Barn swallows and purple martins convert precursors to long-chain polyunsaturated fatty acids: implications for conservation of riparian- vs inland-nesting habitats.

For aerial insectivorous birds, whose populations have declined significantly in North America, long-chain polyunsaturated fatty acids (LC-PUFA) that are more abundant in aquatic-emergent insects than terrestrial insects, are important for the development, somatic growth, and health of young birds. Some bird species, however, can convert shorter chain PUFA that occur abundantly in terrestrial insects into LC-PUFA. Our study aimed to evaluate the ability of two aerial insectivore species to synthesize their own LC-PUFA. We hypothesized that terrestrially associated aerial insectivores rely on higher fatty acid conversion rates compared to those associated with wetlands and riparian habitats. We fed wild barn swallow (Hirundo rustica) and purple martin (Progne subis) nestlings 13C-labelled essential omega-3 (α-linolenic acid; ALA) or omega-6 (linoleic acid; LA) fatty acids to trace internal fatty acid conversion from these dietary precursors. Using compound-specific stable isotope measurements of livers, we estimated conversion efficiency to LC-PUFA. Barn swallow nestlings converted 76% of the omega-3 ALA and 46% of the omega-6 LA precursors to LC-PUFA. Purple martin nestlings converted 88% of the ALA and 44% of the LA. Both species converted five times more ALA to DHA than previously reported in tree swallows (Tachycineta bicolor) and may be more adapted to fluctuations in diet quality and habitat types. Our research highlights the variability in conversion efficiency within the guild of aerial insectivores and the need to better understand which species may be less resilient to sudden changes in nutritional landscapes.