Since the early 1990s, aerial insectivorous birds have shown serious population declines in North America, but it is not clear if factors common to all species within this guild account for these declines. Among sympatric swallows, population trends differ, and this may be due to differences in ecology operating throughout the annual cycle. Although these species all feed on aerial insects, prey taxa can differ tremendously in their “aeroecology” and use by swallows. We examined the potential for dietary differences among three species of swallows, Barn Swallow (Hirundo rustica), Cliff Swallow (Petrochelidon pyrrhonota), and Tree Swallow (Tachycineta bicolor), breeding sympatrically in southern Ontario, Canada. Potential interspecific differences in nestling diet were examined using two endogenous biomarkers, DNA barcoding of nestling feces and stable isotope analysis (δ2H, δ13C, δ15N) of nestling feathers. We found evidence for differences in dietary sources of provisioned young where Barn Swallows provisioned more terrestrial-based prey, Cliff Swallows provisioned an intermediate diet, and Tree Swallows the most aquatic-emergent insect diet. We suggest this information may help to identify potential factors contributing to differential declines of aerial insectivores operating on the breeding grounds, including diet quality.
Diets of migratory aerial insectivorous birds are flying insects, caught almost exclusively on the wing. This guild has been declining throughout North America but causes of declines are generally unknown.
We investigated nestling diets of three species of swallows breeding in the same area in southern Ontario. We thought there would be important differences in the diet among them which would facilitate coexistence.
Nestling diet was determined using two approaches, extracting insect DNA from feces and comparing results to a barcode database of diet species/taxa and measuring diet in birds using naturally occurring stable isotopes of carbon (δ13C), nitrogen (δ15N), and hydrogen (δ2H).
Nestling diets of all three species differed and importantly show that dependence on aquatic emergent insects was greatest in Tree Swallows, intermediate in Cliff Swallows and least in Barn Swallows. These analyses will inform ideas about differing reasons for population declines of swallow species in North America.