Swift sampling of farmland aerial invertebrates offers insights into foraging behaviour in an aerial insectivore

Short communication
The common swift, Apus apus, is an obligate aerial, migratory, insectivorous bird, that has experienced significant declines in the UK since the 1990s. Reductions in the availability of prey during their summer breeding season in the UK are likely to be a key factor in this decline. This short communication aims to contribute new insights into the current foraging behaviours of adult swifts feeding their nestlings , as a means of provoking new conversation and stimulating further work. Food bolus samples are small ball-like structures containing the insect prey that is regurgitated to nestlings. Boluses from adult swifts provisioning their nestlings were collected incidentally at a breeding colony in Suffolk, UK. These were taxonomically identified and compared to corresponding daily insect catches from a nearby Rothamsted Insect Survey suction-trap operating within the foraging area of common swifts. There was a distinction between contents of the bolus samples and the suction-trap samples, whereby larger bodied aerial invertebrates appeared in greater numbers in bolus samples. This was evidenced by the relatively high numbers of agriculturally important species, pollen beetles and cabbage stem flea beetles, in bolus samples compared to low numbers in suction-traps. Smaller invertebrates such as aphids (Aphididae), parasitoid wasps (Hymenoptera), and thrips (Thysanoptera) were not frequent in the bolus samples, relative to the high numbers identified from the suction-trap catch. These results are discussed in relation to swifts providing a pest suppression service, potential impacts of pesticides, and how selective foraging may both buffer and facilitate the challenges swifts face in a modern agricultural landscape.