Strategies in avian breeding cycles

Abstract

Abstract Birds show pronounced seasonal breeding at all latitudes, and many are able to predict and prepare for a forthcoming breeding opportunity, thereby maximizing reproductive potential. The preparatory phase of the breeding cycle is characterized by a complex of sexually motivated behaviours including courtship, selection of nest sites and territoriality. This phase of the breeding cycle is often initiated in response to initial predictive information derived from the decreasing daylengths of autumn or winter. Exposure to short daylengths dissipates long day-induced photorefractoriness allowing the reproductive neuroendocrine system to ‘switch on’. The resulting modest increase in the secretion of reproductive hormones stimulates pre-nuptial sexual behaviour, and in some birds, may be great enough to support short day breeding. For many birds, the increase in daylength in spring above the critical daylength for photoperiodically-induced gonadotrophin secretion is an essential supplementary source of predictive information for breeding resulting in the final maturation of the gonads. The increase in daylength above the critical daylength is also important for determining the date at which breeding terminates, by inducing the development of photorefractoriness. Although photorefractoriness is usually dissipated by exposure to short days, in some species it is dissipated after prolonged exposure to long days. This phenomenon may explain the generation of autonomous rhythms of reproductive activity. These may be used by migratory species, some sub-tropical birds breeding in the autumn, and birds breeding on the equator to generate initial predictive information for seasonal breeding.