The ecological importance of natural dark nights for circadian organization, metabolism and reproduction in birds

The ability to assess and utilise the length of the daily light period has been identified as a key component of the physiological ecology regulating metabolism and reproduction across various vertebrate taxa. While light-dependent responses have been extensively studied, the ecological and physiological significance of natural darkness remains relatively less explicitly emphasized. In fact, studies investigating the role of light in controlling biological processes underscore that the period of darkness is an integral component of the light-dark cycle and an important ecological dimension in shaping the biology of organisms. Therefore, drawing largely from artificial light at night (ALAN) studies, this review synthesizes current evidence on how disruption of light-dark cycles, particularly the darkness at night, perturbs the circadian organization, with cascading effects on metabolism and reproduction, and how metabolism acts as a physiological gatekeeper of reproduction in diurnal vertebrates. Here, much of the evidence that we discuss is from both laboratory and field studies on diurnal birds. Importantly, we underscore the critical ecological value of the natural darkness within the 24-h day, emphasizing that the preservation of the night environment is crucial for maintaining the bioenergetic balance and population viability of species in an overly lit, urbanized environment.