Abstract
Farming activity severely impacts the invertebrate food resources of farmland birds, with direct mortality to populations of above-ground arthropods thorough mechanical damage during crop harvests. In this study we assessed the effects of phenological periods, including the timing of harvest, on the composition and biomass of prey consumed by three species of aerial insectivorous birds. Common Swifts Apus apus, Barn Swallows Hirundo rustica and House Martins Delichon urbica breed sympatrically and most of their diet is obtained from agricultural sources of invertebrate prey, especially from oil-seed rape crops. We categorized invertebrate prey into six functional groups, including oil-seed rape pests; pests of other arable crops; other crop-provisioned taxa; coprophilous taxa; and taxa living in non-crop and mixed crop/non-crop habitats. Seasonality impacted functional groups differently, but the general direction of change (increase/decrease) of all groups was consistent as indexed by prey composition of the three aerial insectivores studied here. After the oil-seed rape crop harvest (mid July), all three species exhibited a dietary shift from oil-seed rape insect pests to other aerial invertebrate prey groups. However, Common Switfts also consumed a relative large quantity of oil-seed rape insect pests in the late summer (August), suggesting that they could reduce pest insect emigration beyond the host plant/crop. Since these aerially foraging insectivorous birds operate in specific conditions and feed on specific pest resources unavailable to foliage/ground foraging avian predators, our results suggest that in some crops like oil-seed rape cultivations, the potential integration of the insectivory of aerial foraging birds into pest management schemes might provide economic benefits. We advise further research into the origin of airborne insects and the role of aerial insectivores as agents of the biological control of crop insect pests, especially the determination of depredation rates and the cascading effects of insectivory on crop damage and yield.
Highlights
Foraging vertebrate insectivores, including birds and bats, are heavily dependent on the abundance and availability of flying insects, and display substantial plasticity towards prey type and feeding micro-habitats [1,2,3,4,5,6,7,8,9,10]
We collected faecal sacs accumulated over 1–10-day intervals under occupied nests of the three species: from three Common Swift nests located in the ventilation holes of a low building, two large breeding colonies of House Martins with 60 and 35 nests respectively located on two four-storey blocks of flats, and four Barn Swallow nests situated in four small farm buildings, two of which housed pigs or piglets
Our study clearly demonstrates that aerial insectivores consume common crop pests of nearby crops, and appear to track crop pest availability, implying potential benefits in terms of pest reduction, reduced crop damage, and improved crop yield, but further studies aimed to evaluate these interactions are needed
Summary
Foraging vertebrate insectivores, including birds and bats, are heavily dependent on the abundance and availability of flying insects, and display substantial plasticity towards prey type and feeding micro-habitats [1,2,3,4,5,6,7,8,9,10]. The community of non-target wild animals occurring within arable landscapes in Europe is undergoing dramatic changes as a result of agricultural practices. These species can be especially vulnerable during crop harvest, when they may suffer direct mortality or be forced to move to adjacent non-crop habitats [17, 18]. This research has implications for understanding predator-prey trophic interactions in communities of organisms living in intensively managed farming landscapes [22, 23]. This research has implications for understanding the dynamic of the processes within food webs and prey-predator trophic interactions in communities of organisms living in intensively managed farming landscapes [22, 23]
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