Levels of polychlorinated biphenlyls, organochlorine pesticides, mercury and lead in relation to shell thickness in marsh harrier (Circus aeruginosus) eggs from Charente-Maritime, France

The resident breeding population of marsh harriers at lʹı̂le dʹOleron, Charente-Maritime, France, has declined by more than 60% over the last decade. Breeding success in this population is lower than reported for other populations of marsh harriers. This paper presents the results of an analysis of contaminants and shell thickness from 13 unhatched eggs from the wild population, and 1 egg from a captive bird. DDE (a principal metabolite of dichloro-diphenyl-trichloro-ethane—DDT) and mercury (Hg) concentrations were below concentrations considered to influence breeding success, and egg lead concentrations were very low. There was a high degree of inter-correlation between contaminants, and egg concentrations of mercury and several polychlorinated biphenyl (PCB) congeners (but not DDE) were significantly negatively correlated with shell thickness. Total PCB concentrations in 3 of 13 eggs (23%), i.e. one or more eggs from 2 of 11 different clutches (18%), were within the range (8–25 ppm wet weight) thought to influence breeding success in a range of bird species. We consider that further and more detailed investigation of the extent to which this population is exposed to PCBs is required.