Maternal yolk androgens in European starlings: affected by social environment or individual traits of the mother?

Social competition among female birds has been shown to positively affect yolk androgen levels, perhaps providing a mechanism to communicate environmental conditions to offspring. Whether this relationship is due to social density or to differences among mothers that breed in different social situations is unclear. We manipulated breeding density to test these alternative explanations. Yolk androgens were measured in clutches of European starlings, Sturnus vulgaris, breeding in consecutive years in outdoor aviaries of different sizes and with varying numbers of breeding pairs. Testosterone (T) levels increased significantly with increasing density. Dihydrotestosterone (DHT) levels increased with the number of nest boxes available. The number of nest boxes monopolized by a breeding male negatively affected androstenedione (A4) levels, positively affected DHT levels and did not affect T levels. Other factors related to social interactions or competition among females (including polygyny) did not influence yolk hormone levels. Within-female yolk androgen levels were highly consistent over two consecutive years even though females were breeding in opposite breeding densities during each year, suggesting that individual characteristics are important determinants of variation in maternal androgen allocation in addition to potential effects of environmental conditions. This within-female consistency of yolk androgen levels across years has important implications.