THE ROLE OF PROLACTIN IN THE DEVELOPMENT OF REPRODUCTIVE PHOTOREFRACTORINESS AND POSTNUPTIAL MOLT IN THE EUROPEAN STARLING (STURNUS-VULGARIS)

Seasonal breeding in many birds, including the European starling, is t erminated by the development of absolute reproductive photorefractorin ess, followed by a postnuptial molt, when photo-induced PRL secretion is at its seasonal maximum. To determine whether this photo-induced in crease in PRL secretion has a causal role in the development of photor efractoriness or molt, European starlings were actively immunized agai nst vasoactive intestinal polypeptide (VIP), the PRL releasing hormone in birds, or against PRL, during a photo-induced breeding cycle. In h alf of the VIP-immunized birds, the photo-induced increase in PRL was completely suppressed. Although these birds became photorefractory, th e rate of gonadal regression was markedly slowed. These birds did not molt. In the remaining VIP-immunized birds, the photo-induced increase in PRL was inhibited but not completely suppressed. In these birds, a nd in those immunized against PRL, gonadal regression was also slowed, but molt progressed as normal. There were no significant differences in concentrations of plasma thyroxine between treatment and control gr oups, indicating that the effects of immunization on gonadal regressio n were not mediated by the induction of hypothyroidism. These results are consistent with the view that in the European starling the seasona l photo-induced increase in PRL accelerates gonadal regression during the onset of photorefractoriness but does not itself cause photorefrac toriness. Further, the seasonal increase in PRL is required for the in duction of the postnuptial molt.