Photostimulation induces rapid growth of song-control brain regions in male and female chickadees (Poecile atricapilla)

To examine the effect of photoperiodic condition on song-control nuclei we examined three groups of male and female black-capped chickadees (Poecile atricapilla). Photorefractory birds were held on long days for several months and had fully regressed gonads. Photosensitive birds were held on short days for several months and had small gonads. Photostimulated birds were transferred to long days for 16 days following several months of short days. Following these photoperiod treatments we determined the volume of the song-control nuclei HVc, Area X, and (RA). There were significant sex differences in HVc and Area X, but not in RA. There were also significant photoperiod effects in HVc and Area X. Thus, sex and photoperiod differences in the song system exist in a species in which both sexes produce learned calls year-round.