Does it matter which egg you hatch from? Hormonal, antioxidant and fatty acid composition in Great Tit Parus major eggs

Environmental variability affects natural populations by affecting individuals both directly and indirectly. In birds, the mother can indirectly influence the environment in which her offspring develops by differential deposition of substances into the egg yolk. This can give rise to long-lasting effects on the offspring phenotype. Here we ask first, is there a consistent female egg phenotype and, second, do yolk components vary within females across the laying sequence? We investigated the concentrations of five steroid hormones and three antioxidants, and the proportion of four groups of fatty acids in the eggs of a wild population of great tits (Parus major) located in Germany. Fresh eggs from 11 entire clutches were collected. We analyzed the variation in yolk components at the within- and between-clutch level across the laying sequence. Dihydrotestosterone, all three antioxidants (vitamin E, lutein, zeaxanthin) and the proportion of the n-6 polyunsaturated fatty acid decreased over the laying sequence. The other steroid hormones (androstenedione, testosterone, corticosterone) and the proportion of the polyunsaturated fatty acid n-3 did not change, while the proportion of saturated and monounsaturated fatty acids increased over the laying sequence. Moreover, for 10 of the 11 yolk components, we found a significant repeatability at the between-clutch level higher than 0.3. These results indicate that the developmental environment for those offsprings coming from the first eggs is of better physiological composition and that mothers consistently differ in their deposition of yolk components. Future experiments will determine how these yolk components influence chick phenotypes.