Conspecific brood parasitism and egg quality in blue tits Cyanistes caeruleus

Avian eggs contain substantial amounts of maternal hormones and so provide an excellent model to study hormone-mediated maternal effects. We review this new and rapidly evolving field, taking an ecological and evolutionary approach and focusing on effects and function of maternal androgens in offspring development. Manipulation of yolk levels of androgens within the physiological range indicates that maternal androgens affect behaviour, growth, morphology, immune function and survival of the offspring, in some cases even long after fledging. Descriptive and experimental studies show systematic variation in maternal androgen deposition both within and among clutches, as well as in relation to the sex of the embryo. We discuss the potential adaptive value of maternal androgen transfer at all these three levels. We conclude that maternal androgen deposition in avian eggs provides a flexible mechanism of non-genetic inheritance, by which the mother can favour some offspring over others, and adjust their developmental trajectories to prevailing environmental conditions, producing different phenotypes. However, the literature is less consistent than often assumed and at all three levels, the functional explanations need further experimental testing. The field would greatly benefit from an analysis of the underlying physiological mechanisms.

The sex steroid hormones that affect development in birds have been thought to be produced exclusively by the embryo or neonate. I used radioimmunoassay to measure the amounts of androstenedione, 5 alpha-dihydrotestosterone, testosterone, 17 beta-estradiol, and corticosterone in the yolk of freshly laid canary (Serinus canaria) and zebra finch (Poephila guttata) eggs. Testosterone was found in both canary and zebra finch eggs, but its contents were much higher in the former than in the latter. The testosterone content of canary eggs in a same clutch increased with the order of laying, regardless of the genetic sex of the offspring that hatched from these eggs. Yolk testosterone was also present in the eggs of female canaries that were kept without a male, indicating that it is of maternal origin. The social rank of juvenile canaries was positively correlated with the concentration of yolk testosterone in the eggs from which they hatched, suggesting that the development of aggressive behavior of offspring might be subject to modification by maternal testosterone. These findings indicate that female songbirds can bestow upon their eggs a dose of hormone that modifies the behavior of offspring. Variable doses of these hormones might explain some of the individual variation in offspring behavior.