Changes in thyroid activity during the annual sexual cycle of a wild avian species, the tree pie Dendrocitta vagabunda.

Thyroid activity was studied during the annual gonadal cycle in both sexes of the tree pie (Dendrocitta vagabunda). The study was conducted once a month throughout the year by evaluating thyroid weight, epithelial height and peroxidase activity. The values of these parameters moderately increased during the progressive phase, maximally increased during breeding, decreased in the regression phase and decreased further during the non-breeding phase in both sexes of birds. The findings indicate that thyroid activity, probably including thyroxine synthesis, varies annually, being highest during breeding compared to other phases of the gonadal cycle and that, there may be a progonadal relationship of the thyroid gland in both sexes of the tree pie.