Diet of nestling Barn Swallows in an agroecosystem: insights from fecal DNA barcoding and feather stable isotopes (δ13C, δ15N)

Globally, grassland and farmland birds have experienced population declines due to habitat loss associated with increasing agricultural land-use intensity. These modified environments can reduce insect availability for aerial insectivorous birds and agricultural development is a leading hypothesis for declines in this guild. Our objective was to investigate the relationship between landscape composition and diet of nestling Barn Swallows (Hirundo rustica) raised in a mixed agroecosystem in southern Ontario, Canada. We used nestling feather stable-isotope (δ13C and δ15N) measurements and DNA barcoding of nestling fecal matter to elucidate nestling diets. Nestling feather isotope values were related to agricultural land use, indicating differences in diet, or diet source that varied with proportion of agricultural crops. In 1 year, we found reduced diet richness in areas with increased amounts of row crop, otherwise measures of taxonomic richness and composition of nestling diet showed no relationship with the proportion of row crop, which suggests similar diets in heavily cropped and less cropped landscapes. Additionally, amount of water in the surrounding landscape was associated with increased diet richness. Overall, isotopic measurements and fecal barcoding suggest that nestling Barn Swallows raised within our agroecosystem were provisioned insects from agricultural food webs for at least part of their diet. There was little evidence to support nestling diet composition or richness changes with increased agricultural intensity, at current (low) nesting densities.