Testosterone Increase in Free-Living Young Blackcaps Sylvia atricapilla and Wood Warblers Phylloscopus sibilatrix During Post-Juvenile Molt with Possible Implications for Juvenile Dispersal

Abstract. Testosterone may affect many aspects of passerine maturation including the expression of plumage coloration and spring migration in adult birds. However, how the testosterone level changes during bird development is not well known. We compared the testosterone profile during juvenile development of a sexually dimorphic species, the Blackcap Sylvia atricapilla, and a monomorphic species, the Wood Warbler Phylloscopus sibilatrix. We first tested whether testosterone influences plumage coloration and increases at the onset of molt in males of the dimorphic but not the monomorphic species. We found that a testosterone level increase occurred in both sexes and species during later stages of molt, and thus was not related to male plumage coloration. We also investigated whether the increase in testosterone level coincides with juvenile dispersal. If testosterone affects dispersal behavior in these species, both sexes should show an elevated testosterone level during dispersal, but this increase should occur earlier in the Wood Warbler, which disperses earlier than the Blackcap. In juvenile Blackcaps, the increase in testosterone level occurred on the 43–68th day after hatching, while in Wood Warblers it occurred on the 32–36th day (i.e. 11–32 days earlier). The increase in testosterone level coincided with the onset of the post-juvenile dispersal in both species. This study provides the first direct evidence of a testosterone level increase during juvenile development in two free-living migratory birds. While not correlated with species plumage coloration, this increase might affect other aspects of behavior, e.g. juvenile dispersal.