Age-Related Effects in Oystercatchers, Haematopus ostralegus, Feeding on Mussels, Mytilus edulis. II. Aggression

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Abstract
This paper describes the changes in aggressive behaviour that occurred in oystercatchers, Haematopus ostralegus, feeding on mussels. Mytilus edulis, as the birds matured. The behaviour of juveniles (first-year) and immatures (second-to fourth-year) was compared with that of adults feeding in the same place at the same time. Data were collected on the estuary of the River Exe [England, .KAPPA.] from August, as adults and juveniles returned from the breeding grounds, through to the following June. When juveniles first arrived on the Exe, they were very aggressive and attacked other birds more often than did the adults. However, their aggressiveness declined through the winter, both in absolute terms and relative to that of the adults. Young birds then remained less aggressive than adults for several years, when they became adult themselves. When they first arrived, juveniles were only slightly less successful in aggressive encounters than adults. Their success declined during the winter, both absolutely and relative to that of the adults. Young birds then remained less successful than adults for several years. Compared with adults, more of the attacks made by juveniles throughout autumn and winter were attempts to steal mussels found by other birds. However, the juveniles became less successful at stealing as the winter progressed. Compared with adults, fewer of the attacks made against juveniles involved attempts by other birds to steal a mussel. Compared with adults, juveniles in autumn had a high proportion of the mussels they found for themselves stolen from them, but they also stole from other birds a high proportion of the ones they ate themselves. Both proportions declined during the winter, and were similar to the less variable adult values by February. Stealing was uncommon in all age-groups in spring and summer when small mussels were being eaten. It is suggested that the declining aggressiveness and success in encounters of juveniles during autumn and winter was partly responsible for the increased vulnerability to interference while foraging that was recorded over the same period (Goss-Custard and Durell 1987a). Some reasons for the changes in aggressive behaviour are discussed.