# Assessing the impact of transport infrastructure construction on breeding birds: A long term before-during-after experiment on female bustards

Soumaya Belghali, Pierrick Devoucoux, Daniel Bizet, Nathalie Hiessler, Thomas Menut, Olivier Scher, Vincent Bretagnolle, Aurélien Besnard

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## To cite this version:

Soumaya Belghali, Pierrick Devoucoux, Daniel Bizet, Nathalie Hiessler, Thomas Menut, et al. "Assessing the impact of transport infrastructure construction on breeding birds: A long term before-during-after experiment on female bustards." Biological Conservation, 2025, 305, pp.111058. 10.1016/j.biocon.2025.111058. hal-05039978

HAL Id: hal-05039978

https://hal.science/hal-05039978v1

Submitted on 18 Apr 2025

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# Assessing the impact of transport infrastructure construction on breeding birds: A long term before-during-after experiment on female bustards

Soumaya Belghali<sup>a,b,*</sup>, Pierrick Devoucoux<sup>c</sup>, Daniel Bizet<sup>d</sup>, Nathalie Hiessler<sup>b</sup>, Thomas Menut<sup>e</sup>, Olivier Scher<sup>b</sup>, Vincent Bretagnolle<sup>f</sup>, Aurélien Besnard<sup>a</sup>

## ARTICLE INFO

### Keywords:

BACI design

Carrying capacity

Little bustard

Railway avoidance

Tetrax tetrax

Zone effect

## ABSTRACT

Artificialization has significantly increased in the past century, posing one of the greatest threats to the viability of animal populations. Since 1980, it has been the second-highest cause of decline in farmland bird populations in Western Europe, after agricultural intensification. Impacts occur through direct destruction of natural and semi-natural habitats or their fragmentation, and also indirectly through the alteration of abiotic and biotic conditions in surrounding habitats, a phenomenon known as the zone effect. Most studies on the effect of artificialization on birds focus on existing structures without considering pre-construction conditions, which limits the ability to accurately assess and quantify their impacts and their zone effect. These studies also often only focus on singing male birds, which are easier to monitor, although females tend to make a greater contribution to population dynamics and may behave differently than males. Taking a different approach, this study estimated the impact of linear transport infrastructure before, during and after its construction on female birds in a breeding population. The case study was the Nîmes-Montpellier high-speed railway (HSR), which crosses the Costières Nîmoises Special Protection Area, home to one of the largest populations in France of little bustards (*Tetrax tetrax*), a highly endangered farmland bird species. Over a 10-year period, female little bustards were monitored before, during and after construction of the HSR within suitable breeding habitat at stratified distances from the railway. The results revealed a significant decline in female density, with a strong avoidance of areas near the HSR, extending up to 775 m after the trains began operating. While there were no major changes in breeding habitat availability in the area, female breeding capacity dropped by 57.8 % in the zone effect area, representing 26 % of the protected area. These findings highlight the need for thorough assessments of the indirect impacts of infrastructure using robust before-during-after-distance-control-impact methods. They also show the importance of considering zone effects in environmental impact assessments and conservation planning to ensure effective protection of threatened species.

## 1. Introduction

The proportion of the Earth's surface that has been altered by human activities has increased markedly over the past century. This is largely due to the conversion of natural and semi-natural areas such as forest, wetland and farmland into built-up, impervious areas, including urban, industrial and commercial areas or energy and transport infrastructure (IPBES, 2018). The projections for the near future indicate that this will

increase and even accelerate (McDonald et al., 2020; Dulac, 2013). Globally, urban areas increased from 350,000 sq km in 1992 to 740,000 sq km in 2015, and this is predicted to reach 1.9 million sq km by 2030 (McDonald et al., 2020). An increase in areas covered by large developments such as linear transport infrastructures is also expected. Worldwide, the length of road and rail networks is expected to increase by 60 % between 2010 and 2050 (Dulac, 2013). Road, rail and parking infrastructure is expected to reach 250,000–350,000 sq km by 2050,

<footnote><sup>a</sup> CEFE, Univ Montpellier, CNRS, EPHE-PSL University, IRD, Montpellier, France</footnote>

<footnote><sup>b</sup> Conservatoire d'espaces naturels d'Occitanie, 34000 Montpellier, France</footnote>

<footnote><sup>c</sup> Egis, 889 Rue de la Vieille Poste, 34965 Montpellier, France</footnote>

<footnote><sup>d</sup> Centre Ornithologique du Gard, Parc Kennedy, 285 Rue Gilles Roberval bâtiment C, 30900 Nîmes, France</footnote>

<footnote><sup>e</sup> Biotope, 24 Rue Paul Doumer, 34140 Meze, France</footnote>

<footnote><sup>f</sup> Centre d'Etudes Biologiques de Chizé, UMR 7372, CNRS & Université de La Rochelle, 79360 Villiers-en-Bois, France</footnote>

<footnote>* Corresponding author at: CEFE 1919 Route de Mende, 34090 Montpellier, France.
E-mail address: soumaya.belghali@cefe.cnrs.fr (S. Belghali).</footnote>
roughly the size of the United Kingdom or Germany (Dulac, 2013).

Currently, a million species are threatened with extinction (IPBES, 2018). Artificialization is one of the major threats to biodiversity through its adverse effects on the viability of certain populations (McDonald et al., 2008). The impacts of artificialization can be direct, through mortality (i.e. collisions with infrastructure) and through the conversion of land from natural or semi-natural habitats to inhospitable impervious areas (Borda-de-Agua et al., 2017; McDonald et al., 2020). It can also be indirect, through the alteration of abiotic and biotic conditions in surrounding habitats and through habitat fragmentation and alteration of connectivity, limiting the suitability of a large area around the infrastructure (Elmqvist et al., 2013).

A review by McDonald et al. (2020) showed that 66 % of studies quantifying the impact of urban expansion on biodiversity only considered the area that was directly altered. However, several studies have demonstrated that the extent and intensity of the zone effect, i.e. the area of indirect impact through the alteration of abiotic and biotic conditions, on animal populations should not be overlooked. A review by Benítez-López et al. (2010) showed that the zone effects of urban areas and of a wide range of infrastructure, including roads, railways, power lines, hydroelectric dams and wind farms, was noticeable on a large number of taxa, including mammals, amphibians, reptiles and birds. Yet the loss of functional habitat due to the zone effect varies in extent and intensity depending on the biological and ecological attributes of the species studied, as well as on the type of infrastructure, the traffic intensity it generates and environmental conditions around the infrastructure (Rabdeau et al., 2021; Benítez-López et al., 2010; De Jonge et al., 2022). A detailed examination of the zone effect, in extent and intensity, is thus essential to provide robust ecological impact assessments of infrastructure projects.

Like many other species, farmland birds of Western Europe are threatened by land artificialization, which has been identified as the second-highest cause of their population decline, after the intensification of agricultural practices (Rigal et al., 2023). Farmland birds are among the most rapidly declining terrestrial vertebrate species in Europe (Donald et al., 2001; Reif and Vermouzek, 2019; Stanton et al., 2018). Between 1980 and 2016, their populations declined by 56.8 % across the continent (Rigal et al., 2023). The little bustard (Tetrax tetrax) has experienced one of the strongest population declines in farmland birds in recent decades. The species has disappeared in at least 15 countries in the last century; in Europe, the remaining populations in France, Spain, Portugal and Sardinia are all undergoing a rapid decline (Morales and Bretagnolle, 2021; Gendre et al., 2018; Quaintenne et al., 2022). The main drivers of these population declines are the intensification of agricultural practices and the increase in the density of linear infrastructure (e.g. Silva et al., 2023, in Portugal). In addition to the direct loss of habitat due to land cover change, a zone effect of transport infrastructures has been demonstrated in male little bustards. A sharp decrease in male density has been recorded in proximity to linear infrastructure in Spain, Portugal, France and Sardinia, with an avoidance effect perceptible up to 1 km from roads (Alonso et al., 2020; Devoucoux, 2014; Santangeli and Dolman, 2011; Santos et al., 2016; Suárez-Seoane et al., 2002), high-speed railways (Devoucoux, 2014; Malo et al., 2017) and power lines (Marques et al., 2020; Silva et al., 2010).

However, most of the studies investigating the impact of infrastructure on little bustard populations have been conducted on already existing structures, i.e. with no initial baseline prior to their construction and without a sampling design based on stratified distances from the infrastructure (e.g. Santangeli and Dolman, 2011; Santos et al., 2016; Suárez-Seoane et al., 2002, Alonso et al., 2020, Marques et al., 2020; Silva et al., 2010). This precludes a robust quantification of the zone effect and the local loss of individuals resulting from the construction of the infrastructure. A more comprehensive assessment can be provided with before-during-after-control-impact (BDACI) approaches (Smith et al., 1993).

Furthermore, existing studies on little bustards have analyzed the impact of infrastructure only on leks of singing males (e.g. Santangeli and Dolman, 2011; Santos et al., 2016; Suárez-Seoane et al., 2002). Yet females do not use the same habitats as lekking males during the breeding season, and they behave differently (Devoucoux et al., 2018; Morales et al., 2013). While females visit leks for mating, they incubate eggs and rear young alone in fields with higher vegetation than those of leks to protect eggs and chicks. Females also select fields located away from males to avoid their harassment (Morales et al., 2013; Silva et al., 2014; Tarjuelo et al., 2013). Consequently, the impacts of infrastructures on females may differ from those on males. As female survival and fecundity contribute most to population dynamics and population viability in this species (Mañosa and Bota, 2023), it is crucial to quantify the impact of infrastructure construction on females to assess the effects on breeding populations in this species.

This study used a before-during-after-distance method to quantify the zone effect, in extent and intensity, of a high-speed railway (HSR) built between 2013 and 2016 in southern France on the breeding females of one of the largest French populations of little bustards. To this end, we modelled the change in density of breeding females in suitable nesting habitats at different distances from the infrastructure in three periods – before, during and after its construction – and estimated the loss in female abundance link to the building and operation of the railway line.

## 2. Methods

### 2.1. Case study area and infrastructure

The study was conducted in the south of France in the Costières Nîmoises Special Protected Area Natura 2000 site, which has been crossed by the high-speed railway line "Contournement Nîmes-Montpellier" (CNM) since 2016 (Fig. 1). The protected area encompasses approximately 13,000 ha and is a flat, open landscape dominated by small fields (median = 1.17 ha, ranging from 0.01 to 36.32 ha, Devoucoux, 2014) with a mosaic of mixed crops, fallow and unplowed land. It is fragmented into several small disconnected zones of farmland separated by urban areas. It was designated a protected area in 2006, mainly due to the presence of one of the largest populations of little bustards in France (277 males sighted in 2004, 679 in 2012; unpublished data).

The Nîmes-Montpellier HSR stretches for 86 km and is largely constructed on elevated backfill reaching around 5 m in height. The construction of the line began in December 2013 and was completed in 2016 (backfill, railway tracks on an embankment, concrete and catenaries). The trains started operating at the end of 2017.

### 2.2. Land use survey in the protected area

Land use was visually recorded annually between 2010 and 2018 and every two years after 2018, field by field throughout the protected area (about 10,500 fields). We extracted data for the year or closest year of each female census, i.e. in 2012, 2016 and 2020. Land use was classified into the following categories: herbaceous wasteland, pastures and alfalfa, vineyards, arboriculture, crops, horticulture, water, buildings and earthworks.

### 2.3. Sampling method of quadrats on favorable nesting habitat

In 2011 and 2012, 488 quadrats of around 1 ha (mean 0.92, range: 0.18–3.30) were first sampled in different habitat types to determine the nesting habitat selection of female little bustards in this study area (Devoucoux et al., 2018). This initial study identified herbaceous wasteland, pastures and alfalfa as the most selected habitats used by nesting females.

For each period (before, during and after HSR construction), we sampled between 160 and 250 quadrats in favorable little bustard
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Map of the high-speed railway (red) crossing the Costières Nîmoises Natura 2000 site (gray) in the south of France and quadrats of favorable nesting habitats monitored before impact (orange), during construction (green), and after trains began operating on the railway line (blue). Interactive map is available on the online Zenodo repository (doi:https://doi.org/10.5281/zenodo.14834520). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

nesting habitats, regularly distributed over a band between 0 and 1000 m from the HSR line, using a 50-m distance class stratification (Fig. 1).

For our pre-HSR construction dataset we selected only the 162 quadrats covered by one of the three favorable nesting habitat types of the 488 quadrats surveyed in 2011–2012 and spread over the first 1000 m of the railway line.

In each of the following periods, during (2016) and after (2021) HSR construction, 247 and 241 quadrats, respectively, were randomly sampled in the favorable nesting habitats only, in successive 50-m-wide bands up to 1000 m from the HSR.

Sampled quadrats were thus not the same (with few exceptions) for each studied period because their cover changed from one year to another due to rapid agricultural rotations in the area. Sampled quadrats were relatively small, around 1 ha each (mean 0.92, max 3.30 and min 0.18), to avoid double counts of flushed birds and to be representative of the field size in the area (Devoucoux, 2014).

## 2.4. Female flush count protocol

Females were counted using the flush count protocol, which is the only protocol known to accurately record female little bustard abundance (Wolff et al., 2002). Each quadrat was searched by five people walking side by side at the same speed, 10 m apart. Observers walked two consecutive 50-m-wide transects (i.e. one 100 m transect if the quadrat measured 1 ha). Walking speed was high to minimize disturbance duration and to cause little bustards to fly away. When a little

bustard was flushed, its exact location was recorded and its age and sex were determined by one of two experienced observers equipped with binoculars standing at each end of each transect. As 1-year-old males can be difficult to distinguish from females (see Jiguet and Wolff, 2000, for criteria), if there was any doubt regarding the sex of the flushed individual, the bird was excluded from the analysis. Only females inside the quadrats were counted. Fieldwork was carried out during the female nesting season, i.e. in May or June. Quadrats were sampled from 7 am to 12 am and from 3 pm to 8 pm to avoid high temperatures, which can be detrimental to unprotected eggs and chicks.

The censuses were authorized each year by the French Ministry of Ecology via an application for exemption from the deliberate disturbance of protected species (Cerfa no. 13616).

## 2.5. Data analysis

### 2.5.1. Changes in landscape characteristics between distance class and time period

The changes in landscape characteristics were examined using two metrics as a function of distance class (100 m by 100 m) from the HSR for each period (before, during and after the construction of the HSR). For each distance class and period, we calculated (1) the surface area and proportion of three types of land use: habitat suitable for the nesting of the little bustard (herbaceous wasteland, pastures and alfalfa, see Devoucoux et al., 2018), other agricultural habitats (vineyards, arboriculture, crops, market gardens), and habitats unsuitable for the species
(water, buildings, earthworks); (2) the median area of agricultural fields with their centroids located in the different 100 m distance classes.

## 2.5.2. HSR impact on the little bustard population

2.5.2.1. *Impact on female density and distance of zone effect.* To assess the impact of the railway line in extent and intensity on the little bustard density, we analyzed how the density of females changed in favorable nesting habitats with distance from the HSR line in each period (before, during or after HSR construction). To do this, we fitted generalized additive models (GAMs) with Poisson error distribution and a logarithm link function using the *mgcv* package (Wood and Wood, 2015) in R v.4.1.2 (R Core Team, 2022) after testing for overdispersion (none was detected as the deviance/degrees of freedom ratio was close to one). The response variable was the counts of individuals in favorable nesting quadrats. The distance from the HSR line was included as the smoothing term (with a maximum number of splines of k = 4) in interaction with the period. The area of the quadrats was added as an offset (in ha, log-transformed). Due to the consistency of the landscape metrics across the different distance classes and time periods (see “Results”), we did not include them in the model.

The distance of the zone effect during and after construction was determined as the distance where the maximum density of little bustards predicted by the model was estimated.

2.5.2.2. *Decline in female abundance.* To determine the effect of the construction and then the operation of the HSR on the number of females, we predicted female abundance for each time period in three zones of interest: the zone immediately in the vicinity of the HSR (100 m, with 197 ha, 102 ha and 206 ha of suitable nesting habitat before, during and after HSR construction respectively), the zone effect of the HSR (775 m, see “Results”, with 1185 ha, 1095 ha and 1291 ha of suitable nesting habitat before, during and after HSR construction respectively) and the study zone (1 km, with 1446 ha, 1409 ha and 1587 ha of suitable nesting habitat before, during and after HSR construction respectively). We estimated total female abundances, the decline after HSR construction and the 95 % confidence intervals for each zone of interest using a parametric bootstrap. For each period, we generated 1000 bootstrap samples from the predicted bustard density distribution for each field containing suitable nesting habitat in the zone of interest, using the distance of its centroid to the HSR. We then multiplied the predicted density of female little bustards for each field by the area of the corresponding field to predict the estimated abundance per field. The total abundance in the zones of interest was then obtained by summing these predicted abundances. Finally, we calculated the differences in female abundance across the three periods based on these sums.

# 3. Results

## 3.1. Changes in landscape characteristics

Prior to HSR construction (in 2012), the proportion of favorable nesting habitat for little bustards in the study area was about 0.20–0.30, other agricultural habitat was about 0.60, and unsuitable habitat was about 0.07–0.10. These proportions did not change along the distance classes from the future HSR (Fig. 2). Compared to this period, the proportion of unsuitable habitats increased during the construction (in 2016) and after construction (in 2020) in the first 100 m from the HSR line, while no change was noticeable beyond 100 m. The median area of agricultural fields did not change along the distance classes from the HSR or with time period (Fig. 3). In the monitored area, the median size of all fields was 1.0 ha (mean = 1.6 ha, min <0.1 ha, max =35.9 ha).

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Proportion of habitats with cover favorable to little bustard nesting (herbaceous wasteland, pastures and alfalfa) (light green), other types of agricultural habitats (vineyards, arboriculture, crops, market gardens) (medium green), and unsuitable habitats (water, buildings, earthworks) (dark green) by 100 m distance classes from the HSR inside the Costières Nîmoises Natura 2000 site and by monitoring period (before-during-after construction). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Median area (ha) of agricultural fields with centroids within 100 m distance classes from the HSR inside Natura 2000 site “Costières Nîmoises” and by monitoring period: before (orange), during (medium green), after (dark green) construction. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
## 3.2. HSR impacts on female little bustards

### 3.2.1. Impact on female density and distance of zone effect

We detected 45 females in 2011–2012 (in 162 quadrats), 82 females in 2016 (in 247 quadrats), and 28 females in 2021 (in 241 quadrats).

Before HSR construction (in 2011–2012), the density of females in the quadrats (all habitats suitable for nesting) was constant in the 1 km zone on either side of the future infrastructure (Fig. 4). During (in 2016) and after (in 2021) HSR construction the density was much lower than before HSR construction close to the HSR. Female density was close to 0 after HSR construction over the first 100 m. The density increased with distance from the HSR up to about 650 m during HSR construction and up to 775 m after HSR construction.

### 3.2.2. Decline in female abundance

During HSR construction, the total number of females decreased by 75.8 % in the zone immediately in the vicinity of the HSR (100 m), but was relatively stable in the zone effect (775 m) and the study zone (1 km) compared to before construction (Table 1). After train traffic began, the total number of females decreased by 94.1 % and fell to almost 0 in the vicinity of the HSR (100 m), decreased by 57.8 % in the zone effect (775 m), and by 50.7 % in the study zone (1 km) (Table 2).

## 4. Discussion

Using a before-during-after-distance approach, our analysis found that female little bustard density in habitat favorable to nesting strongly decreased in proximity to the HSR. This decrease was noticeable up to 650 m during construction of the HSR, and up to 775 m after train traffic began. Female abundance declined by approximately 60 % in this 775 m zone around the HSR.

The strong effect of the railway line we observed on female density is consistent in term of extent with results obtained in studies conducted on male little bustards (mostly on the zone effect of roads using different metrics than our study), as well as other grassland bird species (e.g. great bustard *Otis tarda* and passerines). Arcos and Salvadores (2005) estimated a circa 600 m zone effect of roads on male little bustards. Silva et al. (2004) found that the density of little bustards during the breeding season was higher 1200 m from inhabited houses (associated with high road density) compared to areas closer to these houses. Bretagnolle et al. (2024) detected an effect of built-up areas up to a distance of 700 m. However, it should be noted that these studies were conducted on males only and were not specifically designed to assess the zone effect; they did not use a sampling design based on stratification of distance from

infrastructure. Studies on other farmland bird species have also shown zone effects of linear transport infrastructures over similar distances. A study on great bustard males and females with chicks, using a before-during-after-distance method like ours, also found that the zone effect was noticeable as soon as highway construction began, persisted when it was opened to vehicle traffic, and was detectable up to a distance of about 1 km (Torres et al., 2011). Zone effects of >1 km from roads have also been found for other grassland passerine species in Massachusetts (USA) (Forman et al., 2002).

In terms of the strength of the HSR effect, our results show that eight years after the beginning of HSR construction and after trains began operating, female density fell to almost 0 in the vicinity of the HSR. Within the first 650 m during the construction of the HSR, and then within the 775 m zone around the HSR, we observed a pronounced decline in abundance while the total area of favorable habitat was unchanged. The density of nesting females was indeed strongly negatively affected in plots closed to the HSR while the plots we studied were covered by favorable habitats. This decrease in density will probably be permanent (see below for the suspected causes of the decline). Such a decrease in density in favorable habitats therefore correspond to a loss of nesting capacity in the study zone, especially as we did not observe any increase in the surface of suitable habitats to little bustard nesting into the SPA. It attests to the significance of the HSR impact on carrying capacity within the protected area and, more specifically, on nesting capacity. A zone effect of 775 m means that 3510 ha, i.e. 26 % of the total protected area, is affected by the HSR. As there is a limited area available in the study zone to accommodate the required, well-located offsetting plots the question of the irreversibility of the impact on the overall carrying capacity of the SPA can legitimately be raised.

The total abundance in the study area (0–1 km from the HSR) did not decline during construction, suggesting that individuals may have moved away from the railway but likely stayed in this 1-km buffer zone. This is indicated by the slight (but not significant) increase of density at distances >400 m compared to before construction. Yet after construction was completed (presence of backfill and catenaries) and the line began operating, abundance fell by 60 % in the zone effect and by 50 % in the entire study zone. This suggests either that individuals moved to more favorable habitats farther away, or that the population declined due to a demographic impact of the HSR (e.g. density-dependent phenomena, decrease in fecundity, mortalities by collision, decrease of immigration because of loss of connectivity between population cores...).

It is not possible to assert that the decline we observed is entirely explained by the impact of the HSR. The impact of infrastructure can be

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Density of females by hectare on nesting habitat according to distance from the high-speed railway and period as predicted by a GAM. Solid lines are the predicted density, shaded areas are the 95 % confidence intervals.</figcaption></figure>
<caption>**Table 1**</caption>

Median of predicted female little bustard abundance in suitable nesting habitat close to the HSR, in the zone effect of the HSR, and in all the study zone, before, during and after its construction and distance of the zone effect of the HSR by period.

<table><caption>**Table 2**</caption><thead><tr><th rowspan="2">Time period</th><th colspan="4">Median of predicted female abundance</th></tr><tr><th>In the zone immediately in the vicinity of the HSR (100 m)</th><th>In the zone effect of the HSR after train traffic began (775 m)</th><th>In the study zone (1 km)</th><th>Zone effect distance from the HSR</th></tr></thead><tbody><tr><td>Before HSR construction</td><td>62.2 [38.7; 100]</td><td>362 [270; 493]</td><td>461 [344; 620]</td><td>None</td></tr><tr><td>During HSR construction</td><td>15.2 [7.76; 30.5]</td><td>343 [263; 447]</td><td>475 [380; 592]</td><td>Up to 650 m</td></tr><tr><td>After HSR construction</td><td>3.69 [0.62; 20.7]</td><td>153 [99.7; 238]</td><td>228 [159; 329]</td><td>Up to 775 m</td></tr></tbody></table>

Decrease in female little bustard abundance on suitable nesting habitat close to the HSR, in the zone effect of the HSR and in the study zone, before, during and after HSR construction.

<table><thead><tr><th rowspan="2">Period compared to before HSR construction</th><th colspan="4">Decline in female abundance</th></tr><tr><th>In the zone immediately in the vicinity of the HSR (100 m)</th><th>In the zone effect of the HSR after train traffic began (775 m)</th><th>In the study zone (1 km)</th><th>Zone effect distance from the HSR</th></tr></thead><tbody><tr><td>During HSR construction</td><td>-75.8 % [-89.7; -43.8]</td><td>-5.38 % [-36.0; 43.1]</td><td>2.25 % [-28.1; 49.1]</td><td>Up to 650 m</td></tr><tr><td>After HSR construction</td><td>-94.1 % [-99.1; -66.4]</td><td>-57.8 % [-74.9; -27.0]</td><td>-50.7 % [-68.7; -21.8]</td><td>Up to 775 m</td></tr></tbody></table>

cumulative with other local perturbations such as urbanization more generally or changes in agricultural practices. However, the population in the study area had been increasing in the decades prior to the start of HSR construction, suggesting that it was particularly favorable for the species and no major changes aside from the HSR were observed during the study period in the area (<1 % of the protected area was urbanized, and no major change in agriculture practices was noted). Other factors may be contributing to global population declines and could partly explain the decline observed in the study zone. Influences of large-scale factors, such as climate change and shifts in resource availability, on little bustard population dynamics are poorly understood and using population trends outside the study zone as a control may not be relevant. Firstly, local impacts—such as the increasing urbanization observed in southern France—are likely to differ between protected, as those we studied, and non-protected areas. Secondly, the French Mediterranean little bustard population is made of several subpopulations that are probably largely connected by dispersal (Devoucoux, 2014), so that what happens in an area may have larger scale consequences on the metapopulation dynamics.

Within the first 100 m of the HSR, the proportion of suitable nesting habitat was halved during construction, but returned approximately to its pre-construction level after the end of construction and entry into operation. Beyond 100 m, neither the proportion of suitable nesting cover, other agricultural cover and unsuitable habitat, nor medians of agricultural plot surface area changed along the distance classes from the HSR. These remained remarkably stable before, during and after construction. This rules out the possibility that the zone effect we observed was due to a change in available nesting habitat or in landscape composition. This is another indication that the zone effect was due to avoidance of the HSR. It is possible that this avoidance was due to a simple visual barrier effect of the HSR, which sits on an elevated bank of several meters. The little bustard is indeed an open-landscape species that needs long-distance visibility (Silva et al., 2007). Several studies have demonstrated that both females and males select large open areas and avoid high vegetation and inhabited houses (Silva et al., 2014, 2010, 2004).

We also found that after trains began operating, the avoidance effect increased both in extent and intensity. The progressive increase in impact distance over the years may be related to the fact that the little bustard is a philopatric species (Bretagnolle et al., 2022), resulting in avoidance being delayed over the years. Nonetheless, train traffic also generates a multitude of disturbances, including varying degrees of

noise, vibration, light, movement, etc., which may also explain the increase in zone effect with train operation. These railway disturbances, which have similar effects to road disturbances (Barrientos et al., 2019), may result in an increase in avoidance behavior compared to before train traffic, as has been shown for road traffic for many farmland bird species (passerines and galliformes) in Europe (Wiącek, 2023). Disturbance may directly affect little bustard behavior and health given the very high sensitivity of the little bustard to stress. Indeed, significant behavioral and physiological stress responses of little bustards to human activities and traffic noise have been demonstrated (Tarjuelo et al., 2015; Barrero et al., 2021). Disturbance can also disrupt both inter- and intraspecific interactions. It can affect females' ability to assess male quality by their song, as well as communication between females and their chicks, and can make it more difficult to detect predators (Leonard and Horn, 2008; Quinn et al., 2006). By affecting other species and altering ecosystem functioning, the HSR may also gradually affect site quality (e.g., food availability, predation levels), potentially influencing its selection by bustards and altering local population density. All these mechanisms could in turn affect the survival of females and their reproductive success.

This study is the first to quantify the impact of infrastructure on little bustard females during breeding season using an adequate sampling design. Most studies on these impacts on bird species have focused on singing males, which are easier to monitor (Benítez-López et al., 2010). However, focusing on female little bustards, which incubate and rear young without male assistance, offers a more accurate assessment of how infrastructure impacts ecological processes directly related to population dynamics and viability. The loss of carrying capacity for nesting females in the core of the protected area that we observed is likely to have a major impact on the productivity of juveniles in one of the largest populations in the south of France. In the medium to long term, this type of perturbation to demographic parameters in the core population could have negative consequences for this population's dynamics and viability, and also for the metapopulation (Oro et al., 2012). Although there may be other local and global pressures affecting the dynamics of Mediterranean little bustard metapopulation, we cannot exclude that the strong impact of the HSR has had repercussions farther away from the protected area. Indeed, this large, and probably highly productive population, may be a source population for many small subpopulations in the area. National counts of singing males since 2000 show a shift in population size after 2012. After this period, many small population nuclei around the protected area have been found to be
declining (COGard, 2020). The French Mediterranean population, mainly present in the region surrounding the study area – a population that had been increasing between 2000 and 2012 – decreased 6–10 % between 2012 and 2020 (Gendre et al., 2018; Quaintenne et al., 2022; unpublished data). It would be valuable to conduct further studies to directly assess population dynamics by estimating demographic parameters (e.g. female survival and fecundity) and dispersal processes (e. g. juvenile dispersal and population connectivity), in order to better understand the long-term impact and the scale of such large infrastructure on populations. It would also be of interest to carry out studies to estimate the impact of potential collisions with trains and catenaries to get a full understanding of the impact of the HSR. Studies have shown that the little bustard is a species with high collision mortality (Silva et al., 2010), and the risk of collision with HSR is very high (Malo et al., 2017).

## 5. Conclusion

The before-during-after-distance approach we used allowed the first robust quantification of the effect of major linear transport infrastructure on a population of little bustards, particularly in terms of carrying capacity. The results indicate that solely assessing the surface area of favorable habitats is not sufficient, as the zone effect extends far beyond the artificialized area. These findings highlight the urgent need to examine the indirect effects of artificialization (McDonald et al., 2020) using appropriate methodologies. Considering the extent of the zone effect we observed, our results are an argument for the systematic integration of these effects during ecological impact assessments and conservation planning. Accurately quantifying the effect on bird numbers at a given site allows the impact of potential projects to be better understood in the decision-making process. In the case of existing projects, it allows better tailoring of conservation efforts. Detailed information on the extent and distance of the zone effect is essential to define the best location and size for conservation or offsetting measures to ensure that these are effective for the species.

## CRediT authorship contribution statement

**Soumaya Belghali:** Writing – review & editing, Writing – original draft, Visualization, Methodology, Investigation, Formal analysis, Data curation, Conceptualization.
**Pierrick Devoucoux:** Writing – review & editing, Validation, Methodology, Investigation, Data curation, Conceptualization.
**Daniel Bizet:** Writing – review & editing, Validation, Project administration, Conceptualization.
**Nathalie Hiessler:** Writing – review & editing, Validation, Project administration, Funding acquisition, Data curation.
**Thomas Menut:** Writing – review & editing, Validation, Project administration, Investigation, Data curation.
**Olivier Scher:** Writing – review & editing, Validation, Supervision, Project administration, Funding acquisition.
**Vincent Bretagnolle:** Writing – review & editing, Validation, Methodology, Conceptualization.
**Aurélien Besnard:** Writing – review & editing, Validation, Supervision, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Conceptualization.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgements

We thank all the people involved in the fieldwork over years from COGard and Biotope who collected the data used in this study. This research was made possible thanks to the financial support of Oc'Via, Oc'Via Maintenance and the Association Nationale Recherche

Technologie (ANRT). We thank Elise Bradbury for improving the English, Killian Gregory for the scientific discussions and two anonymous reviewers for their constructive comments on earlier version of the manuscript.

## Data availability

Data will be made available on request.

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