Nest composition and yolk hormones: do female European starlings adjust yolk androgens to nest quality?

The nest is a key element of avian reproductive fitness. It provides the developmental environment for the embryo and nestling thereby affecting their quality and survival. Nests are often constructed by the male and nest quality, elaboration, and ornamentation vary among males in a species. Therefore, male nest-building behaviour is likely under sexual selection and females may use male nest-building behaviour and nest features as cues in mate choice and their own reproductive investment. We tested this hypothesis in European starlings (Sturnus vulgaris). Male starlings present green plants to females during their courtship display and furbish their nests with variable quantities of fresh green plants. The highest frequency of plant incorporation into the nest occurs during pair formation and oogenesis. We determined nest composition and measured yolk testosterone concentrations in eggs. Yolk testosterone concentrations were positively correlated with the amount of green plant material in nests. Because males use green nest material in courtship and green nest material positively affects nestlings’ development, we suggest that female European starlings use male nest-building effort, ritualised courtship display of green plants, and nest characteristics in mate choice, and adjust yolk testosterone levels in the eggs to the quality of the nest to optimise offspring growth.