AS the US writer Nora Ephron wrote in the movie Heartburn, ‘If you’re looking for monogamy, you’d better marry a swan’. Some species of bird do, indeed, pair for life, yet other species—including most songbirds—are ‘socially monogamous’. This means that, although they will form a pair for the entire breeding season, to help one another raise their young, the male (and, often, also the female) may also sneak off and try to mate with other birds.
In the male’s case, this strategy is likely to mean he will have more offspring—even if he might never see some of them. For the female, mating with two or three males does not increase the number of chicks she can have, as she will still lay the same number of eggs. However, by mating with several males, she does benefit from having a range of young with different genetic qualities inherited from their fathers, some of which
