Background/Question/Methods
Migratory birds face a conflicting set of energetic demands between surviving the winter, preparing to migrate and preparing to breed. Physiological mechanisms that minimize conflicts, perhaps through diversifying hormone function, would, if achieved, be advantageous to both survival and breeding success. We hypothesized that the primary male breeding hormone, testosterone, enhances both migratory and breeding preparation prior to departure on spring migration, resulting in early arrival and higher breeding success for males with high testosterone. We measured circulating testosterone in non-breeding male American redstarts (Setophaga ruticilla) in Jamaica, West Indies throughout winter and during arrival at breeding territories in New Hampshire, U.S.A. We further predicted that males from high quality winter habitats (as measured by stable-carbon isotopes in claws) would arrive earlier than those from inferior habitats and have higher testosterone. Lastly, we experimentally manipulated testosterone during migratory preparation in captive male dark-eyed juncos (Junco hyemalis) to test the prediction that males with elevated testosterone would reach peak migratory condition before controls and males in whom the ability of testosterone to bind to receptors was blocked (T-blocked).  
Results/Conclusions
Our results from free living redstarts demonstrate that testosterone increases prior to spring departure from winter quarters and that male redstarts with higher testosterone arrive at the breeding grounds earlier than those with lower testosterone and early arrivers are more likely to successfully breed. Further, early arriving males were more likely to be from high quality winter habitats. Our experimental study indicated that testosterone enhances migratory preparation through its anabolic and erythropoietic effects. Males with experimentally elevated testosterone achieved peak migratory condition (measured by mass, food intake, Zugunruhe, haematocrit, and fat stores) and peak breeding condition (measured by cloacal protuberance development) approximately six days before controls. T-blocked males did not begin migratory or breeding preparation until twelve days after controls. This work reveals a potential physiological mechanism by which winter to breeding seasonal interactions occur in migratory birds.