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. In four species, 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. We compared reproductive behavior of blue tits breeding in edge territories with and without street lights to that of blue tits breeding in central territories over a 7 year period. Under the influence of street lights, females 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 neighbors 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 behavior and on individual mating patterns. It may have important evolutionary consequences by changing the information embedded in previously reliable quality-indicator traits.

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In several bird species, the period around dawn seems important for extrapair behavior. For example, a study on great tits (Parus major) showed that females that emerged earlier from their roosting place during the peak of their fertile period were more likely to have extrapair young in their brood. We investigated the potential effect of female emergence times on extrapair behavior in the blue tit (Cyanistes caeruleus). First, we tested the relationship between natural female emergence times from the nest-box and the presence or frequency of extrapair offspring in the brood, using 4 years of data. Females progressively emerged earlier from the nest-box as egg laying approached, with the earliest emergence 2 days before the start of laying. However, we found no relationship between female emergence time and the occurrence of extrapair young in the brood. Secondly, in 2 breeding seasons, we experimentally advanced female emergence times by supplying the roosting females with additional light in the early morning. Although the experiment had inconsistent effects on the occurrence of extrapair young in the brood, we found no evidence that female emergence time during peak fertility is directly linked to extrapair paternity. Interestingly, females exposed to artificial light were more likely to return to breed in the next year.

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