Exposure to artificial light at night accelerates but does not override latitude-dependent seasonal reproductive response in dark-eyed juncos

In the modern era of industrialization, illuminated nights have become a common defining feature of human-occupied environments, particularly cities. Artificial light at night (ALAN) imposes several known negative impacts on the neuroendocrine system, metabolism, and seasonal reproduction of species living in the wild. However, we know little about the impact of ALAN on populations of birds that either live year-round in the same location or move to different latitudes across seasons. To test whether ALAN has differing impact on reproductive timing of the bird populations that winter in sympatry but breed at different latitudes, we monitored sedentary and migratory male dark-eyed juncos that were or were not exposed to low intensity (~2.5 ± 0.5 lux) ALAN. All groups were held in common conditions and day length was gradually increased to mimic natural day length changes (NDL). We assessed seasonal reproductive response from initiation to termination of the breeding cycle. As expected based on earlier research, the sedentary birds exhibited earlier gonadal recrudescence and terminated breeding later than the migratory birds. In addition, resident and migrant birds exposed to ALAN initiated gonadal recrudescence earlier and terminated reproduction sooner as compared to their conspecifics experiencing NDL. Importantly, the difference in the reproductive timing of sedentary and migratory populations was maintained even when exposed to ALAN. This variation in the seasonal reproductive timing may likely have a genetic ground or early developmental effects imposed due to different latitude of origin. This study reveals first that latitude-dependent variation in reproductive timing is maintained despite exposure to ALAN, and second that ALAN accelerated reproductive development across both migrants and residents. The results corroborating relationship between latitude, population, and ALAN impact on seasonal reproductive timing, may provide a potential mechanism to test the fitness of a population and its range expansion to exploit urban environment.