Seasonal Reproduction: Circannual Rhythms and Photoperiodism, Birds

Abstract

To better adapt to seasonal environmental changes, organisms have evolved ~1-year circannual clock. Birds typically lay eggs in spring through summer because of the abundance of food and mild climates. This reproductive strategy is known as seasonal reproduction. Seasonal changes in photoperiod are crucial cues for seasonal reproduction in birds inhabiting non-tropical regions. Birds detect light information of photoperiod, via deep brain photoreceptors. Photoinduced thyroid-stimulating hormone, originating from the pars tuberalis of the pituitary gland, prompts the local activation of thyroid hormone in the hypothalamus, which in turn leads to morphological changes in neurons and the secretion of gonadotropin-releasing hormone.