The photosensitive weaver bird (Ploceus philippinus) has a long juvenile phase when, while the bird is insensitive to photostimulation, the gonads respond to injected gonadotrophins. It is suggested that the immature status of the hypothalamo/hypophyseal complex may be responsible for the presence of a long pre-photoresponsive phase in this bird. Rowan ('25, '29) was first to demonstrate the importance of light in the regulation of sexual cycles and migration of birds. Since then, the photoresponse of a number of birds has been experimentally studied (see reviews by Burger, '47, '49; Wolfson, '60; Farner, '58, '64; Marshall, '60; see also Thapliyal and Saxena, '64). As a result, it is now generally accepted that exposure of adult birds to constant long days causes their gonads to recrudesce, reach a maximum and then, after a period of activity, to regress and remain regressed as long as the light stimulus is maintained. To explain these results, it is proposed that the effect of light or of any other external or internal stimulus, is transmitted to the adenohypophysis through the neurosecretory cells of the hypothalamus. These cells secrete neurohumoral substance( s) into the hypophyseal portal system. The adenohypophysis in turn then regulates the activity of the gonads (Markee, Evertt, Sawyer, '52; Scharrer, '58; Benoit, '61) and in the weaver bird also controls the plumage pigmentation cycle (Thapliyal and Saxena, '61).
The neuroendocrine mechanism, responsive to light, is established quite early in the life of a bird (Sturnus vulgaris, Bissonnette, '32; Zonotrichia coronata, Miller, '48; Zonotrichia capensis, Miller, '63; Melopsittacus undulatus, Vaugien, '53; and Poephila castanotus, Marshall and Serventy, '57). Moreover, sexual behavior appears in immature birds of some species (see review by Bullough, '45) and juvenile gonads respond to injected gonadotrophins (darkes, Schockert, '31; pigeons and doves, Riddle and Polhemus, '31; pigeons, Evans and Simpson, '34; Lahr and Bates, '41;