Abstract

Parasite loads vary among feeding sites in European oystercatchers, Haematopus ostralegus, hibernating in the Exe Estuary, in Great Britain. This variation might be the result of active or passive selection of non-parasitised mussels (Mytilus edulis) because they are the preferred food of the oystercatcher in the area and are also the intermediate hosts of one of the main parasites of the bird. Parasite loads (Renicola sp. and Psilostomum brevicolle) of the mussels were studied in relation to morphological parameters known to influence the choice of prey by the oystercatcher: length of mussel, meat content, and thickness of shell. Smaller mussels generally carry the lightest parasite loads, but the number of metacercariae of the two parasites potentially consumed in a day varies with feeding site and mussel size. Thus, an oystercatcher will ingest fewer metacercariae of Renicola sp. and P. brevicolle by selecting smaller mussels from bank 1, which is one of the least visited sites in the estuary. However, the birds will have to select mussels of 37 mm or more on banks 30 and 31, which are among the most used sites. We suggest that there is no active selection of non-parasitised mussels, but rather a kind of passive selection as the mussels chosen for their energetic value happen to be also the least parasitised.