Partitioning the effects of livestock farming on the diet of an aerial insectivorous passerine, the Barn Swallow Hirundo rustica

Abstract

Capsule The presence of farm animals increases the amount of larger insect prey (mainly large Diptera and Coleoptera) and decreases the proportion of smaller prey (Hymenoptera). Aims To determine the relationships between presence of farm animals and diversity of diet of nestling Barn Swallows from 52 colonies located at sites with and without livestock. Methods Diet was determined on the basis of faecal analysis. We used two approaches to quantify livestock based on their spatial distribution – a breeding site-focused approach and a village-focused approach. Results Our results show a relationship between the presence of livestock and the size, number and composition of insect prey detected in the diet of nestling Barn Swallows. The results from the two approaches we used in our study were broadly consistent, although effects varied according to the number and types of animal being raised. The presence of domestic animals increases the amount of larger prey (mainly larger Diptera and Coleoptera) and decreases the proportion of smaller prey (Hymenoptera species). The effect of individual livestock species, mainly pigs and poultry alone, was considerably higher than the effects of pooling livestock species from Principal Component Analysis. Conclusion Our results imply that Barn Swallows were foraging beyond the immediate area of the farm buildings in which they nest, suggest considerable adaptability in the species' diet and further emphasize the ecological role that livestock play in providing a source of prey of a required size rather specific identity. Furthermore, a statistically significant positive effect of average mass of prey (which is a direct consequence of the presence of cattle at a breeding site) on colony size suggests that livestock farming provides a more profitable prey community, which may ultimately increase local populations of Barn Swallow.