The influence of crop type (pasture, silage and cereal) on the abundance of aerial invertebrates and the density of foraging barn swallows Hirundo rustica was investigated in lowland mixed farmland in southern Britain. After taking weather and other confounding factors into account aerial invertebrate abundance over pasture fields was more than double that over silage, and more than three and a half times greater than that over cereal fields. Pasture fields also hosted approximately twice as many foraging barn swallows as both silage and cereal fields. The results suggest that past reductions in the availability of pasture will have reduced the numbers of aerial invertebrates, and may have contributed to barn swallow population declines, and possibly those of other aerial insectivores. Maintaining grazed pasture, especially that which is closest to farm buildings and other potential nest sites, will help to maintain breeding barn swallow populations on lowland farmland.
Karl L. Evans et al. Agriculture, Ecosystems and Environment 122 (2007) 267–273