Proximate and ultimate factors explaining floating behavior in long-lived birds.

Avian populations are composed of reproductive individuals coexisting in space and time with young, non-breeding conspecifics or "floaters". Despite the fact that first breeding can be delayed for years and can exert profound effects on fitness, many aspects of the behavior, ecology and physiology of young floaters remain poorly understood. By means of combining endocrine, behavioral and life history information from a population of black kites (Milvus migrans) monitored long-term, we tested several hypotheses suggesting endocrine function as a determinant of floating status. Sexual function in non-breeding males, estimated through determination of systemic testosterone and progesterone levels, was similar to that in reproductively active conspecifics. Floating females, on the contrary, displayed an endocrine pattern of circulating estrogens and progesterone that was parallel in timing but reduced in magnitude as compared to breeders. Our results suggest that floaters are not physiologically constrained to reproduce, but the cost-benefit balance of attaining complete gonadal function is sexually dependent. While young, unmated males could increase their breeding prospects by attaining sexual maturity regardless of their social environment, natural selection would favor females relying on social cues to mature. Consistent with the sexual roles of socially monogamous species, gonadal recrudescence and testosterone production would allow unmated males to access breeding resources (e.g. through male-male competition and extra-pair fertilizations). Unmated females, on the contrary, would reduce physiological costs by means of delaying ovarian maturation until establishing pair bonds with a male providing access to breeding resources.