Timing of copulations and the pattern of paternity in relation to laying order in tree swallows Tachycineta bicolor

It is well established that extra‐pair young are present to varying degrees in nests of most avian species. However, the timing of extra‐pair copulations during a female's fertile cycle is not well known, nor is how these copulations translate into the distribution of extra‐pair young within the laying sequence. We determined the rates of successful within‐pair copulations where at least one cloacal contact was achieved, and the number of extra‐pair copulations/attempts in 17 pairs of tree swallows Tachycineta bicolor during the mornings encompassing the female's fertile period. We also examined nestling paternity with respect to laying order in eighteen mixed‐paternity tree swallow broods. The rate of successful within‐pair copulations peaked three days prior to a female laying her first egg, and decreased steadily throughout the laying period. We therefore predicted that extra‐pair young would be more likely to hatch out of later‐laid than earlier‐laid eggs within a female's brood. We found no evidence of an increased frequency of extra‐pair young with laying order. Instead, extra‐pair young appear to be randomly distributed within the laying sequence of tree swallows.