Time of thyroidectomy variably affects seasonality in female American tree sparrows (Spizella arborea)

Female American tree sparrows (Spizella arborea) were injected with Na131I before, at, or after the onset of photostimulation in order to study the effect of time of thyroidectomy on three components of seasonality: thyroid-dependent photoperiodic ovarian growth, photorefractoriness, and postnuptial (prebasic) molt. Thyroidectomy before or at the onset of photostimulation abolished all components of seasonality; birds exhibited only minor thyroid-independent photoperiodic ovarian growth. Thyroidectomy on day 7 of photostimulation blocked the transition from photosensitivity to photorefractoriness; although birds showed thyroid-dependent photoperiodic ovarian growth, they neither exhibited ovarian regression nor initiated postnuptial molt. Thyroidectomy on day 14, 21, or 28 of photostimulation had no remarkable effect on any component of seasonality. We conclude that separate mechanisms control photoperiodic ovarian growth and photorefractoriness/molt and that, early during photostimulation, the thyroid has a codependent role in programming female tree sparrows for vernal as well as autumnal seasonal events.