Extra-pair paternity variation in two passerine birds breeding in a gradient of urbanisation

Urbanisation has been increasing worldwide in recent decades, driving environmental change and exerting novel selective pressures on wildlife. Phenotypic differences between urban and rural individuals have been widely documented in several taxa. However, the extent to which urbanisation impacts mating strategies is less known. Here, we inferred extra-pair paternity variation in nestbox-breeding great tits (Parus major) and blue tits (Cyanistes caeruleus) in a gradient of urbanisation in Warsaw, Poland, over 3 breeding seasons. Urbanisation was quantified as the amount of impervious surface area (ISA), light pollution, noise pollution and tree cover within a 100 m radius around each nestbox. We successfully genotyped 1235 great tits and 1306 blue tits with a genotyping-by-sequencing method. Extra-pair paternity was inferred by computing a genomewide relatedness matrix on 9379 SNP markers in great tits and 12958 SNP markers in blue tits. We report higher extra-pair paternity in blue tits breeding in more urbanized areas (e.g. with higher ISA, light pollution and noise pollution, and lower tree cover). However, no such trend was found in great tits. Late-stage survival of individual nestlings in both species was not associated with paternity or urbanisation proxies, thus we were not able to detect fitness benefits or drawbacks of being within-pair or extra-pair offspring in relation to the inferred degree of urbanisation. Our results contribute to the growing body of knowledge reporting on the effects of urbanisation on avian ecology and behaviour and confirm species-specific and population-specific patterns of extra-pair paternity.