The Annual Cycle of Thyroid Activity in White-Crowned Sparrows of Eastern Washington

Abstract

It is now evident that many species of plants and animals have long period physiologic cycles. In birds, the most striking of these cycles have a period of one year. Familiar examples are the annual cycles in organs associated with reproduction and migration (Farner, 1955, 1958, 1959). Less familiar are the annual cycles in such organs as the liver (Oakeson, 1953) and the thyroid gland (H6hn, 1950) whose relation with reproduction or migration is not clear. Some annual physiological cycles are caused by cyclic factors in the environment. For instance, day length has been demonstrated to be the main cause of the annual occurrence of testicular development in certain passerine birds (Farner, 1959). On the other hand, internal mechanisms have been suggested to be the sole cause of some physiological cycles including annual cycles (Webb and Brown, 1959). The present paper is concerned with the annual cycle of thyroid activity in a passerine species. Despite the many investigations of annual cycles of thyroid activity in passerine birds, there is still no agreement concerning the form of a typical thyroid cycle. Our first objective, necessarily, has been a quantitative description of the principal features of the thyroid cycle in the bird selected for investigation, Zonotrichia leucophrys gambelii, a race of the White-crowned Sparrow. This race and other representatives of the genus Zonotrichia have already been used for many studies of annual physiological cycles. A good opportunity was thereby provided to examine the relation of the thyroid cycle not only to cyclic environmental factors but also to other physiological cycles. Evidence has been obtained which suggests that the thyroid cycle in Z. 1. gambelii is caused mainly by a cyclic environmental factor, very probably temperature. While our investigations were in progress in Washington, somewhat similar studies were carried out independently on the same race in California and Alaska by Oakeson and Lilley (1957, 1960). This now permits an important series of comparisons.