Light and Environment: Regulation of Seasonal Reproduction in Wild Birds

Reproductive seasonality in birds represents a key ecological adaptation that ensures synchronization between breeding activity and optimal environmental conditions for offspring survival and development. Photoperiod is the primary cue regulating the hypothalamic–pituitary–gonadal (HPG) axis, through brain photoreceptors and pineal melatonin secretion. Increasing day length induces thyroid hormone activation by hypothalamic type 2 deiodinase (DIO2), stimulates gonadotropin-releasing hormone (GnRH) secretion, and promotes gonadal growth, whereas prolonged exposure to long days triggers photorefractoriness, which has been linked to increased hypothalamic type 3 deiodinase (DIO3) expression in several studies, although the causal role of this enzyme remains under investigation. Secondary environmental modulators, such as temperature, food supply, precipitation, and social interactions, also play crucial roles in fine-tuning reproductive timing. Moreover, anthropogenic factors like artificial light at night can disrupt circadian and seasonal regulation, causing mismatches between breeding and food availability. Evidence from diverse species, including passerines, galliforms, waterfowl, and raptors, demonstrates both conserved mechanisms and ecological plasticity, with tropical and urban species showing more opportunistic breeding strategies. These findings highlight the multifactorial and flexible nature of avian reproductive cycles, underlining their vulnerability to climate change and habitat anthropization. Considering this, this review aimed to understand the neuroendocrine and environmental control of seasonality and to offer an integrative perspective on how light, hormones, and environmental factors interact to shape seasonal reproduction in wild birds.