The photo-gonadal response of a migratory bunting: Evidence of an external coincidence system

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Abstract
Male redheaded buntings (Emberiza bruniceps) were subjected to continuous darkness (DD) or short days (8L/16D) after photo‐stimulation (15L/9D) for either 15 or 30 days. Two groups of birds (four each), after photo‐stimulation for 30 days, were continued on a regimen of 15L/9D. Another two groups (of four each) of photo‐sensitive birds were also subjected to DD or 8L/16D; they served as controls. The effect of transfer to DD or 8L/16D on the size of testes was found not be dependent upon the size attained at the time of transfer. The rate of testicular regression was nearly the same under both the treatments, i.e., in those birds transferred either to DD or to 8L/16D. The birds that were maintained on 15L/9D maintained enlarged testes; this excluded the possibility of the onset of photo‐refractoriness during the treatment period. On the other hand, testes were not photo‐stimulated in the photo‐sensitive birds subjected to DD or 8L/16D. These results are consistent with the Bünning‐Pittendrigh avian external coincidence model, but could be explained alternatively by a relatively simple internal coincidence model. Nevertheless, the application of the law of parsimony suggests the acceptance of the hypothesis of the external coincidence model.