The Natural Termination of the Refractory Period in the White-crowned Sparrow

In a number of the avian species in which testicular development can be evoked by increased daily photoperiods it has been found that such development cannot be sustained indefinitely and that there is a period following each cycle of development during which increased daily photoperiods are ineffective. This period is commonly designated as the refractory period. Much of the available information concerning this phenomenon has been reviewed by Burger (1949) and Miller (1954). In the course of experiments in our laboratories on the photoperiodic control of gonadal and migratory cycles in the Gambel race of the White-crowned Sparrow, Zonotrichia leucophrys gambe& it has been necessary to ascertain quite precisely the time of natural termination of the refractory period. The results of our studies of this phenomenon now prove to be supplemental to those obtained by Miller (1954) on this subspecies in his investigation of the refractory period in three subspecies of Z. leucophrys and in the Golden-crowned Sparrow, Z. coronata. However, since our experiments involved numbers of 2.1. gambe&i considerably greater than were available to him, it would appear useful to publish some of our data and observations. The investigations reported here were supported in part by funds provided for biological and medical research by the State of Washington Initiative Measure No. 17 1.