Lead Toxicity in Mute Swans

for obvious reasons, the visits of the adult birds are nee ·1 b · f
· Id · essart y ne
This wou !11ean, tn consequence, that exposure of sub-adult birds·
to t~e tdm1ttedly a~ _Yethundefined lead source at The Lough i~
re 1 at1ve y gre .at~r an . 1s t us reflected in the blood picture.
Although 1t 1s unlikely that the source of increased lead th
· L · · was e
River ee - s.ince 1t would be washed away by the swift river flow
- g.rab samplmg and dredging has failed to confirm that the birds
obtained lead at The ~ough. The few lead weights found there and
.zero trace lead levels m .the wate: are _ . pe~plexing and may reflect the
madequac~ of t~e sampling techniques. Further research is obviously
necessary tn this area .. Ne:ertheless, preliminary results from grab
sampl.es at ~he Lough indicate that the sub-littoral is silty and has
very little gnt. Thus any spent lead or lead shot having fallen into the
"":ater or bee.n caught ~n the vegetation would soon be picked up by
blfds searching for gnt. ·
The cygnets at The Lough, as expected, had low levels in their
blood (8 .28-29.80µg/100mls) compared to adult or sub-adult birds.
However, cygnets contaminated at The Lough had demonstratively
higher levels than those of adults at 'country sites'. The only cygnet
value lower (8.28µg/100mls) was from a very young bird -
introduced into The Lough from Kanturk - where its parents had
been killed . Note that the other cygnet in the sample had hatched at
The Lough.
However, blood lead levels are known to fluctuate and it has been
suggested that this may be due to temporal variations in exposure.
(Birkhead 1983). Indeed the degree· of exposure as indicated by the
blood picture is not now regarded as a totally reliable correlation as
the elevated blood lead levels following an acute exposure are not
necessarily maintained for more than seven to ten days.
A good example of this problem was noted in The Lough sample.
The very first bird examined on 4th October 1983 had a blood lead
level of 354µg/ 1 OOmls. Some three weeks later this elevated level had
more than halved (31st October 1983 the blood lead level was
104µg/1 OOmls). Comparable variation in blood lead levels has also
been noted by Birkhead (1983). Because of the interpretation
difficulties which these fluctuations cause, it has been recommended
that further assessment of the damage caused by high lead levels be
obtained by measuring zinc protoporphygrin in the eryth~ocyte (Bush
et al 1982). This is because one of the susceptible enzymes is
ferrochelatase which catalyses the conversion of protoporphrin into
haem (Lee 1981). This inhibition of protoporphyrin synthesis causes
anaemia, a characteristic of lead poisoning.
Although only two corpses :vere av~ilable for post:morte'!:
examination, they proved sufficient to illustrate the differer. .. ~
between a lead poisoned bird and one with very low, presumably non-
- 145 -