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Plasma Thyroid Hormone Concentrations in a Wintering Passerine Bird: Their Relationship to Geographic Variation, Environmental Factors, Metabolic Rate, and Body Fat

Winter acclimatization among passerine birds involves metabolic adjustments that allow for high rates of thermogenesis. In previous studies, we observed geographic variation in the basal metabolic rate (BMR) of overwintering cardinals along a latitudinal gradient at two different longitudinal transects. Because thyroid hormones (THs) are important for metabolic adjustments in endotherms, we determined whether geographic variation in BMR can be explained by variation in thyroid status. We measured total plasma TH (thyroxine [T4] and 3,5,3′‐triiodothyronine [T3]) concentrations by radioimmunoassay in birds from two latitudinal transects extending from approximately 31° to 42°. Birds from both transects had higher plasma THs in the late afternoon than in the early morning. Plasma T3 increased with latitude, while plasma T4 varied such that the southernmost birds and the northernmost birds had higher hormone concentrations than birds at the intermediate latitude. There was no correlation between plasma TH concentrations and BMR. To test whether thyroid status influences metabolic parameters in winter‐acclimatized captive cardinals, we fed cardinals diets supplemented with T4 (5 μg T4 g−1 food), the goitrogen methimazole (1 mg g−1 food), or both. Plasma T4 concentrations were altered by most of the treatments, but we observed no significant effects on any metabolic parameter. We conclude, therefore, that there is latitudinal variation in metabolic parameters in cardinals but that this variation is not explained by variation in plasma TH concentrations.