Changes in Blood Levels of Thyroid Hormones in Two Species of Passerine Birds

Abstract

Annual changes in plasma levels of thyroxine (T4) and triiodothyronine (T3) were compared in White-crowned Sparrows (Zonotrichia leucophrys gambelii) and House Sparrows (Passer domesticus) using serial blood samples from birds maintained in outdoor aviaries and from feral birds. Gonadal size was monitored by monthly laparotomy, and plumage was examined for molt at frequent intervals. Hormone levels were examined in relation to cycles of reproduction and molt, and to changes in ambient temperature. Thyroxine levels increased during postnuptial molt in both species, although in White-crowned Sparrows the increase was similar to that noted in late April and early May, which may have been associated with premigratory activity. However, in House Sparrows, thyroxine levels were highest throughout the three-month period of molt (mid-August to mid-November). They did not increase immediately after gonadal regression, giving no support for the hypothesis of a thyroid-gonad antagonism. Cold winter temperatures and changes in day length appeared to have little effect on T3 and T4 levels. In both species T3 levels showed little seasonal variation, and changes in T4 levels were independent of those in T3.