DISTRIBUTION OF OYSTERCATCHERS HAEMATOPUS-OSTRALEGUS OVER A TIDAL FLAT IN RELATION TO THEIR MAIN PREY SPECIES, COCKLES CERASTODERMA-EDULE AND MUSSELS MYTILUS-EDULIS - DID IT CHANGE AFTER A SUBSTANTIAL HABITAT LOSS

A storm surge barrier and secondary darns were built in the Oostersche lde estuary (The Netherlands) resulting in a 30% decrease in intertida l area. If the birds that previously fed behind the secondary dams wer e able to establish themselves in the intertidal areas outside that re mained, their densities should have increased in these areas. To test this, the densities of Oystercatchers, Haematopus ostralegus, were stu died in relation to their main prey, Cockles Cerastoderma edule, and M ussels Mytilus edulis, before and after the reduction in tidal area. A s the tide ebbed Oystercatchers moved quickly through the higher part of the intertidal area towards the preferred feeding areas below mid t idal level. The distribution of the birds then became related to their food supply. As the number of birds in the whole area increased, the birds gradually spread out from the preferred feeding plots towards th e less preferred ones, leading to a sequential pattern of filling of t he feeding area. This pattern did not change after the dams were built . After the main loss of intertidal habitat had occurred, densities of Oystercatchers feeding on Cockles were within the range predicted by prey biomass-bird density relationships that had been measured before the environ mental changes. On mussel beds, however, densities of Oyst ercatchers became much higher after 1987/1988 at a given total prey bi omass. This was caused by an increase in cockle biomass on the mussel beds due to an abundant spatfall in 1985. It is argued that because th e harvestable fraction of Mussels represent only a small proportion of the total mussel population on the bed, whereas a large proportion of the cockle population is harvestable, a given combined biomass of Coc kles and Mussels supports higher densities of Oystercatcher as the pro portion that consists of Cockles increases.