The incidence of lead poisoning among whooper and mute swans Cygnus cygnus and C. olor in Scotland

A number of incidents were reported between 1980 and 1986 involving the deaths of substantial numbers of whooper swans Cygnus cygnus in Scotland. Post-mortem examinations identified this to be due to lead poisoning in 47% of these cases. Gizzard contents indicated that most of the lead came from shotgun pellets. In contrast, examinations of mute swans Cygnus olor found dead in Scotland over the same time interval indicate that only 13% died of lead poisoning, and most of these were isolated birds from a wide variety of locations. Mean lead levels in kidney tissue of poisoned whoopers was 223 µg g-1 dry weight compared to 661·9 µg g-1 dry weight in mutes; both these values were considerably lower than the values recorded among poisoned mutes in England. Blood lead levels in whoopers in Iceland had a median value of 31 µg 100 ml-1 compared to 48 µg 100 ml-1 in Scotland, indicating that the lead was being ingested in their wintering grounds in Scotland. The median blood level in Scottish mute swans was 52 µg 100 ml-1. Both of these values from Scotland are high when compared to the ‘maximum acceptable level’ of 40 µg 100 ml-1 suggested by Birkhead (1983). Differences in feeding location, diet, and requirements for grit between whoopers and mutes are considered responsible for different levels of exposure to lead in the environment, and susceptibility to poisoning. Two sites in NE Scotland have been identified as having densities of lead pellets high enough to be potentially dangerous to waterfowl. Both of the sites can be associated with high-intensity shooting and are used frequently by swans for gritting. The relatively low incidence of split shot in poisoned swans in Scotland, compared to the situation in England, is thought to reflect the tackle and techniques of game-fishing rather than coarse fishing in Scotland.