Meire P.M. (1996) 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? ARDEA 84 (A): 525-538

A storm surge barrier and secondary dams were built in the Oosterschelde estuary (The Netherlands) resulting in a 30% decrease in intertidal area. If the birds that previously fed behind the secondary dams were able to establish them¬selves in the intertidal areas outside that remained, their densities should have increased in these areas. To test this, the densities of Oystercatchers, Haematopus ostralegus, were studied in relation to their main prey, Cockles Cerastoderma edule, and Mussels Mytilus edulis, before and after the reduction in tidal area. As the tide ebbed Oystercatchers moved quickly through the higher part of the intertidal area towards the preferred feeding areas below mid tidal 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 the less preferred ones, leading to a sequential pattern of filling of the 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 pre¬dicted by prey biomass-bird density relation-ships that had been mea¬sured before the environmental changes. On mussel beds, however, densities of Oyster¬catchers became much higher after 1987/1988 at a given total prey biomass. 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 the 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 Cockles and Mussels supports higher densities of Oystercatcher as the proportion that consists of Cockles increases.