Cuckoldry and sociality: a comparative study of birds.

The frequency of copulations with multiple mates in a sample of 79 bird species was used to test the hypothesis that cuckoldry is a cost of social living to some males. A disproportionately large percentage of females of colonially breeding species is engaged in copulations with multiple males, and the percentage of extrapair copulations also appears to be relatively larger than in solitarily breeding taxa. Males are able to reduce the apparent cuckoldry cost of social living by increasing the frequency of within-pair copulations, particularly among colonially breeding taxa. The frequent copulation paternity guard is not as efficient as mate guarding in preventing cuckoldry, and therefore a positive relationship exists between uncertainty of paternity (as determined by the percentage of extrapair copulations) and the degree of sociality. The frequent copulation strategy appears to be a best-of-a-bad-job strategy adopted by males unable to guard their mates.