Lead poisoning and other causes of mortality of 115 trumpeter (Cygnus buccinator) and 21 tundra (C. columbianus) swans from northwestern Washington (USA) from 1986 to 1992 are reported. Necropsies were performed on all 136 swans, liver lead analysis conducted on 110, and differentiation between lead and steel shot pellets recovered from gizzards in 97 swans. Shot pellets were detected in 44 (32%) of 136 gizzards. Lead shot was recovered from 32 (33%) of 97 gizzards and steel shot from 16 (16%). Mean intensity of lead shot in gizzards was nearly five times greater than steel shot. Thirty-nine (35%) of 110 livers had lead concentrations diagnostic of lead poisoning (>6 ppm, wet weight). Mean (±SE) weight for 61 non-lead poisoned trumpeter swans was 9.8 (±0.30) kg, significantly heavier (P < 0.05) than 30 lead poisoned trumpeters (x̄ = 6.8 ± 0.23 kg). There was no significant difference (P > 0.05) in weights between lead poisoned (n = 9) and non-lead poisoned (n = 12) tundra swans. Lead poisoning was the primary cause of death, accounting for 29% of the mortalities. Other causes of mortality identified were aspergillosis (17%), illegally shot (11%), and other traumatic factors (12%). The cause of death for 43 swans was not determined. Lead poisoning from the ingestion of lead shot continues to be a principal cause of mortality in swans overwintering in northwestern Washington.
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