Age-Related Effects in Oystercatchers, Haematopus ostralegus, Feeding on Mussels, Mytilus edulis. III. The Effect of Interference on Overall Intake Rate

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Abstract
This paper examines the effect that interference from other birds had on the average intake rate of young oystercatchers, Haematopus ostralegus, feeding on mussels, Mytilus edulis, in winter. Previous parts (Goss-Custard and Durell 1987a,b) showed that juveniles became more efficient at foraging during autumn and winter but, at the same time, they became less aggressive and successful in encounters. As a result, they were increasingly prone to interference as the winter progressed. This paper examines how much of an effect this reduction in intake rate at high bird densities had on their ability to feed in winter. The number of juveniles using the mussel bed increased from August to October, but decreased by over half from October to November as birds moved during low tide to other habitats, including nearby grass fields. Thereafter, there were more juveniles on the mussel bed on cold days when the earthworms in the fields were more difficult to catch. The intake rate of both juveniles and adults feeding on mussels in winter decreased as bird density increased. In juveniles, the decrease began at lower densities of birds. The density of foraging birds in the parts of the mussel bed where juveniles fed was highest during the first and last fifths of the highly variable exposure period. Bird density was highest on neap tides and lowest on spring tides at all stages of the tidal cycle. The potential maximum rate at which a bird could feed was assumed to be set by its intake rate in the absence of competitors. In winter, bird density was high enough on neap tides for interference to depress the average intake rate of juveniles through the tidal cycle to 59% of their potential maximum. The corresponding values for mid and spring tides were 74% an 82%, respectively. The intake rate of adults was depressed on neap tides to 64% of their potential maximum. But, because adults were less prone to interference than juveniles, their intake rate was rather less affected on mid tides (86%) and spring tides (91%). The findings are discussed in relation to the overwinter ecology and survival of juvenile oystercatchers. It is suggested that interference was sufficiently important in young oystercatchers in particular to make their mortality rate dependent on population size.

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