ENS BJ (1994) The career decisions of the Oystercatcher Haematopus ostralegus. LIMOSA 67 (2): 53-68.

In this paper it is attempted to summarize major findings and insights gained during a seven year study of the social behaviour of the Oystercatcher (Ens 1992). My study has concentrated on the career decisions, which relate to access to scarce resources vital for reproduction and survival, to be distinguished from reproductive decisions, which deal with the various ways these resources may be allocated. Costs and benefits of career decisions are measured in terms of expected number of chicks fledged over a lifetime, and are strongly influenced by the intensity of social competition. Establishment of a territory stands out as the major career decision in the life of an Oystercatcher. Competition for space of high quality is intense and only few Oystercatchers settle in high quality territories. That so many Oystercatchers settle in territories of poor quality is probably due to the large number of nonbreeders queuing for territories of high quality. Thus, prospecting nonbreeders are faced with a dilemma: settling immediately in a territory of poor quality, or waiting a long time for a high quality territory with the associated loss of potential breeding years and the risk of premature death. Queues arise because dominance relationships are sitedependent. Formation of the pair bond is treated as a second career decision, though the observation that male and female cooperate in territorial defence suggests this is not justified. Divorces are rare but may occur when one of the partners has the option to switch to a mate of higher quality at low cost, for instance when a neighbouring rival has died in winter. Extra-pair copulations, where a male or a female copulates with a bird other than its mate, are interpreted as attempts to locate available mates of higher quality. The general insight to emerge from the study is that individual Oystercatchers should be thought of as prisoners of their historically acquired local social relationships. Though the birds are physically free to move, the costs of moving territory or changing partner (i.e., the costs of establishing new social relationships and foregoing the benefits of the current social order) will usually not outweigh the benefits associated with the new social position. Fundamental as this insight may be, the problem is only partly solved, as my theorizing did not include the reverse effects of behaviour on the society of the Oystercatcher. Yet, I am convinced that in order to understand how populations are linked to their resource base, it is necessary that both population biologists and behavioural biologists become more fully aware of the two-way links between a society (i.e., a population the social organisation of which is not ignored) and the social behaviour of the individuals comprising it (fig. 13). All animals that build up social relationships will become prisoners of those relationships Eurasian Oystercatcher Haematopus ostralegus

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