High testosterone prior to song crystallization inhibits singing behavior in captive yearling dark-eyed juncos (Junco...

In passerine birds, song is considered crucial in advertising reproductive and territorial status to conspecifics. Variation in the quality and frequency of song may be influenced by hormonal effects during the individual's development. This variation in turn may affect the function and potency of song. We studied the influence of testosterone on vocal production in first-year male dark-eyed juncos (Junco hyemalis), using subcutaneous silastic implants filled with testosterone. Subjects were visually but not acoustically isolated from one another and after capture had no exposure to female or adult male models. Implants were administered when subjects were in the plastic song phase (i.e., after they had begun to sing but before song was fully crystallized). Control males (C males) received empty implants. Experimental males were of two classes: TI males received one dose of testosterone (a single 10-mm implant), and TII males received two doses. Testosterone implants kept plasma levels high well into the breeding season, whereas in nature, levels normally drop after territorial acquisition and pair formation. Control males sang at higher rates than testosterone-treated males of both classes and had the greatest number of song types. This inhibitory effect of testosterone on vocal production suggests that disturbance of seasonal profiles of testosterone in birds may interfere with the production of species-typical song.