Extra-Pair Parentage and Alternative Reproductive Strategies in Dutch Barn Swallow Hirundo rustica Populations

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Abstract
Barn Swallows are known to use alternative reproductive strategies but resulting levels of extra-pair parentage differ widely between studies. In this one-year pilot study (2021) we examined extra-pair parentage in 50 broods across four Barn Swallow populations in The Netherlands, with a focus on whether these levels complicate the estimation of individual (lifetime) reproductive success based on field observations of breeding and provisioning behaviour. We found that 31.3% of nestlings from 56.0% of broods were of extra-pair origin. Of these nestlings, 94.9% were related to the social female but sired by an extra-pair male, while in 0.5% of cases the opposite occurred, involving an extra-pair female. Another 4.6% of nestlings were unrelated to either social parent, indicating ‘egg dumping’. Based on these results we found that visual observations overestimated the average reproductive output of cuckolded males by as much as 48%. Additionally, we observed that extra-pair offspring were not randomly distributed within broods, and there were no consistent differences between breeding locations. The complexity of alternative mating systems is further illustrated by the observation of a polygynous male with offspring in broods of four different females, and broods of two females that each contained offspring from three different males; while their social males may also engage in extra-pair copulations themselves. Overall, we conclude that an accurate assessment of individual reproductive success requires full-scale genetic sampling of local breeding populations.

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