum of 500m apart to avoid overlap of bird counts. To minimize a potential edge effect [START_REF] Ries | A predictive model of edge effects[END_REF], the center of all study plots were situated at least 100m from the edge of forest alleys. We used 100m fixed-radius point-counts that lasted 5 minutes to allow for a good estimation of relative bird abundances, following the methodology detailed in [START_REF] Blondel | La méthode des indices ponctuels d'abondances (IPA) ou des relevés d'avifaune par stations d'écoutes[END_REF][START_REF] Jiguet | French citizens monitoring ordinary birds provide tools for conservation and ecological sciences[END_REF] Repeating point-counts throughout the breeding season improved estimation of species richness and relative abundances of birds [START_REF] Dettmers | Influence of point count length and repeated visits on habitat model performance[END_REF][START_REF] Drapeau | Refining the use of point counts at the scale of individual points in studies of bird-habitat relationships[END_REF][START_REF] Bonthoux | Point count duration: five minutes are usually sufficient to model the distribution of bird species and to study the structure of communities for a French landscape[END_REF]. Points were recorded in the reverse order during the second count to avoid introducing temporal bias between different stations. Since all of the plots were located in core forest habitats with low visibility, birds were mostly detected through their song. Individuals could be distinguished when several birds continuously sang at different locations in the plot.

We excluded observations of raptors, i.e., Golden Eagle (Aquila chrysaetos), Bonelli's Eagle (Aquila fasciata), Short-toed Snake Eagle (Circaetus gallicus), Peregrine Falcon (Falco peregrinus), Common Kestrel (Falco tinnunculus), Eurasian Sparrowhawk (Accipiter nisus), corvids (Corvus corrax and Corvus corrone), Common Cuckoo (Cuculus canorus), and Eurasian

Woodcock (Scolopax rusticola) as these species were poorly surveyed by the method used and/or have life cycles that make them irrelevant to the objective of the study. Game birds (Alectoris rufa, and Phasianus colchicus) were not recorded since their presence in a particular station may have been due to game releases in the area.

To test the hypothesis that the wild boar impact on birds may result of direct predation, we established a classification system that grouped the bird species encountered during the study into two guilds according to the nesting habitat (i.e., ground nesting vs. tree/shrub nesting birds). To test the hypothesis that wild boar may impact birds through food competition, birds were grouped into two guilds according to ground dependence for foraging and gleaning (i.e., ground foraging vs. tree foraging birds). Ground nesting birds were separated from ground foraging and tree foraging guilds to avoid potential overlap of effects (i.e., predation and food competition). Location of the nest-site were taken from literature [START_REF] Gregory | Population trends of widespread woodland birds in Europe[END_REF][START_REF] Flitti | Atlas des oiseaux nicheurs de Provence-Alpes-Côte d'Azur[END_REF] and our own observations (See Table S1). Four indicators were derived: an indicator for ground nesting birds (GN, n=201 birds), and an indicator for tree nesting birds (TN, n=339 birds), an indicator for ground foraging birds (GF, n=234 birds) and an indicator for tree foraging birds (TF, n=105 birds).

At the bird community level, we used species diversity as bird community indicator. Species diversity was calculated using the Shannon index. The Shannon index reflects the number of different species in a community and simultaneously takes into account how evenly the individuals are distributed among those species [START_REF] Lyashevska | How many dimensions of biodiversity do we need[END_REF].

Vegetation data

This study measured the effect of wild boar on birds, controlling for vegetation characteristics (shrub density, time since last clear-cut). Vegetation data were collected during spring 2012, after each bird count session. Measurements were taken at three sampling areas at each point-count station, thus resulting in 78 sampling points. One was located at the point-count station itself, the other two at random distances from the station (<50m). Mean values were calculated for each variables listed. Shrub layer density of vegetation layer (0-1m) was estimated using a pole graduated into 10cm sections and using a protocol adapted from [START_REF] Holt | Breeding and post-breeding responses of woodland birds to modification of habitat structure by deer[END_REF]. The number of sections obscured by vegetation was recorded when viewed from a distance of 10m. An index of shrub density (S, a continuous variable) was derived using the number of segments obscured/10 segments, from 0 to 1. We used these scores to define two classes of shrub layer density: values equal or smaller than 0,5 were considered as "low density" (S-), those equal or greater than 0,6 as "high density" (S+); this two level factor was used for GLM analysis (see below, statistics section).

As avian community structure is largely determined by the growth stage and associated structural characteristics of vegetation [START_REF] Fuller | The distribution of breeding songbirds within mixed coppiced woodland in Kent, England, in relation to vegetation age and structure[END_REF], vegetation age was controlled in each plot, using local forest management archives. Time since last clear-cut (Cut) was used as a continuous variable for statistical purpose (see below, statistics section).

As cervids are known to have an impact on birds (see [START_REF] Allombert | A natural experiment on the impact of overabundant deer on songbird populations[END_REF][START_REF] Holt | Breeding and post-breeding responses of woodland birds to modification of habitat structure by deer[END_REF][START_REF] Martin | Browsing down our natural heritage: Deer impacts on vegetation structure and songbird populations across an island archipelago[END_REF], signs of browsing were carefully searched in each sampling area. We considered only shrub species or woody plants because we could not, with herbaceous species, distinguish deer browsing from that of lagomorphs present in the study area. We recorded the browsing pressure by assessing the number of eaten twigs versus all available twigs at a height <1.80 m, following a methodology similar to those described elsewhere [START_REF] Picard | Evaluation de la charge en gibier par analyse floristique[END_REF][START_REF] Chevrier | The oak browsing index correlates linearly with roe deer density: a new indicator for deer management[END_REF]. A "browsing sign" is one bite by the animal on the tree, being evident as a fresh cut on the twigs. As the browsing pressure was <1% in all the sampled plots, the cervids impact on the avifauna was considered negligible in this study. All the assessments were done by the same person (FR). Great Tit; Eurasian Blue Tit

Statistics

We tested for the effects of 1) wild boar foraging (Ikf), 2) shrub cover (S), 3) timing of spot-counts (Tim) and 4) time since last clear-cut (Cut) (the factors of variation) on birds abundances, and species diversity (the dependent variables) using general linear models (GLM). For the count data (bird abundances), each model was tested assuming normal (Gaussian) and Poisson error. We selected the best model using the Akaike Information Criterion (AIC). However, when the AIC of two competing models differed by less than 2, we examined the residual sum of squares and retained the simplest model in line with the rules of parsimony. We checked for the homoscedasticity and normality assumptions of residuals. Raw data were not transformed, and real birds counts were used throughout the paper. Timing of spot-count was entered as a two-level factor (early vs. late) in all the models. Wild boar foraging and shrub cover were entered as a two-level factors (Ikf-vs. Ikf+; and S-vs. S+, respectively) to investigate potential interactions between factors. Entering wild boar foraging and shrub cover as continuous covariates led to the same conclusions. Time since last clear-cut was entered as a continuous covariate (Cut) in all models. Birds were grouped in functional guilds (See Table S1). We fitted several models including interactive effects of Ikf, S, Tim and Cut. As a control of potential bias in our study, we tested the effects of 1) S and 2) Tim (the factors of variation) on Ikf (the dependent variable) using GLM.

Statistical evaluation was carried out with the "Statistica" software using the functions "GLM" (normal error) or "GLZ" (poisson error). Best results were obtained assuming normal error. Nine bird species that were at least observed in one third of point-counts, were included in statistical analysis. All results are expressed as mean ± SEM. Differences were considered significant at P < .05.

Results

General results

Wild boar foraging traces were observed in all surveyed transects. Maximum value of wild boar indicator of kilometric foraging (Ikf) was of 1 with a high mean value of 0,60 ± 0,03, indicating a relatively strong pressure of wild boar foraging in the study area considered as a whole. Twenty-five spot-counts showed few wild boar traces (Ikf-), and 27 showed many traces (Ikf+, see methods section). GLM analysis showed no relationships between Ikf and timing of spot-counts (Tim) or shrub cover (S) (Adjusted R²=0,02; F=1,31; p=0,28). We analyzed the abundances of 9 bird species and 520 individuals (Table S1). Counts led to an average of 3,9 ± 0,3 SE ground nesting birds (range 0-8); 6,5 ± 0,3 SE tree nesting birds (range 3-13); 4,5 ± 0,3 SE ground foraging birds (range 1-9); and 2,0 ± 0,2 SE tree foraging birds per spot (range 0-7).

Wild boar impact on birds

We tested the effects of Ikf, Tim, S and Cut on the abundance of ground nesting birds. Abundances of TN, GF and TF birds were also tested as supplementary variables. The best model included the variables Ikf, S and Tim; Cut, TN, GF and TF were rejected as explanatory variables (Table 1). The observed changes were well explained by the GLM and were very statistically significant (Adjusted R²=0,70; F=17,89; p<10 -6 ). We found that the abundance values of GN birds were negatively correlated with Ikf (Figure 1a). Entering IkF as a two-level factor showed that there was a significant decrease of 44.7% in ground nesting birds abundances in areas strongly foraged by wild boars (Figure S2). GLM revealed the role of Ikf as being the main factor driving bird abundance changes, and Tim and S as being secondary factors, in this order (Table 2). Test of categorical variables interactions as explanatory variables were not significant (Table 2). The effect of Tim showed that birds were more numerous in late spring. We observed that the negative effect of wild boars on birds abundances was slightly greater in early spot-counts than late, but this result

GN

was not statistically significant. Shrub cover was positively correlated to ground nesting birds abundances. The negative effect of Ikf on birds abundances was marginally greater in S-than in S+ areas, but this result was not statistically significant (Figure S3). Mean values of GN birds in relation to hypothesis decomposition are summarized in Table S2.

Table S2

Mean values of GN birds in relation to hypothesis decomposition.

Table S3

Model selection for TN, GF and TF birds When two models had similar AIC and SS Residual scores, the most parsimonious model was chosen (see methods). We tested the effects of Ikf, Tim, S and Cut on the other guilds. Results of model selection are summarized in Table S3. By contrast to ground nesting birds, abundances of birds belonging to tree nesting (Figure 1b) or ground foraging (Figure 1c) guilds were not well explained by IkF or the other studied variables; the observed changes were not statistically significant (Adjusted R²=0,05;F=1,4;p=0,24;Adjusted R²=0,01;and F=1,1;p=0,40,respectively). The best model describing the tree foraging birds abundances included Ikf and Tim as explanatory variables (Table S3), but the observed changes were barely significant (Adjusted R²=0,12; F=2,0; p=0,07). However, we noted that tree foraging birds slightly tended to be more numerous in areas strongly foraged by wild boars, a not significant result (Figure 1d).

We then tested the effects of Ikf, Tim, S and Cut on bird species diversity. The best model describing species diversity included the variables Tim, IkF*S, Tim*S; Cut was rejected as an explanatory variable (Table S4). The observed changes in species diversity were statistically significant (Adjusted R²=0,33; F=4.6; p<10 -3 ). We found that the main factor affecting bird species diversity was the interaction of IkF*S (Figure 2, Table S5).

The present data indicate that, in accordance with H1, GN birds were less abundant in areas strongly foraged by wild boars (Figure 1a). Thus, in accordance with H2, TN birds abundance was not affected by wild boar foraging (Figure 1b). In addition, GF birds abundance was not correlated to wild boar foraging (H3, Figure 1c). Mean ±SE 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0

I kf 0 1 2 3

Shannon

Mean ±SE 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0

I kf 0 1 2 3

Shannon a b

Table S4 Model selection for species diversity When two models had similar AIC and SS Residual scores, the most parsimonious model was chosen (see methods).

Discussion and conclusions

Wild boar foraging as an indicator of ecological change

Surprisingly, the use of foraging traces as an indicator of wild boar impact on ground nesting birds has never been tested until now [START_REF] Massei | The environmental impact of wild boar[END_REF][START_REF] Barrios-Garcia | Impact of wild boar (Sus scrofa) in its introduced and native range: a review[END_REF]. We found that wild boar foraging traces provided a simple indicator of ground nesting birds abundance. Our results evidenced the usefulness of this method since it was highly correlated with GN birds abundances, but not with other avian guilds (tree nesting or ground foraging birds). The protocol described in this study has numerous advantages. First, the measurements do not depend on biological experts for birds species determination: in other words, anyone can perform an inventory of wild boar foraging traces. Wild boar foraging traces are easily spotted, and the probability of detection is likely constant among different observers using a standardized protocol. Second, wild boar foraging traces are rapidly measured; this may prove useful for monitoring programs designed to track annual changes of wild boar environmental impact on ground ecosystems at large spatial scales. Third, measuring wild boar foraging traces is less climate dependent than measuring bird abundances. Finally, the monitoring of wild boar impact on the avifauna do not requires the previous knowledge of wild boar population size.

Understanding and measuring the environmental impact of wild ungulates in forest ecosystems remains a challenge for researchers and wildlife managers. In practice, for management to succeed, clear goals are required and the success of their application must be evaluated [START_REF] Morellet | Indicators of ecological change: new tools for managing populations of large herbivores[END_REF]. The goal will often be a compromise such as maximising the revenue from hunting while minimising damage to forestry/biodiversity [START_REF] Williams | Analysis and management of animal populations[END_REF][START_REF] Nichols | Monitoring for conservation[END_REF]. Wildlife population trends are often assessed by species counts; surveillance monitoring frequently requires several years to amass enough data to provide strong evidence of a decline in state before action is taken. Species sampling methods remain labor intensive and time-consuming, and may be subject to many biases (Kéry & Scmidt, 2008;[START_REF] Renwick | Composite bird indicators robust to variation in species selection and habitat specificity[END_REF]. As a result, there is an increasing demand from policy makers to improve management by incorporating relevant indicators that are simple, reliable and rapidly assessed. By recording a set of IECs, the aim is to achieve management objectives, rather than exert a routine surveillance monitoring [START_REF] Cederlund | Managing roe deer and their impact on the environment: maximizing the net benefits to society[END_REF][START_REF] Dale | Challenges in the development and use of ecological indicators[END_REF][START_REF] Morellet | Indicators of ecological change: new tools for managing populations of large herbivores[END_REF]. To manage wild ungulates, it is thus suggested to track over time the variations of at least three categories of indicators of ecological changes [START_REF] Morellet | Indicators of ecological change: new tools for managing populations of large herbivores[END_REF]: one describing animal performance, another describing herbivore impact on environment, and the last describing relative animal abundance.

As pointed by [START_REF] Dale | Challenges in the development and use of ecological indicators[END_REF], the focus of ecological management programs becomes narrow by selecting only one or few indicators, and may lead to oversimplified understanding of the ecological interactions involved; the management of large ungulates is greatly improved by the recording of a set of indicators [START_REF] Morellet | Indicators of ecological change: new tools for managing populations of large herbivores[END_REF]. To avoid poorly informed management decisions, indicators should take into account multiple levels in the ecological network in order to effectively monitor the multiple levels of complexity of ecological systems [START_REF] Dale | Challenges in the development and use of ecological indicators[END_REF][START_REF] Niemeijer | A conceptual framework for selecting environmental indicator sets[END_REF]. Thus, a key challenge is to find a mix of measures which give easily identified signals, can be used to track the ecological conditions at reasonable cost, and cover the spectrum of ecological variation. The monitoring of our indicator of wild boar foraging responds to these criteria; and thus managers dispose rapidly and easily of a diagnostic of wild boar potential environmental impact on avifauna. However, if management interventions are required (i.e., if the manager estimates that wild boar impact on ground nesting birds may conflict with conservation interests), the accuracy of management will be greatly improved by studies carried out to estimate wild boar population size and trends. As such tools of wild boar abundance do already exist, based on fecal drops [START_REF] Acevedo | Estimation of European wild boar relative abundance and aggregation: a novel method in epidemiological risk assessment[END_REF], one goal of this study (to complete the set of indicators for wild boar management) was fulfilled. However, to be fully acknowledged as a new IEC, our indicator should be ideally tested over a long period of time, in reference localities where true wild boar population size is known [START_REF] Dale | Challenges in the development and use of ecological indicators[END_REF][START_REF] Morellet | Indicators of ecological change: new tools for managing populations of large herbivores[END_REF]. With a set of IECs, one can envisage an adaptive management program (as defined by [START_REF] Nichols | Monitoring for conservation[END_REF] of wild boar game harvest compatible with bird conservation. The final step is then to set new hunting quotas with the aim of provoking an adjustment in the population-habitat system in order to approach the predefined goals. At least during the first years of monitoring, this is equivalent to a trial-and-error process or adaptive management [START_REF] Nichols | Monitoring for conservation[END_REF][START_REF] Morellet | Indicators of ecological change: new tools for managing populations of large herbivores[END_REF].

Effects of wild boar foraging on birds abundance

To our knowledge, our study is the first to correlate bird abundances to an index based on the frequency of wild boar foraging traces. Our results showed a significant decrease of 44,7% in ground nesting birds abundances in areas strongly foraged by wild boars; in contrast, tree nesting birds (out of reach of large ungulates) were not affected by wild boar gleaning. This result was expected since ground nesting birds are highly impacted by other over-abundant ungulates [START_REF] Allombert | A natural experiment on the impact of overabundant deer on songbird populations[END_REF][START_REF] Holt | Breeding and post-breeding responses of woodland birds to modification of habitat structure by deer[END_REF], and wild boars are known to be highly opportunistic mammals that prey on anything near the ground: depredations on birds and nests [START_REF] Saniga | Nest loss and chick mortality in capercaillie (Tetrao urogallus) and hazel grouse (Bonasa bonasia) in West Carpathians[END_REF][START_REF] Saniga | Ecology of the capercaillie (Tetrao urogallus) and forest management in relation to its protection in the West Carpathians[END_REF][START_REF] Schaefer | Video monitoring of shrub-nests reveals nest predators: Jays are the most common predators, but carnivorous mammals and some other species also predate nests[END_REF][START_REF] Giménez-Anaya | Food habits of wild boars (Sus Scrofa) in a mediterranean coastal wetland[END_REF][START_REF] Carpio | Predation of simulated redlegged partridge nests in big game estates from south central spain[END_REF], reptiles and amphibians [START_REF] Jolley | Estimate of herpetofauna depredation by a population of wild pigs[END_REF], small mammals [START_REF] Lozano | Increase of large game species in Mediterranean areas: Is the European wildcat (Felis silvestris) facing a new threat[END_REF][START_REF] Wilcox | Wild pigs as predators in oak woodlands of California[END_REF], and other fauna and flora communities are substantial (see for review [START_REF] Barrios-Garcia | Impact of wild boar (Sus scrofa) in its introduced and native range: a review[END_REF]. The general assumption that all clutches of birds that nest on the ground represented potential prey for wild boars is thus probably true. This predation effect may explain the observed changes in bird distribution, as predation risk is known to influence prey-birds use of space [START_REF] Thomson | Fear factor: prey habitat selection and its consequences in a predation risk landscape[END_REF][START_REF] Cresswell | Non-lethal effects of predation in birds[END_REF]. We noted a cross-effect of wild boar foraging and habitat on birds, i.e. the effects of wild boar foraging on species diversity were more important in areas with low shrub cover. Other studies pointed that the density of potential nests and predation rates is dependent on vegetation structure complexity and that foliage density near the nest may reduce predation impact [START_REF] Martin | Habitat and Area Effects on Forest Bird Assemblages: Is Nest Predation An Influence[END_REF][START_REF] Chalfoun | Habitat structure mediates predation risk for sedentary prey: experimental tests of alternative hypotheses[END_REF]. In addition to resource competition, predation risk is an important factor determining spatial distribution of birds [START_REF] Suhonen | Predation Risk Influences the Use of Foraging Sites by Tits[END_REF] and offspring success [START_REF] Martin | Fitness costs of resource overlap among coexisting bird species[END_REF][START_REF] Thomson | Fear factor: prey habitat selection and its consequences in a predation risk landscape[END_REF][START_REF] Chalfoun | Assessments of habitat preferences and quality depend on spatial scale and metrics of fitness[END_REF]. Interestingly, GN birds abundances were influenced by the timing of spot-count. As two of three species composing the GN birds guild are long-migrant species, this result is not surprising. Further research is needed to investigate the potential interactions between migration strategy and wild boar impact, i.e., if long-migrant birds are more impacted than partial-migrants.

We then explored the hypothesis that wild boar may compete with ground foraging birds for food resource. Previous studies showed that wild boars negatively impacted rodents populations as a result of direct competition for seeds [START_REF] Focardi | Competition for acorns among wild boar (Sus scrofa) and small mammals in a Mediterranean woodland[END_REF] or legumes (Carpio et al., 2014a).

Although wild boars are omnivorous and prey invertebrates, diet consist mainly of plant matter including above-ground green material [START_REF] Fournier-Chambrillon | Diet of the wild boar (Sus scrofa L.) inhabiting the Montpellier garrigue[END_REF][START_REF] Baubet | Diet of the wild boar in the French Alps[END_REF].

The most obvious direct effect of rooting by wild boars is the reduction in plant cover [START_REF] Singer | Effects of wild pig rooting in a deciduous forest[END_REF][START_REF] Massei | The environmental impact of wild boar[END_REF]. Surprisingly, ground gleaning birds in our study were not statistically affected by wild boar foraging. This strongly support the idea that wild boar mediated effects observed in this study resulted mainly from direct predation on birds and nests or increased predation risk, rather than food competition. In a remarkable long-term study, [START_REF] Wesolowski | Wood Warbler Phylloscopus sibilatrix: a nomadic insectivore in search of safe breeding grounds[END_REF] showed that wood warblers abundance in Bialowieza forest was inversely correlated with small mammal predator densities and were only weakly correlated with caterpillar abundance; the nomadic behavior of wood warblers appears to be a consequence of their attempts to breed in safe places. Although most studies focused on the direct negative effects of wild ungulates on habitat alterations and invertebrate availability, large herbivores may affect many species of plants and animals through indirect effects [START_REF] Suominen | Effects of large herbivores on other fauna[END_REF][START_REF] Barrios-Garcia | Impact of wild boar (Sus scrofa) in its introduced and native range: a review[END_REF]. Large ungulates concentrations may have both positive and negative effects on insectivorous birds;

positive effect being probably mediated by nutrient input through the ecosystem [START_REF] Mathisen | Cascading effects of moose (Alces alces) management on birds[END_REF][START_REF] Mathisen | Contrasting responses of two passerine bird species to moose browsing[END_REF]. This type of complex interaction may explain the observation in our study, that tree foraging birds showed light increases in areas strongly foraged by wild boars. However, one should note that ground nesting birds (Bonelli's warbler, that are also tree foraging birds) were subtracted from this guild. As a result, this guild of "tree foraging birds" consisted of only two species, which require a careful interpretation. Future research is thus needed to better understand these mechanisms or confirm this result.

As final words, none of the birds species observed in this study are listed as threatened species; however wild birds trends are commonly used as indicators of environmental health and biodiversity [START_REF] Gregory | Wild bird indicators: using composite population trends of birds as measures of environmental health[END_REF]. As common bird species shape the ecosystems, even relatively small declines in their ranks can disrupt ecosystem structure [START_REF] Gaston | Commonness, population depletion and conservation biology[END_REF]; as a consequence, monitoring bird indicators may give understanding on other taxas. Numerous studies underlined the decline of common palearctic birds in Western Europe (Birdlife international, 2004;[START_REF] Gregory | Wild bird indicators: using composite population trends of birds as measures of environmental health[END_REF]EBCC, 2015). Recently, [START_REF] Gregory | Population trends of widespread woodland birds in Europe[END_REF] showed that birds nesting on the ground or in low vegetation declined more than those nesting higher up in shrubs and trees. Species trends at the European scale are thus more significantly correlated to nest-type (tree/shrub vs. ground/low vegetation) than to other ecological factors (migration, diet, region), a report that is true for both farmlands and forest birds. It is also suggested that predation may be an important factor influencing bird species trends in Europe [START_REF] Fuller | Living on the edge: British and Irish woodland birds in a European context[END_REF][START_REF] Gregory | Population trends of widespread woodland birds in Europe[END_REF]. Despite these reports, there are actually few managing programs that monitor the potential impacts of wild boar at large scales. As wild boar populations will continue to grow in Europe [START_REF] Massei | Wild boar populations up, numbers of hunters down? A review of trends and implications for Europe[END_REF], the development and use of new impact indicators may be critical for future bird and environmental management.

Figure S1 :

 S1 Figure S1: Location of the study area in southeastern France, showing layout of forest massif Of Siou Blanc les Morrières.

Figure 1 :

 1 Figure 1: Variations of forest bird mean abundances in relation to wild boar foraging (Ikf). Predicted trends are presented as black lines with 95% confidence intervals (dashed lines). Bars indicate the standard error of mean. (a) ground nesting (GN) birds (b) tree nesting (TN) birds (c) ground foraging (GF) birds (d) tree foraging (TF) birds.

Figure 2 :

 2 Figure 2: Variations of bird species diversity (Shannon) in relation to wild boar foraging (Ikf) and shrub density. Predicted trends are presented as black lines with 95% confidence intervals (dashed lines). Bars indicate the standard error +mean. (a) bird species diversity in areas with low shrub cover. (b) bird species diversity in areas with dense shrub cover.

Table S1 Classification of species in relation to foraging and nesting dependence

 S1

	Species acronym Species name	Ground or Tree/shrub	Migration strategy
			dependence
			Nesting	Foraging
	(a) Species
	Cya cae (n=55)	Eurasian Blue Tit (Cyanistes	Tree/shrub Tree/shrub Partial migrant
		caeruleus)
	Eri rub (n=69)	Robin (Erithacus rubecula)	Ground	Ground	Partial migrant
	Fri coe (n=111)	Common Chaffinch	Tree/shrub	Ground	Partial migrant
		(Fringilla coelebs)
	Lus meg (n=51)	Nightingale (Luscinia	Ground	Ground	Long migrant
		megarhynchos)
	Par maj (n=50)	Great Tit (Parus major)	Tree/shrub Tree/shrub Partial migrant
	Phy bon (n=81)	Western Bonelli's Warbler	Ground	Tree/shrub Long migrant
		(Phylloscopus bonelli)
	Syl atr (n=43)	Eurasian Blackcap (Sylvia	Tree/shrub	Ground	Partial migrant
		atricapilla)
	Syl mel (n=26)	Sardinian Warbler (Sylvia	Tree/shrub	Ground	Partial migrant
		melanocephala)
	Tur mer (n=34)	Blackbird (Turdus merula)	Tree/shrub	Ground	Partial migrant
	(b) Guilds	Species name
	Ground Nesting	Common Nightingale; Robin; Western Bonelli's warbler
	(GN, n=201)
	Tree Nesting (TN,	Blackbird; Common Chaffinch; Great Tit; Eurasian Blackcap; Eurasian Blue
	n=339)	Tit; Sardinian warbler
	Ground Foraging	Blackbird; Common Chaffinch; Eurasian Blackcap; Sardinian warbler
	(GF, n=234)
	Tree Foraging
	(TF, n=105)

Table 1 :

 1 Candidate

	Guild			selected variables			AIC	SS
	Ikf	Tim	S	Cut	TN	GF	TF		Residual
	Ground Nesting (GN)
	x	x	x	x	x	x	x	161.7	52.1
	x	x	x					164.1	52.2
	x	x	x	x				165.1	51.8
	x	x						183.4	81.5
	x		x					187.9	83
	x			x				196.8	109.7
	x							197.7	115.3
		x						208.7	161.7
			x					214.3	167.4
						x		221.6	197.9
					x			221.9	199.7
							x	222.1	200.4
				x				222	200.4

models explaining ground nesting birds abundance, with the Akaike information criterion values (AIC) and the selected variables : wild boar foraging (Ikf), timing of spot-count (Tim), shrub density (S), time since last clear-cut (Cut), tree nesting birds abundance (TN), ground foraging birds abundance (GF), tree foraging birds abundances (TF). The interactions between wild boar foraging and timing (IkF*Tim), wild boar foraging and shrub density (IkF*S), wild boar foraging, timing and shrub density (IkF*Tim*S) were also tested, but gave higher scores of AIC(data not shown). Bold characters indicate the selected model. Lines indicate standard error of mean. Results are highly significant (p<10 -6 ); see Table

2

and statistical analysis.

Table 2 :

 2 β coefficients and statistical parameters of the most parsimonious GLM model to explain ground nesting (GN) birds abundance using wild boar foraging traces (Ikf), Timing (Tim) and shrub density (S) as predictors. Bold characters indicate statistically significant results.

	Estimates (sigma-restricted parameterization)
		Level of effect Column	GN param.	± SE	GN	GN	GN β	± SE
	Effect					t	p
	Intercept		1	4,0	0,2	24,2	0,000000
	IkF	(-)	2	1,2	0,2	7,0	0,000000	0,59	0,08
	Tim	early	3	-0,8	0,2	-4,8	0,000018	-0,41	0,08
	S	(+)	4	0,8	0,2	4,9	0,000012	0,41	0,08
	IkF*Tim	1	5	0,1	0,2	0,6	0,581136	0,05	0,08
	IkF*S	1	6	0,1	0,2	0,3	0,744490	0,03	0,08
	Tim*S	1	7	0,1	0,2	0,3	0,753062	0,03	0,08
	IkF*Tim*S	1	8	0,0	0,2	0,2	0,831509	0,02	0,08

IkF(+) areas strongly foraged by wild boars. Black filled circles indicate areas with high density of shrubs; Open grey squares indicates areas with light shrub density. Lines indicate standard error of mean. Results are highly significant (p<10 -6 ); see Table 2 and statistical analysis.

Table S5 β coefficients and statistical parameters of the most parsimonious GLM model to explain species diversity using wild boar foraging traces (IkF), Timing (Tim) and shrub density (S) as predictors

 S5

	676
	677
	678
	679
	680
	681
	682
				selected variables			AIC	SS
		Ikf	Tim	S	Cut	Ikf*S	Tim*S		Residual
	Species diversity
			x			x	x	20,7	3,5
		x	x	x	x	x	x	21,7	3,5
			x	x			x	25,1	4,2
			x			x		25,3	3,5
						x		28,3	4,4
			x					30,3	5
							x	31,2	4,2
		x						32,6	5,3
				x				38,1	5,9
					x			38,6	6
	684
	685
	686
	687
	688
	689
	690
	Estimates (sigma-restricted parameterization)
		Level of effect	Column	Sp. div.	± SE	Sp. div.	Sp. div.	Sp. div. β	± SE
	Effect			param.		t	p
	Intercept		1	1.7	0.0	38.49080	0.000000
	Tim	early	2	-0.1	0.0	-1.92837	0.060277	-0.24	0.13
	S	(+)	3	0.1	0.0	1.27066	0.210526	0.16	0.12
	IkF	(-)	4	0.1	0.0	1.33765	0.187885	0.17	0.13
	Tim*S	1	5	0.1	0.0	1.91937	0.061437	0.24	0.13
	Tim*IkF	1	6	0.0	0.0	0.63077	0.531456	0.08	0.13
	S*IkF	1	7	-0.1	0.0	-2.34528	0.023587	-0.30	0.13
	Tim*S*IkF	1	8	0.0	0.0	0.51398	0.609837	0.06	0.12
	692
	693
	694
	695
	696
	697
	698
	699
	700
	701
	702
	703

This work was funded by the CIRAD, UPR BioWooEB (provided analysis tools).

Author contributions:

Conceived and designed the experiments: FR. Analyzed the data: FR and JMR. Contributed analysis tools: JMR. Wrote the paper: FR.

Submission declaration:

All authors agree with the contents of the manuscript and its submission to the journal. Neither the manuscript nor any parts of its contents are currently under consideration or published in another journal. All authors disclose any potential conflict of interest with other people or organizations that could inappropriately influence their work.