The male great skua may be out at sea during the fight and come home to find a new mate waiting for him. Female birds can sit out a battle just as coolly, says ornithologist Clive Minton of Melbourne, Australia. Guys do the home wrecking in species like blue ducks, African black ducks, and song sparrows, reports Bruno J. Ens of the Institute for Forestry and Nature Research in Den Burg, the Netherlands. He, Black, and Sharmila Choudhury of the Wildfowl and Wetlands Trust in Slimbridge, England, have reviewed field studies of birds with long-standing pair bonds. Among house wrens, the males do most of the invading, although females sometimes try to chase away other females and claim their mates.
Oystercatchers are equal-opportunity mate usurpers.
Divorce on the grounds of desertion is even more difficult to observe. The frequency of deserting females does not surprise Dhondt, who compares relative investments in reproduction. To find out who benefits from a divorce, Dhondt has tallied the number of subsequent offspring of divorced male and female blue tits. Female black-capped chickadees tend to dump their mates for males of higher social standing, say Ken Otter of the University of Copenhagen and Laurene M. Ratcliffe of Queen's University at Kingston in Ontario. Older usually means more desirable in both females and males, says Ens.