Extra-pair copulations in a predominantly monogamous bird: observations of behaviour

The mating behaviour of indigo buntings, Passerina cyanea was studied. A total of 535 attempted matings in 3 years were observed. Of the 413 copulations that occurred within the female's presumed fertile period (from 12 days before to the day before the last egg was laid), 53 (12·8%) were extrapair copulations (EPCs). The rate of within-pair copulations (WPCs) per pair-hour, EPCs per female-hour and territorial intrusions per h all peaked 3 days before the first egg was laid. Intruders were attracted to copulations, and males attempted to copulate with their mates more after an intrusion. Nearly all intruders and males attempting EPCs were territorial neighbours. More than half of the males attempting EPCs had one or more females nesting on their territories. Although males behaved as if they were guarding their mates from possible EPCs, they often left their females unguarded. Females resisted EPCs significantly more than WPCs, and attempted EPCs were less likely to make cloacal contact than attempted WPCs. However, some forced EPCs did make cloacal contact. The difficulties of estimating the rate of extra-pair fertilizations from observations of copulations are discussed. Finally, in the light of these results the possible role of nesting density, mate guarding, synchrony of breeding, male parental care and polygyny on the rate of EPCs is examined.