Variation in seed abundance predicts yolk fatty acid composition in a wild population of birds

Bird embryos develop inside eggs, which contain maternal substances that can shape offspring phenotype and fitness. Yolk fatty acids are a key energy source for the developing embryo, with omega-6 (ω-6) and omega-3 (ω-3) polyunsaturated fatty acids (PUFAs) obtained exclusively from the diet. Therefore, food availability during the breeding season is expected to influence yolk fatty acid composition and, consequently, embryo development and growth. However, the effects of dietary variation on yolk fatty acids remain unexplored in wild birds. We investigated inter-annual variation in yolk fatty acid composition of free-living great tits ( Parus major ) in relation to fluctuations in beech ( Fagus sylvatica ) fructification—their preferred food—across two years differing strongly in seed abundance. We hypothesized that differences in beech seed availability would alter yolk fatty acid composition between years of high and low fructification, with corresponding correlations to the fatty acid profiles of available seeds. Hence, we analyzed fatty acids from seeds of beech and two conifers ( Picea abies and Pinus sylvestris ), which account for most of the tree cover in the study area, and from 112 eggs collected from 107 nests over two years. Beech seeds contained higher proportions of saturated (SFA), monounsaturated (MUFA), and ω-3 PUFAs, and lower proportions of ω-6 PUFAs than conifer seeds. Correspondingly, egg yolks had more SFA, MUFA, and ω-3 PUFAs in the year of high beech abundance, and more ω-6 PUFAs in the year of low abundance. Notably, the proportion of the conifer-associated ω-6 PUFA pinolenic acid was 64.25 times higher in years of low compared to high beech fructification, suggesting marked inter-annual variation in yolk fatty acid composition, likely reflecting shifts in maternal diet. Future experiments need to establish causal links between diet and yolk composition and assess the fitness consequences of different seed resources for breeding females and their offspring.