We retrospectively conducted a survey of blood lead levels in Mute Swans (Cygnus olor) admitted to a Royal Society for the Prevention of Cruelty to Animals wildlife centre with injuries caused by collision with power lines or other overhead cables. Of 72 birds admitted over a three-year period, four birds were dead on arrival and 14 were euthanased on arrival due to severe injuries or burns. Of the surviving 54 birds, 43 were sampled to determine blood lead levels and 63% of these had elevated lead levels (>1.21μmol/l). We also blood sampled 260 swans admitted with symptoms consistent with exposure to lead that had not been involved in collisions and 620 birds admitted for all other reasons that had also not been in collisions. There was no significant difference in the proportion of collision birds with low (<1.21μmol/l) or intermediate (>2<5μmol/l) lead levels compared to non-collision birds. However, a significantly larger proportion of birds admitted following collision with power lines had moderately elevated blood lead levels (1.21-2μmol/l) compared to non-collision birds and a significantly smaller proportion had high blood lead levels (>5μmol/l). Adults accounted for 72% of admissions following collision and there was no significant difference between the proportion of collision and non-collision birds that were adults. Our data suggest that birds with elevated but moderate blood lead levels suffer an increase risk of collision, while those with intermediate to high levels have a much reduced risk of collision, possibly because they are too weak to fly. Inexperience does not appear to be a significant factor in determining the risk of collision.
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Vol. 28 • No. 3