Illuminated nights accelerate the migration‐linked phenology in a passerine finch redheaded bunting

The excessive use of artificial light is altering the natural light-dark cycles, consequently impacting animal behavior and physiology. Dim light at night (dLAN) can disrupt migratory patterns, alter hormone levels, and impact breeding success in birds. The present research aims to address the effects of dLAN on the metabolic and reproductive tissues of migratory redheaded bunting (Emberiza bruniceps). For this, buntings under short winter-like days (10L:14D) were exposed to either dark nights (D = 0.00014 W/m²) or dLAN (D = 0.058 W/m²), and their locomotor activity, body mass, fat score, food intake, testicular volume, and plasma testosterone levels were measured. The histological architecture of the muscle, intestine, testis, and liver tissues was assessed. Birds exposed to dark nights confined their activity to the daytime only, whereas the dLAN group showed nocturnal activity and initiated Zugunruhe (nighttime restlessness). The body mass, food intake, fat score, and testicular volume significantly increased under dLAN. Histomorphometry revealed increased muscle width, epithelium thickness, and lumen diameter in the testis, proximal and distal muscularis thickness in the intestine, hepatic lipid droplet size, and decreased proximal villi length and intestinal diameter under dLAN. Further, plasma testosterone levels also increased under dLAN. Our results suggest that dLAN can induce migration-linked phenotypes even under non-stimulatory short days leading to mistimed seasonal activities.