TIMING OF SEASONAL BREEDING IN BIRDS, WITH PARTICULAR REFERENCE TO NEW-ZEALAND BIRDS

A model to explain the timing of seasonal breeding in birds is present ed. It is assumed that, despite the wide range in egg-laying seasons, there are common physiological mechanisms which underlie seasonality i n birds and that most, if not all, birds are photoperiodic. Birds are considered to possess an internal rhythm of reproduction which is sync hronized with seasonal changes in the environment by external factors, particularly the annual cycle of daylength. The rhythm consists, at l east in part, of regular changes in the photoperiodic response between states of photosensitivity and photorefractoriness. Avian breeding se asons effectively start in autumn when birds become photosensitive, re gardless of when egg-laying occurs. The timing of breeding is then inf luenced by the rate of increase of hypothalamic 'drive' and by the sen sitivity of the hypothalamus and pituitary gland to inhibitory feedbac k from gonadal steroids. If sensitivity is high, gonadal growth will n ot occur until the threshold daylength for photostimulation is exceede d after the winter solstice. Egg-laying then starts in late winter, sp ring or summer. Alternatively, steroid feedback may be relatively low and gonadal growth may be sufficiently rapid once the birds become pho tosensitive that breeding occurs in late autumn or winter. The time of egg-laying in birds may also be strongly influenced by supplementary information, such as social cues, food availability, temperature and r ainfall and, in some species, this information is more important than daylength in determining the timing of breeding. The review also inclu des the first summary of the breeding seasons of New Zealand birds. Th e pattern of egg-laying is exactly the same in native birds, in birds introduced to New Zealand and in other Southern hemisphere birds from similar latitudes, with a broad peak of egg-laying occurring from Sept ember to December. In addition, annual cycles of steroid hormone conce ntrations in the North Island brown kiwi, the yellow-eyed penguin and the kakapo are consistent with results from many studies on Northern h emisphere birds. This model for the timing of breeding in birds can be applied to New Zealand birds and it is concluded that the physiologic al control mechanisms for the timing of seasonal breeding in New Zeala nd birds are similar to those of other birds.