Combining fatty acid and stable isotope techniques to trace lipids in birds

Aerial insectivorous songbirds have been declining for decades in North America. Among many interacting factors, prey quality is important for the growth and health of nestlings. Aquatic-emergent insects provide more beneficial omega-3 fatty acids than terrestrial insects, but aerial insectivores may be able to convert shorter and less unsaturated fatty acid precursors to longer and more unsaturated chains (LC-PUFAs). Using fatty acid profiling and stable isotope techniques, I assessed the diet quality and fatty acid conversion efficiency of aerial insectivores, and using captive warblers I evaluated the incorporation of dietary LC-PUFAs into blood and the metabolism of carbohydrates versus lipids. I found that purple martin (Progne subis) and tree swallow (Tachycineta bicolor) nestlings had more aquatic-emergent diets (lower feather δ2H) than barn swallows (Hirundo rustica). As adults provision nestlings with aquatic-emergent insects, lakeshore purple martins and tree swallows had higher levels of omega-3 LC-PUFA EPA in blood plasma than inland populations. Yet, barn swallow nestlings provisioned with more terrestrial insects had among the highest levels of the omega-3 LC-PUFA DHA. Using compound-specific stable isotope analysis, I confirmed that barn swallow nestlings were able to efficiently convert fatty acid precursors into LC-PUFAs, as were purple martin nestlings. Fatty acid conversion may allow nestlings to supplement diets low in LC-PUFAs. Even with a high-quality diet, LC-PUFAs may not be equally incorporated into tissues nor metabolized. Using the yellow-rumped warbler (Setophaga coronata) as a model species, I found that plasma reflected LC-PUFAs faster than red blood cells, and that omega-3 DHA was incorporated faster than omega-6 ARA. I also confirmed that δ13C in exhaled breath can be used to distinguish between C4 and C3 dietary sources of metabolized carbohydrates and lipids. Admittedly, further understanding of fractionation of exhaled breath is needed, but studies may use this technique to trace fatty acid metabolism during nestling growth. My thesis has shown how diet quality, fatty acid conversion and incorporation can influence nutrient acquisition, indicating that birds with low conversion efficiency and poor-quality diets may be vulnerable. Wetland habitats and aquatic-emergent insects rich in LC-PUFAs may be crucial for such species.