Lead Toxicity in Mute Swans

mammals' (Finley et al. 1976). This relates to the use of the gizzard which
breaks down the food by abrasion with grit and small stones which have
been collected by the bird. Lead shot is dissolved in the gizzard by
digestive enzymes, leading to the absorption of lead salts across the
gizzard wall and the intestine. In North America there have been many
'die-offs' of waterfowl attributed to lead poisoning (Bellrose 1959; Hawkins
1965; Anderson 1975), and lead pellets have been found in 19
species of waterfowl. The large numbers of waterfowl dying in each of the
major migratory flyways ( 4% in the Mississippi flyway and 2-3% in the
other fiyWays (Bellrose 1959) clearly highlight the importance of lead as a
mortality factor. In 1973, 21 out of 47 states in the USA responding to a
questionnaire indicated that poisoning from ingested lead occurred in
birds in their state. Fifteen states reported acute poisoning from lead shot
(Thomas 1980). Research in at least eight countries (Canada, Denmark,
Federal Republic of Germany, the Netherlands, Sweden, Switzerland,
United States and the United Kingdom) has shown that lead poisoning
results from wildfowl or clay-pigeon shooting. In Britain, Owen and
Cadbury (1975) have shown that at least 37 (29%) of 128 swans [Cygnus
olor (Gmelin), C.c. cygnus (Linnaeus) and C.c. bewickii Yarrell] found
dead at the Ouse Washes between 1969 ·~nd 1975 died from lead
poisoning. Hunt ( 1977) reported that lead poisoning was responsible for
the deaths of 107 (52%) of 206 C. olor from rivers, lakes and gravel pits
in the English midlands. The birds died from ingesting anglers' split lead
shot and fishing weights. In 1981, the Nature Conservancy Council
reported that between 3000-4000 C. olor were dying annually from
ingestion of angling lead (N C C 1981).
In Ireland, the first published fatality due to lead in mute swans was in
1983 at Cork City Lough (O'Halloran and Duggan 1984). Since then,
further records have been made available to us. Two shelduck, Tadorna
tadorna (L.), died at the Cork City Lough in 1962. Veterinary records of
this incident revealed-ingested lead as the cause of mortality (D.Twomey,
pers. comm). In 1980, 21 mute swans died of lead poisoning on the
southern shore of Lough Neagh, Co. Down, the location of a clay pigeon
shooting site (R. Davidson, pers.comm.).
This paper reports on current Irish work on lead poisoning of swans,
examines relevant fishing practice and considers possible legislation.
CURRENT RESEARCH
Current research aims to establish the occurrence and extent of elevated
lead levels and causes of mortality in Irish mute swans. Blood
samples are collected from swans and methods of sampling and analysis
follow those published elsewhere (O'Halloran and Duggan 1984). In
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