PHOTOREFRACTORINESS IN THE BLACK-HEADED BUNTING EMBERIZA-MELANOCEPHALA - POSSIBLE INVOLVEMENT OF THE THYROID-GLANDS

In the male black-headed bunting, Emberiza melanocephala, photostimula ted testicular and/or body weight growth was followed by the regressio n. Transfer of photorefractory birds from 20L/4D to 23L/1D or from nat ural lighting (12-13 h) to 20L/4D failed to evoke testicular and/or bo dy weight recrudescence. Thyroidectomy suppressed light-induced increa se in the testes and resulted in early regression. Fully developed tes tes of breeding birds also regressed following thyroidectomy; an effec t which was reversed by daily injections of 1 mu g/bird of L-T-4. Trea tment with L-T-4 at doses from 0.5-2.0 mu g/bird/day/30 days had no ef fect on the testes of birds maintained on 12L/12D or following a shift from 12L/12D to 20L/4D. Photoinduced increase in body weight was inhi bited by thyroidectomy; an effect which was reversed by treatment with L-T at dose level 1 mu g/bird/day. The extent to which thyroidectomy decreased body weight of birds depended upon the lipid reserves at the time of operation. It is suggested that in the male black-headed bunt ing (i) breeding is terminated by development of absolute-gonadal and metabolic-photorefractoriness and (ii) thyroid hormones are necessary for sustaining light-induced increase in the gonads and/or body weight and for their maintenance, but not for the development of photorefrac tory state.