INFLUENCE OF SPECIES, AGE AND DIET ON MERCURY CONCENTRATIONS IN SHETLAND SEABIRDS

Chick down, chick feathers and feathers from adults of 5 seabird speci es (Arctic skua Stercorarius parasiticus, great skua Catharacta skua, Arctic tern Sterna paradisaea, kittiwake Rissa tridactyla, and common guillemot Uria aalge) were analysed for mercury. Individual female Arc tic and great skuas' body feather mercury concentrations correlated wi th concentrations in their chicks' down, but not feathers (Arctic skua : r = 0.64; great skua: r = 0.66). This demonstrated that mercury in c hick down originated from the egg, and that mercury in the egg and in adult females' plumage could have the same dietary source. Inter-speci fic differences in mercury concentrations were found for all age class es sampled, and these could be explained partly in terms of dietary sp ecialisation, although physiological variations may also be important. All 3 age classes of great skua showed a direct increase in mercury w ith increasing proportion of bird meat in the diet of individual pairs . In kittiwake, Arctic skua and great skua, adults had higher mercury concentrations than chicks and chick down had higher concentrations th an chick feathers. However, in 2 species (Arctic terns and guillemots) chick down had higher concentrations than adult feathers. Chick down could be sampled for mercury content as an alternative to using eggs i n national biomonitoring schemes. Feathered chicks could be sampled to determine mercury availability around the breeding colony between hat ching and fledging.