Data from: Artificial light at night (ALAN) negatively affects the nest site occupancy but does not influence breeding success in two sympatric owl species

Light pollution affects negatively a broad range of animal species, with severe implications for conservation policy development and strategic planning globally. Birds are one of the most widely used ecological indicator groups in monitoring environmental changes. However, most studies examining the effects of light pollution are focused on diurnal bird species. It would therefore be necessary to study in more detail these effects on species with at least crepuscular or nocturnal animals, since they may be more vulnerable. We investigated the effects of illumination on presence of breeding of the barn owl (Tyto alba) and tawny owl (Strix aluco) in illuminated vs. unilluminated towers and the reproductive output in nest boxes in church towers by comparing the numbers of eggs and chicks fledged. We found the breeding presence was reduced in illuminated towers in both species but no difference in reproduction parameters for either of the species. Our results underscores that light pollution has a negative consequence on the reproduction of barn owls and tawny owls due to reducing breeding site suitability. This raises the threat that artificial light at night may hinder the conservation of such nocturnal bird species whose reproduction may be increasingly connected to human settlements in the future.