Testosterone dynamics of migratory birds during stopover

Birds migrating in the spring must balance energy with hormonal preparations in anticipation of the forthcoming breeding season. We investigated the relationships between testosterone, body condition, sociality, territoriality and fueling rates in Western Subalpine Warbler (Curruca iberiae) males during a trans-Saharan stopover. Baseline testosterone was highly variable in correspondence with the transitional nature of spring stopover. Some individuals reached breeding testosterone levels while others had undetectable levels. Testosterone varied with body condition suggesting an endocrine-energy link during migration. Simulated territory intrusions induced an increase of testosterone up to physiological maxima- a similar pattern to breeding contexts. Testosterone was negatively associated with territorial male density, suggesting a 'dear enemy' effect related to the daily variation in social stability. In repeatedly-sampled individuals, stopover duration and fueling rate were not correlated with baseline testosterone. However, as testosterone decreased, body condition increased. This suggests that stopover territoriality may reduce the reported negative effects of chronically high testosterone. Our data supports the hypothesis that hormonal preparation for breeding may already occur during stopover, and that this is largely linked to body condition. In this system, the endocrine-energy relationship is likely maintained by stopover territoriality. We conclude that male-male social contexts are modulated in similar ways during spring migration as during the breeding life history stage.