Thyroid development in relation to the development of endothermy in the red-winged blackbird (Agelaius phoeniceus)

We investigated the development of thyroid function during the transition t o endothermy in red-winged blackbirds (Agelaius phoeniceus). Thermoregulato ry 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 i n these birds is dependent in part on the development of thyroid function. We assessed thyroid development by measuring changes in thyroid gland histo logy and plasma concentrations of thyroxine (T4) and triiodothyronine (T3) during the nestling period. To gain insight into the role of thyroid matura tion 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-secti onal area) increased during nestling development, with the fastest growth o ccurring just before the development of endothermy. By day 8, it reached th e size typical of that in adults. Follicular cell height of the thyroid gla nds increased in nestlings up to day 6 and then decreased for the rest of t he nestling period. The mean area of individual follicles increased up to d ay 8 of nestling life and then decreased. Individual nestlings were capable of strong endothermic responses at 7 to 8 days of age and had significantl y decreased plasma T4 concentrations following cold exposure, suggesting in creased T4 to T3 deiodination to maintain the plasma concentrations of the more metabolically active T3. The patterns of plasma T4 and T3 during nestl ing development were consistent with those of nestlings of other altricial species of birds that have been studied. Overall, the patterns of thyroid d evelopment observed were consistent with the hypothesis that the functional development of the thyroid is critical to the development of endothermic c apabilities and that thyroid hormones play a role in endothermic responses to cold temperatures. (C) 1999 Academic Press.