POPULATION-DYNAMICS OF 2 SPECIES OF INTESTINAL HELMINTH IN OYSTERCATCHERS (HAEMATOPUS-OSTRALEGUS)

The cestode Micrasomacanthus rectacantha and the trematode Psilostomun brevicolle are the two most common intestinal helminths of oystercatc hers, Haematopus ostralegus, on the Exe estuary, England. Each was pre sent in 57 of 60 birds, in numbers ranging from 1 to 9833 and from 1 t o 182, respectively. Cockles (Cerasroderma edule) were used as paraten ic host by M. rectacantha and as second intermediate host by P. brevic olle. Over 90% of the transmission of M. rectacantha from intermediate hosts occurred in winter, between cockles and juvenile birds. Exposur e to M. rectacantha was highest in cockles from muddy sites, where up to 80% of older cockles were infected with 1-14 larvae. Exposure to P. brevicolle was similarly high at one sandy site. Juveniles also made up >90% of the birds present on the estuary in summer, when significan tly more worms were gravid. Estimates based on the numbers of uterine eggs per individual P. brevicolle showed that 8% of the total Exe popu lation of oystercatchers (ca. 200) contributed 51% of the eggs to the estuary. The results implicate host age, host feeding preference, and host population structure as important factors determining variation i n helminth abundance in birds.