Endogenous biomarkers reveal diet partitioning among three sympatric species of swallows

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 (δ 2H, δ 13C, δ 15N) 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.