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Aquatic insects rich in omega-3 fatty acids drive breeding success in a widespread bird.

Ecologists studying bird foraging ecology have generally focused on food quantity over quality. Emerging work suggests that food quality, in terms of highly unsaturated omega-3 fatty acids (HUFA), can have equally important effects on performance. HUFA, which are present in aquatic primary producers, are all but absent in vascular plants, and HUFA content is also correspondingly higher in aquatic insects. Here, we show that Tree Swallow (Tachycineta bicolor) chicks rapidly accumulate HUFA from food during the nestling period. Using data sampled over 24 years, we also show that Tree Swallow breeding success is positively associated with the availability of HUFA-rich aquatic insects. Variation in aquatic insect biomass during chick development was a strong predictor of fledging success, whereas variation in terrestrial insects had little effect on fledging success. Our results highlight the potential for nutritional mismatches between insectivores and high-quality prey to affect avian reproductive performance.