Irene Di Lecce, Charles Perrier, Marta Szulkin, Joanna Sudyka

Most of what is known about extra-pair paternity in hole-nesting birds derives from studies using artificial nesting sites, such as nestboxes. However, it has rarely been investigated whether inference drawn from breeding events taking place in nestboxes matches what would be observed under natural conditions, i.e. in natural cavities. We here report on a variation in promiscuity in blue tits and great tits nesting in natural cavities and nestboxes in an urban forest in Warsaw, Poland. Specifically, we tested whether breeding density, breeding synchrony and extra-pair paternity (inferred from SNP data generated with a high-throughput genotyping by sequencing method) differed between birds nesting in natural cavities and nestboxes. In both blue and great tits, the frequency of extra-pair paternity was similar between the two cavity types. Breeding density and synchrony was higher in nestboxes than in natural cavities in blue tits, but not in great tits. Moreover, the occurrence of extra-pair paternity in the nest in great tits increased with the number of neighbours and the number of synchronously breeding neighbours within a 120 m radius from the focal nest, regardless of cavity type. In blue tits, the proportion of extra-pair offspring per nest increased with neighbour density within 160 m of the focal nest, both in natural cavities and nestboxes. Our results indicate that providing nestboxes can alter the distribution of individuals in space and time, which potentially influences mating outcomes among breeders outside the social bond. Whilst these parameters should be carefully considered when comparing studies and/or sites, we highlight that conclusions drawn from nestbox studies on natural variation in extra-pair matings might be adequate in some species, and/or sites, but not in others.