A test of absolute photorefractoriness and photo-induced neural plasticity of song-control regions in black-capped chickadees (Poecile atricapillus)

We tested whether male and female black-capped chickadees, Poecile atricapillus (L., 1766), were absolutely photorefractory according to Hamner's (1968) criteria of (i) spontaneous regression of gonads during prolonged long-day exposure and (ii) no subsequent recrudescence of gonads in response to constant light. We initially exposed black-capped chickadees to constant long-day photoperiods. Gonads regressed spontaneously, demonstrating that the birds met Hamner's first criterion for absolute photorefractoriness. Once their gonads fully regressed and the birds were in advanced prebasic moult, we exposed them to one of three photoperiods for an additional 2 weeks: constant light (24 h L), short days (8 h L), and controls (15 h L). Constant light challenge had no effect on gonadal condition or rate of moult, confirming that the birds met Hamner's second criterion for absolute refractoriness. We also compared volumes of song-control nuclei in the three groups and found that males overall had larger HVC, robust nucleus of arcopallium, and area X than females, but that longer days (24 h L) did not increase volumes and that shorter days (8 h L) did not decrease volumes compared with controls (15 h L). These data support the inference that black-capped chickadees do indeed become absolutely photorefractory, and that photorefractoriness precludes photo-induced plasticity of the song-control system.