THE TIMING OF THYROID-DEPENDENT PROGRAMMING IN SEASONALLY BREEDING MALE AMERICAN TREE SPARROWS (SPIZELLA-ARBOREA)

There is convincing evidence that euthyroid male American tree sparrow s are already programmed for seasonal reproduction and postnuptial mol t by Week 4 of photostimulation. To explore more precisely when, durin g early photostimulation, thyroid-dependent programming of seasonal ev ents occurs, photosensitive male tree sparrows were radiothyroidectomi zed or sham thyroidectomized on the first day of photostimulation (Wee k 0) or at Weeks 1 or 3 thereafter. Birds were monitored for testicula r growth and regression over 8 or 12 weeks and then tested for photose nsitivity or photorefractoriness by exposing them to constant light an d exogenous L-thyroxine for 4 weeks. Molt of the primary flight feathe rs was scored periodically, and at the end of the experiment, hypothal ami were saved for cGnRH-I (chicken gonadotropin-releasing hormone I) assay. Because (1) the thyroid is already dysfunctional by Day 4 after radiothyroidectomy, (2) thyroid-dependent photoinduced gonadal growth is programmed growth, and as demonstrated here, (3) testis length at Week 6 on long days (i.e., maximum or near-maximum testis size) is ind ependent of the time of thyroidectomy at or after the onset of photost imulation, we conclude that male American tree sparrows were programme d for photoperiodic testicular growth during the first week of photost imulation. Based on measurements of testis length and hypothalamic cGn RH-I content, only 2 of 11 birds thyroidectomized at Week 1 were alrea dy programmed for photorefractoriness by Week 1, whereas all birds thy roidectomized at Week 3 were so programmed by Week 3. Marked differenc es in molt scores between photorefractory (thyroidectomized and euthyr oid) and photosensitive (thyroidectomized) birds argue that postnuptia l molt also was programmed roughly between Weeks 1 and 3. To explain t hese results, we ascribe organizational-like actions directly or indir ectly to endogenous thyroid hormones and argue that the onset of photo stimulation opens a window, during which time euthyroid male American tree sparrows are programmed for seasonality. A clear dissociation of photorefractoriness from photoperiodic testicular growth in birds thyr oidectomized at Week 1 suggests that separate control circuits mediate these two annually periodic events. (C) 1996 Academic Press, Inc.