The effect of an experimentally created mussel bed on bird densities and food intake of the Oystercatcher Haematopus ostralegus

When an experimental mussel bed was created in 1987 on the mudflats south of Schiermonnikoog, Herring Gulls immediately increased in numbers, consuming starfish and damaged Mussels. The build-up of oyster catcher numbers was more gradual, but persisted for longer. Most Oystercatchers attracted to the mussel bed fed on Mussels. The proportion of Oystercatchers that hammered Mussels was relatively high, which may have been due to the relatively thin shells of the Mussels fished from the subtidal. Intake rates on the experimental mussel bed were on average higher than those achieved by birds feeding on other prey (mainly Macoma balthica and Nel-eis diversicolor), but the difference did not reach statistical significance. As the feeding density on the mussel bed increased, intake rates dropped, but it is not known if this relationship has a causal basis. Previous suggestions that the hammering birds substantially depleted the hammerable Mussels seem less likely in the light of our current analysis. The discussion stresses that the experimental creation of mussel beds is a powerful technique to test in the field distribution models derived from ideal free theory and investigates what lessons can be learned from the current pilot experiment that may be of use in the design elf future experiments.