Fitness consequences of divorce in the oystercatcher, Haematopus ostralegus

We investigated the fitness consequences of divorce in oystercatchers. We made a distinction between two types of divorce: in desertions the disruption of the pair bond is initiated by one of the pair members, and in usurpations by a conspecific individual. Survival and reproduction prospects for oystercatchers are largely determined by their social status (nonbreeder or breeding bird at a site of a specific quality). Changes in social status in relation to divorce showed that birds taking the initiative to leave their mate increased in fitness, relative to birds that were forced to leave their partner. Status of individuals that remained in their territory after divorce was unaffected if their mate was expelled, but declined if their mate deserted. Survival after divorce was significantly lower for birds that were expelled than for those deserting. Divorce rate, and especially desertion rate, was higher among occupants of lowthan high-quality territories. In general, divorce rate increased following elevated mortality. In high-quality territories usurpations increased with increasing breeder mortality, but at low-quality territories this relation was absent. Desertion rates were similarly related to mortality in both territory types. Divorce participants thus differed strongly in their fitness prospects, depending on the type of divorce, the role played in the divorce and the quality of the territory where divorce took place. Studies that do not observe the birds during divorce cannot determine the type of divorce and the role played by the individuals, and this may lead to misleading conclusions on the costs and benefits of divorce.