A survey of blood lead levels in Mute Swans Cygnus olor.

Following bans on the use of most lead angling weights, the incidence of lead poisoning cases in Mute Swans started to fall and the population started to increase. However, surveys of lead levels in blood of rescued swans continue to show that a high proportion of the birds are carrying levels in excess of 1.21 mol/l. Since rescued swans, although rescued for many different reasons, might be a biased sample, a survey was made of apparently healthy birds living in flocks in the summer. These too showed that a high percentage of the birds in most of the flocks sampled had blood lead levels in excess of 1.21 mol/l. No source of this lead has been identified other than lead fishing weights; these may be long-lost leads, current, but illegally used weights or "dust-shot" which it is still legal to use. Although many of the birds sampled are carrying lead burdens that are probably not harmful, others are seriously affected. Except in the most serious cases, it is not possible to recognise birds with elevated lead levels without taking a blood sample for assay.