Role of thyroid in the response of bill pigmentation to male hormone of the house sparrow, Passer domesticus

In the house sparrow (Passer domesticus), seasonal testicular growth was decreased, and fully developed testes regressed after thyroidectomy. Both castration and thyroidectomy resulted in a loss of bill color. Thyroidectomy had no effect on the body weight, the seasonal regression of the testes, or on regressed testes.Administration of testosterone at 25 μg day^?1 bird^?1 level had no appreciable effect on the bill color of either the castrated or thyroidectomized birds; and 50 μg day^?1 bird^?1 was partially effective only in castrates. Further, while 100 μg day^?1 bird^?1 fully counteracted the effects of castration on bill color during October/November and induced partial coloration during December, it could not suppress depigmentation or induce coloration in the bill of the thyroidectomized birds. Testosterone, however, had no effect on the gonads of thyroidectomized, but increased the weight of the vas deferens to the same extent as that of the castrated birds.It is suggested that the thyroid-gonad relationship in the house sparrow varies with the reproductive status of the bird, and thyroid activity is essential for the development of the gonads and pigmentation of the bill.