Taking the short- or long-chain route: conversion efficiency of alpha linolenic acid to long-chain omega-3 fatty acids in aerial insectivore chicks

Food availability and quality are both critical for growing young animals. In nature, swallows (Tachycineta bicolor) and other aerial insectivores feed on both aquatic insects, which are rich in omega-3 long-chain polyunsaturated fatty acid (LCPUFA) and terrestrial insects, which contain considerably less LCPUFA. Carnivorous mammals and fishes must obtain LCPUFA from diet, as they have lost the capacity to convert the precursor omega-3 ALA into LCPUFA. Thus, the relative value of aquatic versus terrestrial insects depends not only on the fatty acid composition of the prey, but also upon the capacity of consumers to convert ALA into LCPUFA. We used a combination of stable-isotope-labeled fatty acid tracers to ask if, and how efficiently, Tree Swallows can deposit newly synthesized LCPUFA into tissue. Our data show for the first time that Tree Swallows can convert ALA into LCPUFA deposited in liver and skeletal muscle. However, high Tree Swallow demand for LCPUFA combined with low ALA availability in natural terrestrial foods may strain their modest conversion ability. This suggests that while Tree Swallows can synthesize LCPUFA de novo, LCPUFA are ecologically essential nutrients in natural systems. Our findings thus provide mechanistic support for our previous findings and the importance of LCPUFA-rich aquatic insects for Tree Swallows and most likely other aerial insectivores with similar niches.Summary StatementA stable-isotope-labeled tracer reveals the mechanism for omega-3 long-chain polyunsaturated fatty acid (LCPUFA) limitation in a wild avian insectivore, showing that LCPUFA are an ecologically essential nutrient.