75 / 156
73
took the average retention time of gunshot in the gizzards
of mallard from experimental studies to be 20 days. He then
divided the length of the hunting season by this 20 day period
to give a turnover correction factor to account for the numbers
of birds ingesting gunshot throughout the season. In the UK
the wildfowl hunting season is at least 153 days (1 Sept until
end of January inland – longer below the high water mark
in all but Northern Ireland). We therefore used a correction
factor of 7.5 (150/20) to account for turnover. By analysing
ring recoveries, Bellrose calculated the absolute difference in
the annual mortality rate of wild mallards in the USA between
ringed ducks dosed experimentally with various numbers
of gunshot and control ducks that were ringed but not
given gunshot. This difference was for the year immediately
following ringing and dosing (detailed in Bellrose 1959, Table
27 and pages 274-276). We assumed that these additional
annual mortality rates would be broadly similar for all wildfowl.
From these calculations, presented in Table 5, we estimate that
73,750 birds of the 16 species presented in Table 3 might die
every winter in Britain from lead poisoning following gunshot
ingestion (this figure would be slightly higher for the UK at c.
75,000, using data from Musgrove
et al.
2013).
This may underestimate mortality for several reasons. It does
not include species of wildfowl for which UK data on the
incidence of gunshot ingestion is not available (
e.g.
some of
the goose species), and does not include mortality caused
by gunshot ingested in the UK outside of the hunting season
(which will occur but likely with a reduced incidence). It also
excludes the sub-lethal effects of lead which can also influence
mortality. These three factors would result in our estimate of
mortality being too low. A few factors could potentially result
in our estimate being too high. We assume that mortality
levels given ingestion of a specific number of gunshot will
be similar in all species to those used by Bellrose (1959) for
mallard, while these may be higher in some species and lower
in others. It is possible that mortality levels could be lower
in the geese and swans ingesting small numbers of gunshot
Table 5: Estimate of numbers of 16 species of wildfowl listed in Table 3 dying of lead gunshot ingestion annually during winter.
Number of
gunshot
ingested
% hunter-
shot birds
with
ingested
gunshot
1
Hunting bias
correction
2
%with
ingested
gunshot
after
correction
for hunting
bias
%with
ingested
gunshot
corrected for
turnover
3
Additional
mortality
rate (annual
probability
of death)
4
% of the
population
estimated as
dying of lead
poisoning
5
Number
of birds
estimated as
dying
6
1
1.89
1.5
1.26
9.45
0.09
0.85
20,039
2
0.525
1.9
0.276
2.07
0.23
0.48
11,230
3
0.081
2
0.041
0.30
0.3
0.09
2,147
4
0.207
2.1
0.098
0.74
0.36
0.27
6,255
5
0.207
2.2
0.094
0.70
0.43
0.30
7,132
6 or more
0.578
2.35
0.246
1.84
0.62
1.14
26,947
Totals
3.487
2.015
15.11
3.13
73,750
1
Assuming incidences from Mudge (1983) for 12 of the 16 species in Table 3;
2
Correction factor based upon the increased likelihood of hunters to shoot wildfowl that
have ingested lead gunshot (Bellrose 1959);
3
Assuming a 150 day hunting season (Britain) and an average 20 day residence time of gunshot in the gizzard – turnover
of 150/20 = 7.5 (see Bellrose 1959);
4
Mortality level is the increase in mortality in mallard caused by ingestion of set numbers of lead gunshot (see Bellrose 1959) – we
assume that the mortality level would be similar in all species;
5
% with ingested gunshot corrected for hunting bias and turnover multiplied by mortality level;
6
Using
wintering wildfowl estimates from Musgrove
et al.
(2011).
Lead poisoning of wildlife in the UK