Involvement of thyroid gland in the development of migratory disposition in the redheaded bunting, Emberiza bruniceps

Abstract

In the redheaded bunting Emberiza bruniceps, thyroidectomy inhibited premigratory fattening and nocturnal restlessness—two characteristics of avian migration—observed in caged birds during the premigratory period (March/April). Thyroxine (T 4) and triiodothyronine (T 3) administration in thyroidectomized birds stimulated locomotor activity and restored the loss in body weight. Annual variations in circulating thyroid hormone concentrations revealed a significant rise in T 3 T 4 ratio prior to spring migration in both years studied. This increase in circulating T 3 T 4 ratio may be associated with the development of migratory disposition in this bird. There was no increase in circulating T 3 T 4 ratio prior to autumnal migration, however, plasma T 4 increased significantly. Different thyroidal mechanisms are most likely involved in spring and fall migratory periods. While T 3 remained low throughout, apart from the characteristic spring rise, high T 4 levels in E. bruniceps were associated with periods of reproduction and molting, the latter coinciding partly with autumnal migration.