Artificial Light at Night Consistently Impacts Avian Physiology and Behaviour: A Meta-Analysis.

Artificial light at night (ALAN) is a major anthropogenic pressure across the tree of life. While ALAN drives population declines in insects and reptiles, birds defy simple patterns. Avian responses to ALAN vary in direction and magnitude, yet the fitness consequences of these responses remain unresolved. We conducted a meta-analysis to test whether ALAN alters avian physiology, behaviour and life-histories, analysing 675 effect sizes from 36 studies across 30 species. We found consistent physiological and behavioural shifts under ALAN, while life-history traits were unaffected. ALAN disrupted physiology through reduced sleep, higher metabolic rate and accelerated reproductive maturation. Behaviourally, daily activity was extended, with earlier onset, later offset, and increased nocturnal activity and foraging effort. Subset analyses revealed stronger ALAN effects in migratory species compared to residents and more pronounced in adults and females than in nestlings and males. Activity shifts, accelerated ageing and sleep disruption were amplified with brighter light. Birds appeared to buffer ALAN effects through physiological and behavioural adjustments, minimising impacts on life-histories, a paradox likely explained by phenotypic plasticity or evolutionary adaptation. We identified key ecological vulnerabilities, emphasising the need for targeted conservation strategies in our increasingly illuminated world.