Promiscuity is surprisingly common in small birds, and both sexes are known to solicit ‘extra-pair’ copulations – those involving individuals other than their mate. The extent of extra-pair copulation within a species can be determined by taking blood samples from chicks and extracting DNA for a paternity analysis.
Though this has been done for only a small proportion of UK birds, the research so far suggests that the reed bunting may be the most promiscuous – studies have found that extra-pair males sired roughly half of all of the chicks that were reared in study nests.
Extra-pair copulation may enable males to father more chicks than would be possible if they remained monogamous. However, the benefits to females are less clear cut, though the behaviour probably increases their reproductive success too in some way, perhaps because it enables them to secure investment from more males, or because it provides genetic benefits for their offspring.