Role of thyroid in sexual and body weight cycles of the migratory redheaded bunting, Emberiza bruniceps.

In the male redheaded bunting, Emberiza bruniceps, kept under natural lighting in northern India (Lat 25 degrees 18'N, Long 83 degrees 01'E), testicular growth began during March/April and full spermatogenesis was achieved in May/June; thereafter, the testes regressed and remained regressed for the rest of the year. Thyroidectomy between November and April partially suppressed photoinduced testicular growth in May/June, but did not prevent testicular regression in July. The extent to which thyroidectomy suppressed photoinduced testicular growth was directly related to the time elapsed between thyroidectomy and photostimulation. Thyroidectomy in mid-late May induced testicular regression, an effect which was reversed by daily im injection of 500-2000 ng thyroxine/bird/day/30 days. Thyroidectomy or treatment of intact or thyroidectomized birds between July and March when the testes were regressed had no immediate effect on testicular growth. Body weight which was about 24 g between December and March, increased to about 28 g during the breeding season. Thereafter, it decreased to 21-22 g between August and October. Thyroidectomy between November and May caused an immediate fall in body weight, an effect which was reversed by daily im injection of 500-2000 ng thyroxine/bird/day/30 days. Neither thyroidectomy nor thyroxine treatment prevented the seasonal fall in body weight in June and July nor affected body weight between August and October. It is suggested that in the male redheaded bunting, under natural lighting, thyroid hormones are necessary for the full development of the testes and an increase in body weight during the breeding season. However, thyroid hormones do not appear to be necessary for the development of photorefractoriness or for the seasonal decrease in body weight.