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Endocrine and Environmental Control of Vernal Migration in Male White-Crowned Sparrows, Zonotrichia leucophrys gambelil

Migratory birds in captivity continue to show migratory restlessness throughout the normal breeding period. We investigated whether reduced endocrine activity of the testes of captive male white-crowned sparrows causes this extension of migratory state. Experimentally elevated circulating levels of testosterone at the time it attains maximum levels in free-living male conspecifics after arrival on their territories did not suppress the migratory activity of captive males. Also, exposure to estradioltreated females did not affect migratory activity. Testosterone treatment at the end of the reproductive period postponed the seasonal termination of migratory activity and the onset of postnuptial molt. It is suggested (1) that unknown environmental or neuroendocrine cues suppress migratory behavior during breeding; and (2) that testosterone indirectly delays the final termination of vernal migratory behavior of captive males by its delaying effect on the onset of photorefractoriness, which ultimately terminates the tendency to migrate to the breeding area.