The role of testosterone in spring nomadic migration of male pine siskins (Spinus pinus)

Our understanding of the endocrine regulation of migration comes primarily from studies of obligate migrants, which make predictable seasonal movements. Less well studied are facultative migrations, which are more variable in timing, distance, and direction. In obligate migrants, an increase in circulating testosterone appears to be important in stimulating the transition to a spring migratory state, though the mechanism by which testosterone exerts these effects remains poorly understood. The pine siskin (Spinus pinus) is a songbird that exhibits spring nomadism, a form of facultative migration. Using captive male pine siskins, we first tested the hypotheses that circulating androgens stimulate (i) physiological preparation and (ii) the expression of behavioral readiness for nomadic migration. We found that when birds were given subcutaneous implants containing exogenous testosterone, they exhibited greater nocturnal migratory restlessness, but generally not greater physiological preparation, compared to control birds. Further, when we inhibited the effects of testosterone by giving subcutaneous implants of androgen receptor antagonist and aromatase inhibitor these birds showed less nocturnal migratory restlessness compared to control birds, though the groups did not differ in physiological preparations. We then tested whether the effects of testosterone are mediated by activation of androgen receptors or estrogen receptors by giving androgen receptor antagonist and aromatase inhibitor separately. This manipulation provided some evidence that the effects of testosterone on migratory restlessness occur via activation of androgen receptors and suggested a role for androgen receptor activation in physiological preparations. Overall, the results indicate a role for testosterone in stimulating spring nomadic migratory behavior in male pine siskins.