Age-Related Effects in Oystercatchers, Haematopus ostralegus, Feeding on Mussels, Mytilus edulis. I. Foraging Efficiency and Interference

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Abstract
(1) This paper examines how foraging efficiency and susceptibility to interference while foraging change with age in young oystercatchers. Haematopus ostralegus, feeding on mussels, Mytilus edulis. To control for changes in behaviour due to changes in the environment (particularly in the size and density of available prey) rather than to changes in the young birds themselves, the foraging 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 one mussel bed on the estuary of the River Exe [England, UK] from August, when adults and juveniles returned from the breeding grounds, through to the following June. (2) The intercept of the curve relating intake rate to bird density (the ''interference curve'') was used as an estimate of foraging efficiency, i.e. the intake rate in the absence of competition. The slope of the curve was used as an estimate of susceptibility to interference, i.e. by how much a unit increase in bird density affected intake rate. Intake rate was measured over a range of bird densities up to a limit of 1000 birds ha-1. (3) In adults, the intercept of the interference curve decreased gradually from August to the following spring and summer, presumably as the food supply deteriorated. The foraging efficiency of juveniles was only 44% that of adults in August, but was similar by February. This suggests that the foraging skill of juveniles that fed consistently on mussels through the winter improved rapidly. Foraging efficiency in juveniles decreased temporarily again in April when inexperienced birds, which had probably fed on other prey during the winter, returned to the bed. The efficiency of immatures in autumn was similar to that of the adults. (4) Adults ate larger mussels than did juveniles in autumn, but thereafter there was little difference. Birds in all age-groups changed from eating large mussels at a slow rate in winter to eating small ones at a fast rate in spring and summer. (5) None of the age-classes suffering interference in August; indeed the juveniles may even have benefited from high bird densities at this time of year because the slope of their interference curve was (non-significantly) positive. Interference increased as the winter approached, particularly in the juveniles. By February, the juveniles were almost twice as badly affected as the adults by interference. Neither the juveniles or the adults experienced interference in spring and summer when they ate small mussels.