Yolk androgens in great tit eggs are related to male attractiveness, breeding density and territory quality

Females can adaptively adjust phenotype of their offspring via deposition of various compounds into eggs, including androgens and other hormones. Here, I investigated how egg yolk androgens (testosterone and androstenedione) related to environmental conditions and parental traits in the great tit (Parus major) across three breeding seasons. Male and female traits studied included age, condition and multiple feather ornaments, both carotenoid- and melanin-based (carotenoid and UV chroma of yellow breast feathers, area of black breast band and white cheek immaculateness). Yolk mass increased with laying temperature, laying date and area of male black breast band. Concentration of androgens increased with breeding density, territory quality and carotenoid chroma of male yellow breast feathers and was higher in mates of 1 year old as compared to older males. Yolk androgens were not related to any of the female traits analysed. These patterns were thus consistent with (1) social and environmental effects on yolk mass and composition and (2) both positive and negative differential allocation strategies of resource allocation in females. Overall, male traits were the most important predictors of egg yolk characteristics in this socially monogamous songbird.