The effects of winter daylengths on the duration of pituitary refractoriness to photostimulation are described.
The mallard, as do probably all wild avian species, enters a postnuptial refractory period during which the gonads display no gametogenesis and the anterior pituitary becomes seasonally unresponsive to photostimulation so far as gonadotrophin secretion is concerned. This phase lasts about three months and is an important synchronizer of the breeding cycles.
Refractory specimens kept under a summer daylength of 16.5 hours remain unresponsive to photostimulation at a time when birds in their natural environment are emerging from their refractory period. When, however, such birds are subjected to a period of eight hours daylengths there is a shortening of the refractory phase and they display a gonadotrophic response to photostimulation.
There is some indication that a phase of winter daylengths is not, however, a necessary pre-requisite for birds to be capable of emerging from the refractory period and recovering some degree of spermatogenetic activity. Thus, even in some specimens kept under the long photoperiods there were spermatogonia multiplications occurring when the experiment was terminated.