EFFECT OF WITHDRAWING LONG DAYS FROM MALE AMERICAN TREE SPARROWS (SPIZELLA-ARBOREA) - IMPLICATIONS FOR UNDERSTANDING THYROID-DEPENDENT PROGRAMMING OF SEASONAL REPRODUCTION AND POSTNUPTIAL MOLT

In previous studies, we withdrew thyroid hormones by thyroidectomy bef ore, at, or after the onset of photostimulation and showed that male A merican tree sparrows (Spizella arborea) are programmed for seasonal r eproduction and postnuptial molt by or before Week 3 on long days. In this corollary study, we withdrew long days before or after the contro l circuits had been programmed. After 1 day to 4 wk on long days, grou ps of thyroid-intact males were returned to short days until Week 7, w hen they were moved to constant light and evaluated for photosensitivi ty or photorefractoriness and postnuptial molt. Long-day controls held 7 wk on long days showed robust testicular growth through Week 6 and then spontaneous testicular regression. Testes of short-day controls a nd of males photostimulated for 1 day remained small. In all other gro ups, photostimulation induced testicular growth, which gave way to reg ression during exposure to short days. Long-day controls tested photor efractory at Week 7 and initiated molt by Week in. All other groups te sted photosensitive and did not molt. Our demonstration that long days are required for expression of seasonal reproduction and postnuptial molt in thyroid-intact male tree sparrows previously programmed for th ese events suggests that long days create a milieu that is permissive for expression.