HORMONAL RESPONSES TO EXTERNAL STIMULI IN BIRDS*

Abstract

Summary. 1 Within the breeding seasons regulated by annual changes in environmental conditions, there exist detailed changes in the pattern of hormone secretion from stage to stage within the season, which are in part responsible for the successive changes in behaviour during the season. 2 In many species of birds the general environmental variables induce greater development in the male reproductive system than in the female, so that males arriving at the breeding grounds in spring are usually more advanced toward full spermatogenesis than are the females toward ovulation. Experimentally, additional illumination alone is capable of bringing about full testicular development, but not full ovarian development. 3 Stimuli arising from courtship behaviour induce the secretion of follicle-stimulating hormone by the pituitary gland of the female in many species of birds. 4 The presence of nesting material and of the nest, and/or participation in nest-building behaviour, further contribute to the advancement of the reproductive cycle, probably contributing to the stimulation of luteinizing hormone secretion by the pituitary. 5 Stimuli provided by the eggs appear to be able to induce prolactin secretion, and to inhibit the secretion of FSH. The prolactin secretion so stimulated probably, in turn, contributes to the maintenance of incubation, and of the brood patch. 6 Stimuli provided by the young are capable of inhibiting or delaying the onset of a new laying cycle, although the physiological basis of this is rather obscure. 7 Recently discovered details of the physiological and anatomical relationships between the brain and the pituitary gland provide insight into the anatomical and physiological mechanisms by means of which external stimuli can reflexly cause changes in hormone secretion. 8 The synchronization of the behaviour of the members of a mated pair, necessary for effective reproduction, is probably in part effected by the effects upon their endocrine glands of stimuli which they provide to each other, and which they receive from the eggs and young. 9 These processes of neuro-endocrine and “psycho-endocrine” regulation provide a link, in certain areas, between the work of physiologists, ethologists, ecologists, and animal psychologists.