Paternal care declines with increased opportunity for extra-pair matings in fairy martins

Following the discovery that extra-pair fertilizations are common in many birds, it has been predicted that male participation in parental care may be influenced by their opportunity for extra-pair copulations. However, such a trade-off between male contribution to parental care and the availability of fertile females has not been confirmed. Here we use a novel remote monitoring technique to show that participation in incubation by male fairy martins, Hirundo ariel, declines with the increasing availability of fertile females in the breeding colonies. Furthermore, male contribution to incubation is most responsive to change in the availability of fertile females in the early morning, when most copulations occur, and also if their clutches are smaller than average. Both of these patterns support the presence of a trade-off between parental and extra-pair copulation effort. We suggest that this trade-off may be widespread among the 90% of bird species where males contribute to parental care.