Avian trait-mediated vulnerability to road traffic collisions

Highlights

• Roadkill numbers and bird abundances near roads were strongly correlated.

• Some species (36%) were more (or less) killed than expected from their abundances.

• Selective mortality was strongly related to foraging behaviour and habitat associations.

• Small woodland passerines that often forage in shrubs and small trees were most vulnerable.

• Bird communities near roads were not depleted in species most vulnerable to road killing.

Abstract

Collision with vehicles is an important source of bird mortality, but it is uncertain why some species are killed more often than others. Focusing on passerines, we tested whether mortality is associated with bird abundances, and with traits reflecting flight manoeuvrability, habitat, diet, and foraging and social behaviours. We also tested whether the species most vulnerable to road-killing were scarcer near (< 500 m) or far (> 500–5000 m) from roads. During the breeding seasons of 2009–2011, we surveyed roadkills daily along 50 km of roads, and estimated bird abundances from 74 point counts. After correcting for phylogenetic relatedness, there was strong correlation between roadkill numbers and the abundances of 28 species counted near roads. However, selectivity indices indicated that Blue tit (Parus caeruleus), Blackcap (Sylvia atricapilla) and European goldfinch (Carduelis carduelis) were significantly more road-killed than expected from their abundances, while the inverse was found for seven species. Using phylogenetic generalised estimating equations, we found that selectivity indexes were strongly related to foraging behaviour and habitat type, and weakly so to body size, wing load, diet and social behaviour. The most vulnerable passerines were foliage/bark and swoop foragers, inhabiting woodlands, with small body size and low wing load. The species most vulnerable to road collisions were not scarcer close to roads. Overall, our study suggests that traits provide a basis to identify the passerine species most vulnerable to road collisions, which may be priority targets for future research on the population-level effects of roadkills.

Introduction

Roads may have negative impacts on wildlife (Spellerberg, 1998, Fahrig and Rytwinski, 2009, Benítez-López et al., 2010), mainly due to habitat fragmentation and road-related mortality (i.e., roadkills) (Pons, 2000, Erritzoe et al., 2003, Barthelmess and Brooks, 2010, van der Ree et al., 2015). Collisions with vehicles can be particularly detrimental, because hundreds of millions of individuals are killed each year, though the effects of this mortality on the long term persistence of the species most affected are still uncertain (Kociolek et al., 2011, Roger et al., 2011, Borda-de-Água et al., 2014, Loss et al., 2015). Therefore, impact assessment and mitigation should be based on a thorough understanding of the species most vulnerable to road mortality, which is generally very limited (Kociolek et al., 2011).

Birds are often killed due to collisions with vehicles (Erritzoe et al., 2003, Summers et al., 2011, Loss et al., 2015), with roadkills representing up to 5%–10% of overall bird mortality in the Western Paleartic (Møller et al., 2011). Many species are affected, with passerines and owls being among the most commonly reported groups (Erritzoe et al., 2003, Benítez-López et al., 2010, Boves and Belthoff, 2012). However, there is great disparity among studies regarding the number and species killed at different sites, or at the same site over time, though reasons for this are not always clear. One possibility is that the number of road-killed individuals is mainly a consequence of bird abundances near roads, because more individuals of common species are likely to be exposed to collision risk than those of rare species. This idea implies that most collisions should involve species that are naturally abundant in habitats bordering roads (Møller et al., 2011, D'Amico et al., 2015), or species that are attracted by resources provided directly or indirectly by roads such as food, hunting perches, or nesting places (Barrientos and Bolonio, 2009, Morelli et al., 2014, Ascensão et al., 2015). It is possible, however, that road mortality risk is also affected by bird traits, with some species being killed more (or less) frequently than expected from their abundances (Erritzoe et al., 2003, Møller et al., 2011, Cook and Blumstein, 2013). This selective mortality might be important, as it could induce long term changes in bird communities, by progressively depleting the most vulnerable species or groups of species. However, little is known about what traits affect bird susceptibility to road mortality (but see, e.g., Møller et al., 2011, Cook and Blumstein, 2013, Lima et al., 2015), and whether communities actually change due to differential collision risk.

Bird morphology is one of the factors that may greatly affect the risk of collision with vehicles. In fact, it is often assumed that birds with heavier bodies, small wings, or a combination of both characteristics (i.e., high wing loadings; Brown and Brown, 2013, DeVault et al., 2015) have higher mortality risk because they are less manoeuvrable (Tennekes, 2009), and have more difficulty in changing direction or reducing flight speed when faced with a moving obstacle (Tennekes, 2009, Martin, 2011, Møller et al., 2011, Brown and Brown, 2013, Legagneux and Ducatez, 2013, DeVault et al., 2015). The type of diet may also be influential, as birds feeding on insects or vertebrates should have higher visual acuity and relative brain size (Garamszegi et al., 2002, Husby and Husby, 2014), and thus presumably lower vulnerability. Also, some birds use foraging techniques that imply flying lower or slower than others, and thus may have higher collision risk (Erritzoe et al., 2003, Kociolek et al., 2011). Other birds make diving manoeuvres or sharp turns, which may also increase collision risk (Erritzoe et al., 2003). Open habitat birds may have lower risk than those of woodland habitats, because they might have higher spatial perception abilities (Lima and Dill, 1990). Finally, flocking behaviour may imply neighbour skills and distance perception (Lima and Dill, 1990, Blumstein, 2006), which could lower their vulnerability (Møller et al., 2011, Cook and Blumstein, 2013). Overall, these studies suggest that functional traits may influence birds' vulnerability to collisions with vehicles, allowing generalization of results and thus yielding practical recommendations to road managers working in different ecological settings (McGill et al., 2006).

In this paper we addressed the general idea that roads cause selective bird mortality, and that this selection is due to morphological, behavioural and ecological traits affecting species vulnerability to vehicle collisions. We further investigate whether passerine communities near roads show evidence for depletion of the most vulnerable species. The study was based on a thorough characterization of bird roadkills through daily surveys carried out during the breeding seasons (March to June) of 2009 to 2011, over a 50-km road network, and compared to local bird abundances estimated through point counts. Specifically, we tested the following hypotheses derived from previous studies on bird vulnerability to road collisions (e.g. Erritzoe et al., 2003, Tennekes, 2009, Martin, 2011, Møller et al., 2011, Brown and Brown, 2013, Legagneux and Ducatez, 2013, DeVault et al., 2015): i) there is strong correlation across species between bird abundances and the number of road-killed individuals; ii) there is selective mortality after controlling for the effects of abundance, with higher roadkill risk for species with lower flight manoeuvrability, species that forage on the ground versus woody vegetation, species that feed on plants versus animals, solitary versus flocking species, and forest versus open habitat species; and iii) bird community composition close to roads is depleted in the species most vulnerable to road kills in relation to those far from roads. Results were then used to discuss the effects of roads on bird communities.