The Role of Breeding Preparation in Migration Phenology and Seasonal Interactions in Migratory Passerines

Migratory birds face a conflicting set of energetic demands between preparing to migrate and preparing to breed. Physiological mechanisms that minimize conflicts, perhaps through diversifying hormone function, would be advantageous to both survival and breeding success. Migratory animals complete different stages of their life-cycle in areas that may be thousands of miles apart. However, events that occur in one location can affect events in another location during a subsequent stage, in what are termed seasonal-interactions . I sought to determine if variation in production of androgens, the primary male breeding hormones, could underlie seasonal interactions between non-breeding and breeding seasons in male migratory birds. I first demonstrated in a captive experiment using male dark-eyed juncos (Junco hyemalis) that elevating the androgen testosterone (T) advanced migratory and breeding preparation, while inhibiting T activity prevented preparation for both. In an observational study, I demonstrated that free-living male American redstarts (Setophaga ruticilla) increase androgen production before departure from a non-breeding site in Jamaica, and that males in better late winter condition have higher androgen. Further, in a hormone manipulation experiment, I showed that males with elevated T in late winter prepare to migrate faster and depart on spring migration earlier. Lastly, I demonstrated that, compared to late arrivers, early arriving male redstarts at a breeding site in New Hampshire had higher androgen, were from higher quality winter habitat, were in superior migratory condition, and ultimately were more likely to successfully breed. Collectively, these results demonstrate that early elevation of androgens could facilitate early arrival at breeding areas, ultimately increasing fitness and potentially providing a mechanism for the interaction between seasons