Influence of agricultural intensification on prey availability and nestling diet in Tree Swallows (Tachycineta bicolor)

Over the last decades, aerial insectivorous birds have been declining in both North America and Europe. Those declines have been hypothetically attributed to a decrease in prey availability caused by agricultural intensification, but empirical evidence remains scarce. Here, we quantify the effect of landscape composition on the abundance and diversity of potential prey of Tree Swallows (Tachycineta bicolor (Vieillot, 1808)) and on nestling diet in southern Quebec, Canada. We collected food boluses from nestlings and compared their composition with spatiotemporally corresponding samples from traps on farms distributed along a gradient of agricultural intensification. The diet of nestlings was mostly composed of Diptera, both in biomass and abundance, but by mid-June, these decreased with increasing proportions of intensively cultivated crops within 500 m of the nests. Trap catches for Diptera and all arthropods combined followed the same trends. Yet, the associations between Diptera subgroups (Nematocera, non-schizophoran Brachycera, Schizophora (Calyptratae), and Schizophora (Acalyptratae)) and landscape composition differed between traps and boluses, suggesting that prey selection was altered by agricultural intensification. Our results suggest that agriculture can alter the availability of preferred prey for aerial insectivores, and further studies should evaluate the impact of prey availability to explain the decline of aerial insectivores. Key words: aerial insectivorous bird, arthropods, agricultural intensification, Diptera, redundancy analysis (RDA), Tree Swallow, Tachycineta bicolor. Depuis quelques decennies, les oiseaux insectivores aeriens sont en declin en Amerique du Nord et en Europe. La diminution de la disponibilite de proies dans l'environnement causee par l'intensification agricole est souvent l'hypothese suggeree pour expliquer ces declins, mais les preuves empiriques le confirmant sont rares. Cette etude quantifie l'effet de la composition du paysage sur l'abondance et la diversite des proies potentielles de l'Hirondelle bicolore (Tachycineta bicolor (Vieillot, 1808)) tout comme sur la diete des oisillons dans le sud du Quebec (Canada). Des becquees alimentaires ont ete prelevees a des oisillons et leur composition a ete comparee a celle d'echantillons recoltes par des pieges a insectes installes le long d'un gradient d'intensification agricole. La diete des oisillons etait majoritairement composee de dipteres tant au niveau de l'abondance que de la biomasse, mais des la mi-juin, ces parametres diminuaient a mesure qu'augmentait la proportion des cultures intensives dans un rayon de 500 m des nids. Les memes relations ont ete trouvees pour les echantillons des pieges a insectes en ce qui a trait aux dipteres analyses seuls, mais aussi pour tous les arthropodes combines. Les associations entre les sous-groupes de dipteres (nematoceres, brachyceres non-schizophores, schizophores (Calyptratae) et schizophores (Acalyptratae)) et les composantes du paysage agricole differaient cependant entre les becquees alimentaires et les pieges a insectes suggerant que la selection des proies est influencee par l'intensification agricole. Nos resultats suggerent donc que l'agriculture peut alterer la disponibilite de proies pour les insectivores aeriens, mais d'autres etudes devraient toutefois etre effectuees afin d'expliquer l'effet de la disponibilite de proies dans le milieu sur le declin des insectivores aeriens. Mots-cles: oiseau insectivore aerien, arthropodes, intensification agricole, dipteres, analyse de redondance, hirondelle bicolore, Tachycineta bicolor.
Introduction Anthropogenic environmental changes are affecting a large number of species worldwide and in many cases result in their decline (Fuller et al. 1995; Wittmer et al. 2007; Cahill et [...]