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. Beginning more than half a century ago, with the experiments in which thyroid tissue was fed to birds (Carlson et al. 1912), a voluminous literature has accumulated that implicates thyroid hormones as the cause for molt (for reviews see Assenmacher 1958, Voitkevich 1966, Payne 1972). Although thyroidectomy appears to inhibit molt and thyroid-inhibiting drugs often alter feather morphology (Voitkevich 1966), histological studies in several species, using epithelial cell height as a measure of thyroid activity, do not support the hypothesis that an increase in thyroid gland function is the primary mechanism responsible for molt and feather replacement (reviews by Wilson and Farner 1960, Payne 1972). Progesterone and prolactin also induce molt, but in experiments with such hormones feathers are usually dropped in an abnormal sequence (Assenmacher 1958, Tanabe and Katsuragi 1962). Using radioimmunoassay, I measured changes in plasma levels of thyroid hormones in order to determine if there were increases in thyroid activity associated with molt or with seasonal cycles in ambient temperature, and if there was a hormonal basis for the temporal separation of reproduction and molt. I compared the White-crowned Sparrow (Zonotrichia leucophrys gambelii), a migratory species having two molts per year, with the House Sparrow (Passer domesticus), a sedentary species which has a single extended molt