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for mallard of 3.6%, pintailAnas acuta 5.4%, and pochardAythya
ferina 9.3% in northern European wetlands. The majority of
studies summarised by Mateo (2009) appear to be of birds shot
by hunters, though in some cases trapped birds were included.
Studies conducted in the UK reported broadly similar levels of
gunshot ingestion to those elsewhere in Europe, although they
vary among sites and species (Table 3).
More recently, Newth et al. (2012) reported lead poisoning
in wildfowl (between 1971 and 2010) in the UK where the
majority of cases of birds dying of lead poisoning (75% of 251)
still had lead gunshot in various stages of dissolution in their
gizzards. The post mortem data used for this study revealed
a small number (13) of lead poisoned birds with >40 pellets
within the gizzard, three of which contained more than 100
pellets, including a Canada goose Branta canadensis whose
gizzard contained 438 pellets, while Spray and Milne (1988)
reported a mute swan with 844 pellets. Species suffering lead
poisoning from ingested gunshot included those feeding in
water and wetlands, as well as grazing species including geese
and swans where a large proportion of time is spent feeding on
agricultural land (Newth et al. 2012). Gunshot ingestion levels
in birds found dead from this paper and WWT’s database are
given in Table 3. Of a subset of 104 whooper swans diagnosed
as having died of lead poisoning, 86% contained shotgun
pellets in the gizzard.
More studies on lead poisoning have been conducted on
wildfowl than other taxa. However, where lead ingestion has
been investigated in other taxa that feed in areas shot-over using
lead gunshot it has generally been found. Table 4 summarises
some of the studies that illustrate gunshot ingestion in a range
of non-wildfowl waterbirds and in terrestrial birds. This is not
comprehensive but illustrative of the range of different birds
that can be affected.
Several methods have been used to estimate the proportion
of wild birds with ingested gunshot in the gizzard or digestive
tract and various biases may be associated with them. Hunter-
shot birds will be subject to the biases involved in hunting,
e.g.young birds are often over-represented in hunting bags.
Also ingestion of lead may remove many poisoned individuals
from a population (via lead-related morbidity and mortality) or
conversely lead ingestion may disable birds sufficiently to make
themmore likely tobeharvested (e.g.Bellrose 1959, Heitmeyer et
al.1993, Demendi and Petrie 2006). In field experiments Bellrose
(1959) found that mallard dosed with lead gunshot were more
vulnerable to being shot than undosed controls – by 1.5 times,
1.9 times and 2.1 times for birds dosed with one, two and four
No. 6 gunshot respectively. Trapping may potentially introduce
biases, but little informationexists. Ingestion levels inbirds found
dead may also be subject to confounding factors. Firstly, “found
dead”studies are biased towards those species most likely to be
visible to humans e.g. large, white or close to human habitation.
The nature of lead poisoning as a debilitating condition may
make affected individuals more prone to disappearing into
vegetation and to scavenging and predation (Sanderson and
Bellrose 1986, Pain 1991). Moreover, gunshot may be ground
down or dissolved in the bird’s alimentary canal and thus not
be apparent on radiographs or at post mortem examination.
While proportions of birds found dead with ingested gunshot
in the gizzard may not accurately reflect the situation in the
wild population, finding gunshot in found dead birds obviously
illustrates the pathway of ingestion.
Despite these biases and confounding factors, any one or all
of these methods can be used to compare the prevalence of
ingestion across space and time. Studies from across the world
have shown that levels of gunshot ingestion are influenced by
factors including species’ feeding habits, gunshot density and
availability (influenced by substrate type and shooting intensity,
duration and season) and grit availability (e.g. Bellrose 1959, Flint
1998, Mudge 1983, Thomas et al. 2001, Demendi and Petrie 2006
– see also reviews cited above).
Several means can be used to establish or infer the provenance
of elevated tissue lead concentrations in birds. Ratios of stable
leadisotopesinmaterialsvaryaccordingtothegeologicalorigin
of the lead. Lead isotope ratios can therefore be compared
between animal tissue samples, lead from ammunition and the
other potential sources that exist in the area where the animal
lived and this can help to identify or exclude some potential
sources of the lead. Lead isotope studies have linked gunshot
ingestion with elevated tissue lead concentrations in a range
of wild birds in a number of studies from around the world.
These studies support ammunition-derived lead as the major
contributor to widespread elevated tissue lead concentrations
in wild birds (e.g. Scheuhammer and Templeton 1998,
Scheuhammer et al. 2003, Svanberg et al. 2006, Martinez-Haro
et al. 2011).
Temporal or spatial correlations between elevated tissue lead
levels in birds and hunting activities can also help establish
the primary source(s) of lead exposure. Studies have compared
Deborah J. Pain, Ruth Cromie & Rhys E. Green