Gonadal Responses of Brown-Headed Cowbirds to Long Daylength

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Experimental work with several species has shown that the gonads of wild birds develop from sexual inactivity into breeding condition when the birds are kept on increased hours of daily light in the winter months before the natural breeding season. The effect of the long daily photoperiods on the experimental birds is thought to be similar to the gradual gonadal development in wild birds during the slowly increasing daylengths of spring. The birds of temperate regions have probably evolved the photoperiodic response rather than a response to other environmental conditions because of the greater predictive value of daylength in timing the breeding season (Farner, 1959, 1964). Energy is conserved by remaining in a sexually inactive condition until a few weeks before the breeding season. The breeding seasons of the birds are thought to be timed to the occurrence of abundant food for their young, and daylength in early spring provides a reliable source of information about the coming of spring. As the time of nesting of host species is determined by daylength, it is of interest to know whether the development of the gonads in spring in brood parasites is determined to the same degree by daylength. The Brown-headed Cowbird (Molothrus ater) was studied to compare the gonadal response of a brood parasite to long daylengths with the photoperiodic responses of nonparasitic species. In addition, the significance of the refractory period as a timing factor in the annual cycle of temperate-zone songbirds was evaluated by a comparison of the breeding seasons and the timing of the refractory period in four species of North American icterids.