Associated with a continued global increase in urbanization, anthropogenic light pollution is an important problem. However, our understanding of the ecological consequences of light pollution is limited. We investigated effects of artificial night lighting on dawn song in five common forest-breeding songbirds. We used automated recording devices set up in twelve locations in the forest: 6 devices were placed in territories affected by streetlamps, 6 devices were in territories without artificial light. We recorded the dawn chorus on all territories on 19 consecutive mornings. We also compared reproductive behaviour of blue tits (Cyanistes caeruleus) breeding in edge territories with and without street lights to that in central territories over a 7-year period.
In four out of five species that were recorded daily, males near street lights started singing significantly earlier at dawn than males elsewhere in the forest, and this effect was stronger in naturally earlier singing species. Under the influence of street lights female blue tits started egg-laying on average 1.5 days earlier. Males occupying edge territories with street lights were twice as successful in obtaining extra-pair mates than their close neighbours or than males occupying central forest territories. Artificial night lighting affected both age classes, but had a stronger effect on yearling males. Our findings indicate that light pollution has substantial effects on the timing of reproductive behaviour and on individual mating patterns. It may have important evolutionary consequences by changing the information embedded in previously reliable quality indicator traits.