Data from: The potential for climate change to intensify nest site competition between two sympatric owl species

Interspecific competition profoundly influences the ecology of species and climate change is expected to alter the strength of interspecific interactions. Using long-term data from sympatric breeding populations of the western barn owl (<i>Tyto alba</i>; hereafter barn owl) and the tawny owl (<i>Strix aluco</i>) in southern Hungary, we explored the associations between the owl species’ (i) breeding phenology (annual median laying dates) and regional weather components (daily precipitation, temperature minimum and maximum), (ii) temporal trends of median laying dates and the associated weather signals and (iii) reproductive output (laying date, clutch size, fledgling number) and the co-occurence of the two species in the same nestbox during the same breeding season. In the barn owl, breeding onset was negatively associated with daily temperature maximum, and advanced by two weeks in the study period, while the tawny owl breeding onset did not change. We found that when the two species used the same nestbox, the breeding of barn owls (occurring after the tawny owl breeding) was delayed by a month, and they produced one more egg and owlet on average, but second clutches were practically absent, compared to cases when no interaction occurred in the same nestbox during the same breeding season. In contrast, when nesting in boxes later occuppied by barn owls, the tawny owl's breeding started a few days earlier with an increased clutch size, although with no difference in the number of fledglings. Our results suggest that climate change could heighten competition for nest sites between the two owl species, as the barn owl's breeding season has shifted closer to the breeding season of the tawny owl through the study period in parallel with the temperature rising.