Extra-pair copulation and sperm competition in the zebra finch

Most birds are monogamous, but recent studies have shown that extra-pair copulations (EPCs) occur frequently^1,2 despite a range of paternity guards, including mate-guarding and frequent copulation¹. Although EPCs are known to result in extra-pair paternity^3-5, there are no previous quantitative estimates of the success of EPCs in fertilizing eggs. We present here estimates of the likelihood of success of extra-pair copulations in a monogamous passerine, the zebra finch Poephila guttata. We show that (1) EPCs occurring under semi-natural conditions in captivity result in extra-pair paternity, (2) sperm from the last male to mate has precedence over previous matings: a single EPC occurring last is disproportionately successful in fertilizing eggs, but EPCs followed by further pair copulations have a low probability of success. These results have important implications for sexual selection theory.