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.
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