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

-Annual changes in plasma levels of thyroxine (T4) and triiodothyronine (T3) were compared in White-crowned Sparrows (Zonotrichia leucophrys gumbeliz) 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 T, and T, levels. In both species T, levels showed little seasonal variation, and changes in T, levels were independent of those in T,. Beginning more than half a century ago, with the experiments in which thyroid tissue was fed to birds (Carlson et al. 19 12) 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 per year. METHODS I captured the birds for the laboratory study with mist-nets in January 1976, on the Sunnyside Game Refuge, Mabton, Yakima Co., Washington, and transported them within 24 h to outdoor aviaries in Seattle (48”N, 122”W). At approximately biweekly intervals I collected serial blood samples in heparinized capillary tubes from the brachial vein in the left wing of 40 birds, 10 adults of each sex and of each species. I collected the samples between 1O:OO and 12:OO and, following centrifugation, froze the plasma at -20°C until analysis. I checked body weight and molt status at each sampling and estimated gonadal weight at monthly intervals by laparotomizing the birds and visually comparing their gonads with a preserved set of standards. I collected additional blood samples from free-living birds, also at the Sunnyside Game Refuge, during the fall, winter, and spring (1976-1977). I sampled birds between 10:00 and 14:00 PST, marked them with numbered aluminum bands (U.S. Fish and Wildlife Service), and released them. A few were recaptured and resampled. Their blood samples were centrifuged in the field, and the plasma removed and frozen immediately on dry ice (for details, see Wingfield and Farner 1976). John C. Wingfield provided me with blood samples from breeding White-crowned Sparrows (Z. 1. gumbelii) collected near Fairbanks, Alaska, 65”N, 148”W (Wingfield and Farner