Extra‐pair paternity in the shag, <i>Phalacrocorax aristotelis</i> as determined by DNA fingerprinting

Abstract

The frequency of chicks resulting from extra‐pair copulation in the shag, Phalacrocorax aristotelis , was measured by DNA fingerprinting. DNA fingerprints were taken from both sexes of 15 pairs and their chicks (28) in a subcolony on the Isle of May, UK. It was found that 18% of the chicks had extra‐pair paternity, and one chick (3.5%) was not the offspring of either member of the pair, implying either a polygynous male whose second female was fertilized by another male or adoption. Although no observations of courtship and copulation were made in the same season, observations in a previous year on the same colony of shags showed that 14.1% of the copulations by males were not with the female with whom that male reared young. The similarities and differences are discussed between these results on the level of extra‐pair copulations and of extra‐pair paternity and those of other studies where both observations of extra‐pair copulations and various measures of the degree of extra‐pair paternity have been made.