Acute and Chronic Lead Exposure in Four Avian Scavenger Species in Switzerland

Abstract

Despite irrefutable evidence of its negative impact on animal behaviour and physiology, lethal and sublethal lead poisoning of wildlife is still persistent and widespread. For scavenging birds, ingestion of ammunition, or fragments thereof, is the major exposure route. In this study, we examined the occurrence of lead in four avian scavengers of Switzerland and how it differs between species, regions, and age of the bird. We measured lead concentration in liver and bone of the two main alpine avian scavengers (golden eagle Aquila chrysaetos and bearded vulture Gypaetus barbatus) over the entire area of the Swiss Alps and two of the main avian scavengers occurring in the lowlands of Switzerland (red kite Milvus milvus and common raven Corvus corax). Of those four species, only the bearded vulture is an obligate scavenger. We found that lead burdens in the two alpine avian scavengers were higher than those found for the same species elsewhere in Europe or North America and reached levels compatible with acute poisoning, whereas lead burdens of the two lowland avian scavengers seemed to be lower. Several golden eagles, but only one red kite with abnormally high bone lead concentrations were found. In all four species, a substantial proportion of birds had elevated levels which presumably represent recent (liver lead levels) or past (bone lead levels) uptake of sublethal doses of lead.

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Archives of Environmental Contamination and Toxicology
https://doi.org/10.1007/s00244-018-0561-7
Acute andChronic Lead Exposure inFour Avian Scavenger Species
inSwitzerland
KathrinGanz1 · LukasJenni1· MilenaM.Madry2· ThomasKraemer3· HannesJenny4· DavidJenny1
Received: 10 May 2018 / Accepted: 10 September 2018
© Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract
Despite irrefutable evidence of its negative impact on animal behaviour and physiology, lethal and sublethal lead poisoning
of wildlife is still persistent and widespread. For scavenging birds, ingestion of ammunition, or fragments thereof, is the
major exposure route. In this study, we examined the occurrence of lead in four avian scavengers of Switzerland and how
it differs between species, regions, and age of the bird. We measured lead concentration in liver and bone of the two main
alpine avian scavengers (golden eagle Aquila chrysaetos and bearded vulture Gypaetus barbatus) over the entire area of
the Swiss Alps and two of the main avian scavengers occurring in the lowlands of Switzerland (red kite Milvus milvus and
common raven Corvus corax). Of those four species, only the bearded vulture is an obligate scavenger. We found that lead
burdens in the two alpine avian scavengers were higher than those found for the same species elsewhere in Europe or North
America and reached levels compatible with acute poisoning, whereas lead burdens of the two lowland avian scavengers
seemed to be lower. Several golden eagles, but only one red kite with abnormally high bone lead concentrations were found.
In all four species, a substantial proportion of birds had elevated levels which presumably represent recent (liver lead levels)
or past (bone lead levels) uptake of sublethal doses of lead.
Even after over a century of research demonstrating the det-
rimental effects of lead on animal and human health, the
problem of lethal and sublethal lead poisoning of wildlife is
still persistent and widespread (Pokras and Kneeland 2009;
Bellinger etal. 2013; Haig etal. 2014; Arnemo etal. 2016).
There are various ways of exposure, but lead poisoning
through ingestion of ammunition, or fragments thereof, is
undoubtedly the major exposure route for scavenging birds
in Europe and elsewhere (Pain etal. 1997, 2009; Scheuham-
mer and Templeton 1998; Church etal. 2006; Kenntner etal.
2007; Helander etal. 2009; Legagneux etal. 2014; Madry
etal. 2015; Carneiro etal. 2016; Jenny etal. 2016). Avian
scavengers are prone to lead poisoning via ingestion of lead
ammunition fragments from wounded animals, carcasses or
offal heaps that are left in the field (Fisher etal. 2006). Lead
ingestion from ammunition has been documented in at least
33 raptor species (Pain etal. 2009).
High lead concentrations in Swiss golden eagles Aquila
chrysaetos have previously been published and has been
shown to result from ingestion of lead bullet fragments
(Madry etal. 2015). In this study, most of the golden eagles
came from the Canton of Grisons, an alpine region in the
southeast of Switzerland. Hunters in this area use an unusu-
ally large bullet calibre (10.3mm), whereas smaller cali-
bres (mainly between 6.5 and 8mm) are used elsewhere
in Switzerland and in Central Europe. Per shot, the larger
bullets used in the Grisons leave more lead in the carcasses.
It remains therefore unclear whether lead exposure is a prob-
lem specific to the Grisons or over the whole Swiss alpine
range.
Hunting in Switzerland shows some potentially important
differences between the Alps and the lowlands with respect
to the availability of lead contaminated carcasses for avian
scavengers. In the Swiss Alps, bullets are frequently used for
large game hunting (red deer, Alpine ibex, chamois), while
* Kathrin Ganz
ganz.kathrin@bluewin.ch
1 Swiss Ornithological Institute, Seerose 1, 6204Sempach,
Switzerland
2 Center forForensic Hair Analytics, Zurich Institute
ofForensic Medicine, University ofZurich, Kurvenstrasse
17, 8006Zurich, Switzerland
3 Department ofForensic Pharmacology andToxicology,
Zurich Institute ofForensic Medicine, University ofZurich,
Winterthurerstrasse 190/52, 8057Zurich, Switzerland
4 Fish andGame Department, Canton ofGrisons, Chur,
Switzerland

Archives of Environmental Contamination and Toxicology
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in the Swiss lowlands, hunting large game with bullets is
much less frequent, and shot is used for smaller animals (roe
deer, small game). Besides the different ammunitions used,
there are also different ways in how hunting is regulated in
different regions. In most of the alpine region of Switzer-
land, hunting is restricted to a few weeks per year and to cer-
tain species, whereas in parts of the Swiss lowlands hunting
is generally permitted all year round. Furthermore, ungulates
occur in higher densities in the Alps and more frequently in
open habitats than in the lowlands. An open question there-
fore is whether scavengers in the different regions are prone
to lead exposure to different extents.
We examined lead concentrations in both liver and bone,
because each of these tissues provides different informa-
tion about lead exposure of the birds. Lead in liver stays
elevated for several weeks after lead ingestion and therefore
indicates relatively acute exposure. Lead in bones is thought
to integrate lifetime exposure, because it is relatively stable
once incorporated (Franson and Pain 2011). However, bone
lead also is leaking from the bones back into the blood-
stream, because bones are remodelled during life (Ambrose
etal. 2000; Pokras and Kneeland 2009). This endogenous
lead mobilization could lead to poisoning many years after
the initial exposure (Wiemeyer etal. 2017). Nevertheless,
bone lead concentrations might be most useful to compare
patterns of lead exposure between populations in different
geographical areas (Franson and Pain 2011). Therefore, the
analysis of both liver and bone lead concentration ensure
that neither past nor current lead exposure events are missed,
as the two measures are complementing each other.
The purpose of this study was to examine the occurrence
of lead in four avian scavengers and how it differs between
species, regions, and age of the bird. We therefore examined
lead concentrations in the two main alpine avian scaven-
gers (golden eagle and bearded vulture Gypaetus barbatus)
over the entire area of the Swiss Alps and two of the main
avian scavengers occurring in the lowlands of Switzerland
(red kite Milvus milvus and common raven Corvus corax).
Additionally, we examined whether there was a correlation
between liver and bone lead concentrations.
Materials andMethods
Sample Collection
The final data set used for analysis included 127 birds (67
golden eagles, 5 bearded vultures, 45 red kites, and 10 com-
mon ravens) collected throughout Switzerland (Figs.1, 2).
This data set consists of previously published liver and bone
Fig. 1 Map of Switzerland with the locations where birds were found dead and their bone lead concentration

Archives of Environmental Contamination and Toxicology
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lead data from 31 golden eagles collected between 2006
and 2013 (Jenni etal. 2015; Madry etal. 2015), data from
three bearded vultures (one of them kindly provided by E.
Bassi, see also Bassi etal. 2016) and newly collected data
from 36 golden eagles, 2 bearded vultures, 45 red kites, and
10 common ravens. Newly acquired samples of dead birds
were obtained from several Cantonal Fish and Game Depart-
ments, gamekeepers, bird care stations, and animal hospitals
between 2009 and 2017. Most dead red kites from the Can-
ton of Fribourg were found by members of the Swiss Orni-
thological Institute studying red kites in that area. Depend-
ing on the stage of wing and tail feather moult the age of the
golden eagles was determined as juvenile (juv, first year),
immature (immat, 1–3years old), subadult (subad, 3–5years
old), or adult (ad, > 5years old) (Haller 1996). Red kites
were classified as juvenile (juv, first calendar year), imma-
ture (immat, second calendar year), or adult (ad, third cal-
endar year or older).
Lead concentrations were measured both in bone and
liver samples whenever possible. Bone samples were pref-
erably taken from the diaphysis of the femur (n = 67) or
humerus (n = 8), because they are structurally similar and
have been shown to contain comparable lead concentra-
tions (Mateo etal. 2003). If these were not available other
bones or bone parts, mainly sternum (n = 17), but also tar-
sometatarsus (n = 5), tibiotarsus (n = 3), r ib (n = 2), furcula
(n = 1), pelvis (n = 1), coracoid (n = 1), an epiphysis, and a
not clearly attributable piece of a long bone were analysed.
Common ravens are huntable in Switzerland and most of the
individuals available for this study had been shot. Therefore,
we preferably analysed an intact tarsometatarsus or tibiotar-
sus to avoid having lead contaminated samples due to shot
fragmentation in the body. Liver samples of common ravens
were only analysed if the bird was clearly not shot.
Five golden eagles and one bearded vulture were found
with traces of illegal poaching. Two golden eagles were
found with a single (FR2, GR4) and another with three
encapsulated lead pellets (BE5); a fourth golden eagle (FR3)
was found with seven pellets distributed in the body and died
most likely from the consequences of poaching. Another
golden eagle (SG6) showed an injury that was likely caused
by a bullet. One bearded vulture was found with six encap-
sulated pellets distributed in the thorax (BGR3).
For golden eagles, reported death causes were intraspe-
cific fights (n = 30), power line or cable collision (n = 8),
poisoning (n = 6), others (n = 11), or unknown (n = 12). Two
bearded vultures died because of power line collisions, one
of liver degeneration, one was an avalanche victim, and one
showed signs of lead poisoning (green stained faeces, bone
lead concentration of 100.04µg/g) but probably died finally
in an intraspecific fight with a golden eagle. The causes of
death reported for red kites were injuries (n = 10), collisions
Fig. 2 Map of Switzerland with the locations where birds were found dead and their liver lead concentration

Archives of Environmental Contamination and Toxicology
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(n = 8), predation (n = 6), others (n = 7), or unknown (n = 14).
Eight of the ten common ravens were shot; one died because
it collided with a power line and for one the cause of death
was unknown.
Sample Preparation
Similar to the previous ones (Madry etal. 2015), samples
from the newly collected individuals were thawed and tissue
adherent to bones was removed with a stainless steel scalpel.
If necessary, the bones were rinsed with ultrapure water to
remove traces of blood. Bone fragments and liver samples
were then dried in an oven at 90°C until constant weight.
On average, 250mg of bone or 150mg of liver mass was
analysed in duplicates. To each sample, 1ml of 65% (v/v)
nitric acid (Trace Select Ultra, Fluka) was added and the
samples stored at room temperature for 30min for prediges-
tion. Before microwave digestion, 4ml of ultrapure water
was added to each sample. The digestion was performed in
a Microwave Digestion System (CEM Discover® Explorer
24) following a standard procedure with three steps. During
the first 5min, the samples were heated up to 220°C, then
hold at 220°C, and under constant pressure (setpoint 290
PSI) for 5min and afterwards cooled down to 60°C before
they were released out of the microwave. After digestion,
the whole samples were transferred to plastic vials (15-ml
tube, Sarstedt) and diluted with ultrapure water by a factor
of 100 before analysis.
Lead Concentration Analysis
Lead concentration was measured using an inductively cou-
pled plasma mass spectrometer (ICP-MS) from Analytik
Jena (Plasma Quant® MS Elite, Jena, Germany). Calibra-
tion curves were prepared in aqueous solutions using ICP-
MS Calibration Mix 1 and 2 (Analytik Jena). To verify the
accuracy of the calibration curve, a control serial dilution
was prepared with ICP Multi Element Standard Solution
XXI CertiPur (Merck). Bone meal (SRM 1486, National
Institute of Standards and Technology) with a certified lead
concentration of 1.335 ± 0.014µg/g served as reference
material. Additionally, with each batch of 30 samples two
250mg samples of a bone pool, consisting of several ground
golden eagle bones, were prepared in the same manner as
the samples and analysed as well. Lead recovery from the
certified bone meal was 107 ± 7% (mean ± standard devia-
tion, SD) and ranged from 99% to 126% (n = 18). Our bone
pool yielded a lead concentration of 13.08 ± 0.64µg/g over
all samples (n = 24). Blanks, prepared in the same way as
the samples, but without material were used to assess back-
ground levels of lead. The limit of detection (LOD) was
calculated as mean(blanks) + 3 * SD(blanks). The LOD,
back-calculated to tissue concentrations, was 0.25µg/g dry
weight for liver and 0.15µg/g dry weight for bone sam-
ples. All bone lead concentrations were above the LOD.
Liver lead concentrations below the LOD were assigned to
0.125µg/g (1/2 LOD) for statistical purposes. For quantifi-
cations, the average of lead isotopes 206Pb, 207Pb, and 208Pb
was used. Bismuth (209Bi) was added to each sample as an
internal standard. Lead concentrations in the tissue samples
are always reported as µg/g dry weight.
Interpretation ofTissue Lead Concentrations
Different publications propose different threshold values to
interpret tissue lead concentrations. We used the threshold
concentrations for liver dry weight proposed for bald Hali-
aeetus leucocephalus and golden eagles by Wayland etal.
(1999) and considered liver lead concentrations < 6µg/g
to be background, 6–30µg/g to be diagnostic of elevated
lead exposure, and > 30µg/g to be diagnostic of lead poi-
soning. These also are the threshold values used in sev-
eral other raptor studies (Clark and Scheuhammer 2003;
Madry etal. 2015; Jenni etal. 2015). In bones, lead con-
centrations < 10µg/g were considered to be background,
10–20µg/g to be indicative of subclinical to clinical poi-
soning, and > 20µg/g to correspond to abnormally high
exposure that is in some cases compatible with severe clini-
cal poisoning (Pain 1996). These threshold values were
originally established for waterfowl (Pain 1996), but have
since been used in various raptor studies (Mateo etal. 2003;
Rodriguez-Ramos Fernandez etal. 2011; Jenni etal. 2015;
Wiemeyer etal. 2017) and been recommended by Franson
and Pain (2011). It is important to note that background lead
concentrations do not equal natural environmental concen-
trations, nor do they imply a “no-effect” exposure. After
decades of anthropogenic emissions, natural environmental
concentrations no longer exist and lead is ubiquitous in the
environment (Pain 1996; Franson and Pain 2011). Further-
more, there is no safe threshold of lead exposure and lead
affects biological systems even at very low concentrations
(Pain 1996; Stroud 2015).
Data Analysis
Because the data did not fulfil the assumptions of paramet-
ric tests, we used the nonparametric counterparts for the
analyses. Bone lead concentration comparisons between
golden eagles from the Grisons and the rest of Switzer-
land, as well as between red kites and golden eagles, were
done with a Wilcoxon rank-sum test. To test whether
there were any significant differences in bone lead con-
centration between age classes in golden eagles and red
kites, Kruskal–Wallis tests were used. There was only
one juvenile golden eagle, and this age class was there-
fore excluded from the age class analysis. Because of tied

Archives of Environmental Contamination and Toxicology
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ranks, we correlated liver with bone lead concentrations
calculating the nonparametric Kendall rank correlation
coefficient.
Results
Fourteen of 46 golden eagle bone samples, but only 1 of 45
red kite bone samples had lead concentrations that are in
some cases compatible with lethal lead poisoning (Fig.1).
Bone lead concentrations in red kites were significantly
lower than in golden eagles (W = 416, p < 0.001; Fig. 3;
Tables1, 2). However, the highest bone lead concentra-
tion found in a red kite (43.02µg/g) was almost as high
as the highest bone lead concentration of a golden eagle
(54.21µg/g). Bone lead concentrations in golden eagles
from the Grisons (n = 19, median = 11.68µg/g) did not differ
significantly from bone lead concentrations in golden eagles
from the rest of Switzerland (n = 27, median = 13.16µg/g;
W = 232, p = 0.60). Three bearded vultures had bone lead
concentrations indicative of background exposure, whereas
the remaining two individuals had bone lead concentra-
tions well above 20µg/g (i.e., 58.9µg/g and 100.04µg/g;
Fig.1). No common raven had bone lead concentrations
that are in some cases compatible with lethal lead poison-
ing (> 20µg/g), but four of ten individuals had bone lead
concentrations indicative of subclinical to clinical poisoning
(10–20µg/g; Fig.1; Table2).
Between age classes of golden eagles, there were no sig-
nificant differences in bone lead concentrations (analysed
without the single juvenile; Kruskal–Wallis test: H(2) = 3.25,
p = 0.20), although there is a tendency that very high values
are found in subadults and adults (Fig.4). In red kites, how-
ever, there was a significant difference in bone lead con-
centrations between the age classes (H(2) = 23.8, p < 0.001).
Comparisons of the mean ranks between groups showed that
adult red kites had significantly more lead in their bones than
juveniles (observed difference: 18.0, critical difference: 13.3,
α = 0.001; Fig.4).
Two of a total of 55 golden eagle liver samples had
concentrations indicative of lethal lead poisoning (GR7:
77.4µg/g, VD2: 80.4µg/g). Three further golden eagles
showed elevated liver lead concentrations (6–30µg/g),
Fig. 3 Bone lead concentrations in a golden eagles, b red kites, and
c common ravens. The threshold values between background lead
exposure (< 10µg/g), elevated lead exposure (10–20µg/g), and con-
centrations sometimes associated with lead poisoning (> 20µg/g) are
indicated with dashed, vertical lines (threshold values according to
Pain 1996). All bone values were above the LOD of 0.15µg/g
Table 1 Sample size, median,
mean and range of bone and
liver lead concentrations in
golden eagles, bearded vultures,
red kites, and common ravens
a Of bearded vulture BGR3 a piece of the coracoid (11.35µg/g) as well as of a rib (100.04µg/g) was ana-
lysed. The latter value is considered here
Species Sample nMedian (µg/g) Mean (µg/g) Range (µg/g)
Golden eagle
Aquila chrysaetos Bone 46 12.54 16.06 0.40–54.21
Liver 55 1.34 4.89 < LOD–80.44
Bearded vulture
Gypaetus barbatus Bonea5 6.50 33.37 0.53–100.04
Liver 2 0.38 0.38 < LOD–0.64
Red kite
Milvus milvus Bone 45 4.08 5.79 0.23–43.02
Liver 34 0.45 0.59 < LOD–3.54
Common raven
Corvus corax Bone 10 6.58 7.89 1.20–17.78
Liver 2 0.32 0.32 0.30–0.35

Archives of Environmental Contamination and Toxicology
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whereas the majority of golden eagle liver samples had
background values (< 6µg/g; Figs.2, 5). Two of three
golden eagles with embedded ammunition had liver lead
concentrations around the threshold for elevated exposure
(FR3: 7.99µg/g, BE5: 5.44µg/g). Whether the lead found
in the liver of these individuals originated from the embed-
ded pellet or from oral uptake of lead is not evident. It has
however been found that lead is absorbed to a much lesser
degree from embedded than from ingested shot (Finkelstein
etal. 2014). The liver lead concentrations in the bearded
vulture (n = 2), red kite (n = 34), and common raven (n = 2)
samples were all at background level (Fig.2).
Bone and liver lead concentrations over all species were
significantly positively correlated (Kendall’s tau = 0.44,
p < 0.001; Fig.6). This also was the case for golden eagles
and red kites analysed separately (Kendall’s tau = 0.34,
p < 0.005 for each species).
Discussion
In this study, we showed that the main avian scavengers in
the Alps (golden eagle and bearded vulture) generally have
higher bone lead concentrations than two important avian
scavengers in the lowlands (red kite and common raven). We
confirmed that golden eagles in the Swiss Alps had a high
lead burden in bone (Mardy etal. 2015) with no noticeable
difference between regions using different ammunition. A
clear increase in bone lead concentrations with age was only
found in red kites.
Lead concentrations were generally higher in the two
alpine species (golden eagle and bearded vulture) than in
the two lowland species (red kite and common raven). The
question is whether this is due to differences in longev-
ity, different exposure, and ingestion or to species-specific
absorption rates in the digestive tract and differences in lead
metabolism.
Both golden eagle and bearded vulture had very high
bone lead concentrations in the Swiss Alps compared with
data from the literature from other areas. The median lead
concentration of 12.54µg/g (dry weight) found in golden
eagles from the Swiss Alps in this study is substantially
higher than values previously reported for golden eagles
elsewhere, although golden eagle bones were not analysed
in many studies and the sample size was often low. In North
America, Clark and Scheuhammer (2003) reported 5 of 9
golden eagles as “Pb-exposed” (defined as lead concentra-
tion > 6.75µg/g), and Wayland etal. (1999) found a maxi-
mum lead concentration of 18µg/g in 49 bald and golden
eagle bone samples (they do not state in which species
this concentration was found, but that there is no signifi-
cant difference between the two species). In Spain, Mateo
etal. (2003) found 0.49 and 4.97µg/g, respectively in two
Table 2 Percentage of individual birds (sample size in parentheses)
with background lead concentrations, elevated concentrations, and
concentrations that are in some cases compatible with acute poison-
ing, respectively, in bones (dry weight)
a Of bearded vulture BGR3 a piece of the coracoid (11.35 µg/g) as
well as of a rib (100.04µg/g) was analysed. The latter value is con-
sidered here
Species nBackground
Pb (< 10µg/g) Elevated Pb
(10-20µg/g) Pb poisoned
(> 20µg/g)
Golden eagle
Aquila chrysaetos 46 35% (16) 35% (16) 30% (14)
Bearded vulture
Gypaetus barbatus 5a60% (3) 0% (0) 40% (2)
Red kite
Milvus milvus 45 78% (35) 20% (9) 2% (1)
Common raven
Corvus corax 10 60% (6) 40% (4) 0% (0)
Fig. 4 Bone lead concentration in a golden eagles and b red kites
according to age class for all individuals of known age class. Golden
eagles: juv n = 1, immat n = 10, subad n = 9, ad n = 25. Red kites: juv
n = 17, immat n = 4, ad n = 14. Median (horizontal line), 25–75% per-
centiles (box), and outliers (dots) are given

Archives of Environmental Contamination and Toxicology
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golden eagles. Lead concentration in our golden eagle liver
samples was mostly below the threshold for elevated expo-
sure (i.e., < 6µg/g), which agrees with findings in Canadian
golden eagles (Wayland etal. 1999; Clark and Scheuhammer
2003), but two of our birds were acutely poisoned. Two of
five bearded vultures had very high bone lead concentrations
(58.90µg/g and 100.04µg/g). In a study from the Alps, three
bearded vultures (20% of samples analysed) were found with
bone lead concentrations higher than 6.75µg/g (Bassi etal.
2016). In contrast, in the Pyrenean population only 1 of 43
individuals showed a bone lead concentration indicative of
chronic poisoning (> 20µg/g) (Hernández and Margalida
2009).
High lead burdens do not seem to be a phenomenon
restricted to the area using large calibres of ammunition
(canton Grisons) but are more or less homogenously dis-
tributed in golden eagles over the whole Swiss alpine range
(Fig.1).
Both golden eagles and bearded vultures seem to be heav-
ily exposed to lead in the entire Swiss Alps compared with
other areas of their distribution. The high lead exposure
of the bearded vultures in the Alps may be aggravated by
the apparently high susceptibility to lead poisoning of this
species. Like other raptors, vultures are in general highly
susceptible to lead poisoning because of a very acidic envi-
ronment in the stomach that allows rapid lead particle dis-
solution (Duke etal. 1975; Houston and Cooper 1975; Fisher
etal. 2006; Ford 2010). The diet of the bearded vultures
consists largely of bones, which need a digestion time of
approximately 24h, at least 3–4 times longer than in raptors
eating mainly meat, and pellets are only rarely regurgitated
(Houston and Copsey 1994; Frey 2011). Therefore, retention
time of lead particles in the stomach is likely also longer
than in other raptors. There are several reports of acutely
lead poisoned bearded vultures found in the wild (Hernández
and Margalida 2009; Gressmann etal. 2013) and also several
cases of bearded vultures lethally lead-poisoned in captivity
because of lead contaminated food (Frey 2011).
In contrast to the two alpine species, bone and liver lead
burden of red kites and common ravens were lower. Red
kite bone lead concentrations also were lower than those
found in other countries. While we found 98% of red kites
(44 of 45) with bone lead concentrations < 20µg/g (dry
weight), this percentage was 79% (68 of 86 birds found
Fig. 5 Liver lead concentrations in golden eagles and red kites.
The first bin on the left indicates values below the LOD, i.e., below
0.25 µg/g. The threshold values between background exposure
(< 6µg/g), elevated lead concentrations (6–30µg/g), and lead poison-
ing (> 30 µg/g) are indicated with dashed, vertical lines (threshold
values according to Wayland etal. 1999)
Fig. 6 Relationship between bone and liver lead concentrations for
all four species. The acutely lead poisoned golden eagle VD2 (femur:
25.63µg/g, liver: 80.44µg/g) is omitted from this graph

Archives of Environmental Contamination and Toxicology
1 3
dead in the wild) in England (Pain etal. 2007) and 92% (11
of 12 birds) in Spain (Mateo etal. 2003). However, about
half of the birds of known age in our study from which we
analysed bones (17 of 35) were in their first months of life
(found dead in July up to 17 August) before their first migra-
tion, and all had very low bone lead concentrations (up to
5.07µg/g). This indicates that the food ingested by young
birds in summer in Switzerland is hardly contaminated with
lead. However, among adults, 36% (5 of 14) had bone lead
concentrations > 10µg/g (d.w.), which is indicative of ele-
vated lead exposure. It remains unclear whether this lead
had been acquired during migration and/or in Switzerland
(most first-year birds migrate to France and Spain in their
first winter, while they are inclined to overwinter in Swit-
zerland as adults; M. Grüebler, personal communication).
From regurgitated pellet analysis, it is known that red kites
ingest lead shot (Pain etal. 2007) and cases of acute poison-
ing have been found (Mateo etal. 2003; Pain etal. 2007;
Berny etal. 2015).
Common ravens often are the first and most frequent
scavengers that arrive at a carcass and have the longest
scavenging duration and the highest carrion intake (Legag-
neux etal. 2014; Nadjafzadeh etal. 2015). It is therefore
likely that they are repeatedly ingesting lead bullet fragments
from hunter-killed game or offal. Even though we had some
ravens with elevated bone lead concentrations, none of them
was as high as those found in the other three avian scaven-
ger species, but admittedly, sample size was low. We are
not aware of any previous study on common raven bone or
liver lead concentrations, but blood lead concentrations were
found to be elevated during the moose hunting season in the
United States and Canada (Craighead and Bedrosian 2008;
Legagneux etal. 2014). We also know of no case of acutely
poisoned common raven from Switzerland, in contrast to
several cases of lead-poisoned golden eagles, bearded vul-
tures, and red kites. Common ravens might not be as sensi-
tive to lead as raptors (large differences in tolerance to lead
poisoning have been found between bird species; Carpen-
ter etal. 2003), retention time in the digestive tract may be
much shorter (rapid regurgitation reduces the amount of lead
absorbed; Pain etal. 2009), pH of the stomach may not be as
low as in raptors (we did not find pH values in the literature),
common ravens may be better able to avoid lead particles
when feeding on carcasses, or lead-poisoned common ravens
are not diagnosed correctly or not found in the field. On the
other hand, common ravens, in contrast to raptors, ingest
grit to aid digestion, and grit may enhance lead absorption
through abrasion of the lead fragments.
Lead in bones is thought to represent lifetime exposure
of the bird to this heavy metal (Franson and Pain 2011).
Assuming that lead ammunition particles are ingested sev-
eral times throughout the life of an individual (Jenni etal.
2015), resulting in repeated sublethal lead poisoning, an
age-related increase in bone lead concentration would be
expected. This was found in some (Pain etal. 2005; Rod-
riguez-Ramos Fernandez etal. 2011) but not all (Wayland
etal. 1999; Hernández and Margalida 2009) studies. In our
study, significantly higher bone lead concentration in adults
compared with juveniles was only found in red kites but not
in golden eagles. As previously suggested, a reason for the
lack of an age effect in golden eagles might be an increased
mortality in immatures and subadults exceeding a certain
bone lead concentration threshold (Madry etal. 2015).
Even though liver and bone lead concentrations corre-
lated in our study, bone lead concentrations seem to be a
poor predictor of liver lead concentrations and vice versa.
Similar to our results, previous studies found either signifi-
cant, albeit weak, relationships between bone and liver lead
concentrations (bald and golden eagles: Wayland etal. 1999)
or no correlation at all (black and turkey vultures: Behmke
etal. 2015, 2017; red kites: Pain etal. 2007). The finding
of a weak or no relationship is not surprising, because bone
lead reflects lifetime exposure of the birds to lead, whereas
lead in the liver reflects recent exposure.
In summary, we demonstrate that lead burdens in the
two alpine avian scavengers reached values all over the
Swiss Alps, which are higher than found elsewhere in
Europe or North America for the same species and which
reached abnormally high levels. This is particularly alarm-
ing for the bearded vulture, which has been reintroduced
from a captive breeding programme and still has a small
and vulnerable, although increasing, population in the
Alps (Schaub etal. 2009; Jenny etal. 2018). The rein-
troduction of the bearded vulture in the Swiss Alps goes
back to a complex and expensive international releasing
and monitoring project (Robin etal. 2004). Due to a very
low reproduction rate, a long lifespan of the reproduc-
tive individuals is crucial for a growing population. Lead
poisoning as a leading mortality factor among bearded
vultures (Jenny etal. 2016) affects the success of the rein-
troduction programme in a serious way. Lead burdens of
the two lowland avian scavengers seemed to be lower than
lead burdens of the two Alpine avian scavengers, and only
one red kite with bone lead concentration that is in some
cases compatible with lethal lead poisoning was found.
In all four species, a substantial proportion of birds were
found with elevated levels, which presumably represent
recent (liver lead levels) or past (bone lead levels) uptake
of sublethal doses of lead. Sublethal lead concentration
in the blood was found to affect flight height and move-
ment rate of golden eagles (Ecke etal. 2017), lead shot
ingestion can slow species recovery (Meyer etal. 2016),
and reproduction during subsequent breeding seasons can
be impaired by mobilisation of lead from bones and dep-
osition in eggs (Pikula etal. 2013). Therefore, the high
bone lead concentrations found in our study raises serious

Archives of Environmental Contamination and Toxicology
1 3
concern about the general health of the avian scavenger
community in Switzerland, despite the fact that all four
species increase in population size in Switzerland.
It has repeatedly been shown that the lead found in avian
scavengers originates from spent hunting ammunition and
that lead shot pellets or fragments from lead bullets (which
partly disintegrate into fragments when striking) are taken
up by the birds from carcasses or offal left unburied (Madry
etal. 2015; Bassi etal. 2016; Jenny etal. 2016). We can
be fairly certain that the lead found in the golden eagles,
bearded vultures, and common ravens originates from Swit-
zerland (or adjoining countries), because these species are
nonmigrants in Europe, whereas red kites, which are partial
migrants, may take up lead in any country on the dispersal
and migration route (mainly Switzerland, France, Spain).
It is certainly concerning that a toxicant, such as lead that
is negatively affecting all body systems, even at the lowest
measurable concentration (Pain etal. 2009), is present in
our wildlife in this magnitude and so widespread. We should
be aware that a bullet might kill—unwantedly—more than
once. There are two main possibilities to reduce the risk of
lead exposure. The first is to burry all gut piles properly,
which is an impractical method (Krone etal. 2009), and
the second is the switch to lead-free ammunition. In several
regions of Switzerland, game wardens and certain hunters
already changed from lead to lead-free hunting ammunition,
and by now lead-based ammunition is forbidden in four fed-
eral states in Germany and some other regions in Europe.
However, more effort towards a voluntary use of lead-free
ammunition or a ban of lead ammunition are needed to
relieve the avian scavengers from the lethal and sublethal
effects of lead.
Acknowledgements The authors thank Fabian von Kaenel for his
help with the ICP-MS measurements. The authors thank everyone
that provided samples for this study. Tissue samples of carcasses were
kindly provided by the Vetsuisse Faculties Bern (FIWI; Marie-Pierre
Ryser, Mirjam Pewsner, Roman Meier) and Zurich (Barbara Vogel), the
authorities of the Cantonal Fish and Game Department of the Grisons
(Werner Degonda, Gieri Derungs) and other Cantonal Fish and Game
Departments, many gamekeepers, various bird care stations (Andi
Lischke, Berg am Irchel; Vreni Mattman, Sempach; Michel Beaud,
Fribourg; Erich Widmer, St. Gallenkappel; Ulrike Cyrus, Wildstation
Landshut; Christoph Meier, Malans) and the Valais Field Station of the
Swiss Ornithological Institute. Enrico Bassi from the Stelvio National
Park and Daniel Hegglin, Stiftung Pro Bartgeier, provided data from
bones of bearded vultures. Lorenzo Vinciguerra, Ueli Schneppat and
René Heim of the Natural History Museums of St. Gallen, Grisons and
Lucerne, prepared bone samples of some golden eagles and bearded
vultures. Most red kite samples were provided by Patrick Scherler and
Martin Grüebler. They also thank Gabriele Hilke Peter, Swiss Ornitho-
logical Institute, for preparing the maps.
Compliance with Ethical Standards
Conflict of interest The authors declare that they have no conflict of
interest.
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