RESUMEN
We investigated the development of thyroid function during the transition to endothermy in red-winged blackbirds (Agelaius phoeniceus). Thermoregulatory capabilities of blackbirds improve markedly over their relatively short nestling period (10-12 days), with the most striking improvements occurring between days 6 and 8. We hypothesized that the development of endothermy in these birds is dependent in part on the development of thyroid function. We assessed thyroid development by measuring changes in thyroid gland histology and plasma concentrations of thyroxine (T4) and triiodothyronine (T3) during the nestling period. To gain insight into the role of thyroid maturation in the context of thermoregulation, we compared plasma thyroid hormone profiles in nestlings exposed to cold temperatures to those maintained at thermoneutral temperatures. The overall size of the thyroid (as cross-sectional area) increased during nestling development, with the fastest growth occurring just before the development of endothermy. By day 8, it reached the size typical of that in adults. Follicular cell height of the thyroid glands increased in nestlings up to day 6 and then decreased for the rest of the nestling period. The mean area of individual follicles increased up to day 8 of nestling life and then decreased. Individual nestlings were capable of strong endothermic responses at 7 to 8 days of age and had significantly decreased plasma T4 concentrations following cold exposure, suggesting increased T4 to T3 deiodination to maintain the plasma concentrations of the more metabolically active T3. The patterns of plasma T4 and T3 during nestling development were consistent with those of nestlings of other altricial species of birds that have been studied. Overall, the patterns of thyroid development observed were consistent with the hypothesis that the functional development of the thyroid is critical to the development of endothermic capabilities and that thyroid hormones play a role in endothermic responses to cold temperatures.