Lead poisoning in swans Cygnus olor

French, Michael C (1991). Lead poisoning in swans Cygnus olor. PhD thesis The Open University.


This study examines some aspects of lead poisoning in the Mute Swan (Cygnus olor). In 4 years I examined 860 carcasses from a study area in East Anglia. Seventy-four per cent of the birds concerned had died of lead poisoning, from ingested fishing weights. Annual aerial surveys showed that the number of live birds in the study area declined by 22% during the 4 year period. When 23 carcasses were re-introduced to their original locations, 5 were re-notified and 16 disappeared without trace. A comparison of gizzard grit size between a group of lead-poisoned birds and a similar number of birds, which had died of other causes, indicated that lead weights, when available are ingested by choice. Results from an environmental survey showed that without ingesting fishing weights swans carry a light burden of lead, resulting from contamination by airborne lead, probably originating from petrol-engined motor vehicles.
An analysis using the bodies of lead-poisoned swans, found freshly dead in the field, showed that lead can be excreted via the bile and lower gut.
Controlled experiments using pigeons (Columba livia) and mallard (Anasplatyrhynchos) showed that:- (1) tissue lead and zinc values monitored over a period of one year became elevated due to changes in physiology during moult; and (2) changes in calcium metabolism during eggshell formation had no detectable effect on blood lead values. Results indicated that in pigeons most of the calcium used in the formation of eggshells is derived directly from the diet, rather than from bone reserves.
As a treatment for lead poisoning, daily injections of thiamin were administered to lead-poisoned mallard. No detectable effect on the progress of the intoxication was found.
Tissue lead values were lower in lead-dosed ducks receiving a diet supplemented with phytic acid than in a similar group receiving no supplementation.
Pigeons dosed with lead scored more "fails" during a simple performance test which measured landing ability than did a control group of birds.
Zinc was tested as a possible fishing weight substitute and, at the levels administered, was found to be non-toxic to mallard. Combined dosing of lead and zinc showed that zinc had a demonstratable protective action against lead toxicity.

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