Diet and Prey Selection of Barn Swallows ( Hirundo rustica ) at Vancouver International Airport

The Barn Swallow ( Hirundo rustica ) is the most widely distributed aerial insectivore in north America, but has declined appreciably in recent decades. reasons for these declines are largely unknown, though presumably relate mainly to changes in prey availability. To help inform conservation priorities for this species, we assessed their diet and prey selection using birds lethally struck by aircraft at Vancouver International Airport (YVR). Esophagi and gizzards of 31 Barn Swallows collected from june 2013 to october 2013 contained insects mainly from the orders Hymenoptera (mean across birds = 40% of insect numbers), Diptera (31%), Hemiptera (15%), and Coleoptera (12%). To assess prey selection, we compared the esophagi and gizzard contents of 20 swallows collected from July 2013 to September 2013 to populations of aerial insects we sampled during the same period using Malaise traps. Barn Swallows selected strongly for insects in the order Hymenoptera (mainly Formicidae, which comprised 29% of diet), and selected against insects in the orders Coleoptera, Diptera, and Lepidoptera. For all prey taxa combined, Barn Swallows displayed strong selection for insects of length 4−8 mm (body length excluding appendages). Conversely, they selected against smaller and larger insects, despite the fact that smaller insects comprised about 80% of all insects sampled in Malaise traps. Combined with past studies, our results suggest that Barn Swallows select among available aerial insects within local feeding sites for taxa that (i) are of intermediate size, (ii) occur at relatively high density, and (iii) have poor flight performance.