Effects of Lead Exposure in Wild Birds as Causes for Incidents and Fatal Injuries

Abstract

Lead is among the most toxic heavy metals, posing significant risks to all living organisms. It is a pervasive and persistent contaminant in the environment. Ingested lead in birds and wildlife induces a range of sublethal effects that disrupt physiological functions and behavior, ultimately resulting in mortality at higher doses or with prolonged exposure. To investigate the relation of lead to accidents and injuries in wild birds, we analyzed lead concentration in 43 wild birds that were admitted as patients to the Wildlife Rehabilitation and Breeding Center (WRBC). The findings reveal a significant dependency between the detected levels of lead in the birds’ bone samples and the reported etiology of their injuries, with variances in the age groups of the patients received at the WRBC in Bulgaria.

1. Introduction

Wildlife population balance and biodiversity are highly affected by various anthropogenic factors, among which we could point out the use of lead ammunition as it affects the living organisms in both direct and indirect ways. As a representative of the group of heavy metals, lead (Pb) is a persistent contaminant in the environment and a toxic agent [1]. Its toxic effects are manifested when Pb is bound to the sulfur-containing components of the enzymes and proteins [2], which results in behavioral disorders and neurological symptoms. Regarding ecosystems and wildlife health monitoring for conservation purposes, institutions like wildlife rescue centers serve simultaneously as units for treatment and rehabilitation of injured animals and, at the same time, as a network for surveillance of emerging ecological changes, environmental contamination and emerging wildlife diseases [3,4].

The risk of lead poisoning in wildlife arises due to:

• Direct ingestion of lead pellets by animals;
• Ingestion of lead through its presence in the food consumed by wild animals;
• Lead exposure due to embedded pellets in body tissues from shooting incidents;
• Accumulation of environmental lead in wildlife due to its presence in metal rich sludge in mining areas, marshlands and other contaminated zones.

Birds in wetland areas are at risk of ingesting lead pellets from surface sediments while foraging for grit or mistaking them for seeds. Granivorous birds may ingest lead residues from gunshots, especially in areas where hunting is intensively practiced [5].

Lead poisoning is estimated to cause the death of around 1 million wild birds per year in Europe and the sub-lethal poisoning of around 3 million more [6]. Wildlife sensitivity to lead poisoning is influenced by various biological and ecological factors, and species-specific susceptibility to the effects of this heavy metal varies among wild animals and birds. Research on the topic suggested that these differences could be influenced by other factors such as birds’ demographics and various causes of accidents [7]. In this regard, Ref. [8] pointed out that lead absorption from the ingested content to the bloodstream is increased in Bearded vultures due to their highly acidic gastric environment. In addition to the direct impacts of lead on health and survival, indirect effects are also likely to occur. These may include increased susceptibility to infectious diseases, parasite infestations (due to the immunosuppressive effects of lead), as well as a higher risk of collisions with power lines and being shot, as lead exposure affects vision, coordination, and muscle strength [9].

Lead poisoning in birds and wild animals through ingestion causes a series of sublethal effects affecting their physiology and behavior, and with higher and/or prolonged exposure, it can lead to death. Studies on birds have shown that lead can impair liver function and reproduction [10,11,12]. Poisoning due to ingested lead pellets also negatively affects bird behavior, leading to significant weight loss and eventual death [13].

Mortality from lead poisoning, including Pb from ammunition used in hunting game animals, is a significant problem for long-lived, slow-reproducing species, such as certain birds of prey [14]. It was found by [15] that raptors and other birds near general migratory routes often fall victim to illegal shooting. Such a cause for mortality in vultures was reported as well by [16], and while illegal shooting could be the direct cause for death, on some occasions, the birds, if not fatally injured, could continue to live with embedded projectiles in their tissues and bones. Ref. [17] argued that lead residual fragments could be an indirect source for lead toxicosis.

Increased mortality in small populations due to the effect of lead pellets on adult individuals poses risks to the species [18]. After ingesting or through embedded lead pellets or fragments, lead can be absorbed through the digestive tract, enter the bloodstream, and quickly accumulate in soft tissues, primarily the liver and kidneys, as well as in bones and feathers [19]. Lead accumulates in bones over a prolonged period, while lead in soft tissues remains for a shorter duration (weeks to months). Lead levels in the blood remain elevated for weeks or months after the ingestion of lead pellets [20].

The issues on adverse lead effects on public health were addressed at the international level long ago, starting in 1921 with the introduction of the White Lead (Painting) Convention [21], which came into force in 1923 and was signed by many European countries shortly after this with a ratification by Bulgaria in 1925. With the persistence of lead in the environment, further legal actions were taken by banning leaded petrol in the EU in 2000 [22], prohibiting the use of lead in new electrical and electronic equipment placed on the EU market in 2006 [23], and member states to gradually remove the use of lead ammunition in wetlands by 2009 [24]. While research has proved the negative impact of Pb as a heavy metal on the animal health and welfare, biodiversity and conservation issues in wildlife, the legal framework of the European Union [25] strives to reduce the risks of lead to wild species, game and domestic animals by imposing a ban on the use of lead ammunition [26]. Analysis of the progress of restriction of Pb exposure with regard to wildlife conservation and biodiversity concerns has shown that 23 countries in Europe have successfully restricted the lead shot use within varying areas (some wetlands, waterbird hunting or all habitats included) [24]. However, over the last years, there were still registered cases of lead intoxication among various wild species in Bulgaria, which provoked the scientific interest to study the effect of Pb on wild birds, especially the potential impact on their organisms, leading to incidents with traumas and injuries.

2. Materials and Methods

The study dataset was prepared from 43 wild birds admitted to the Wildlife Rehabilitation and Breeding Center (WRBC) in Stara Zagora, Bulgaria, during the period 2019–2023. Patient data were obtained through authorized personal access to the WRBC electronic database. All of the birds investigated in this paper are listed as protected species under the specific terms of the national and international legal framework on biodiversity. The total number of 43 birds was grouped into three categories, generalizing their species characteristics as: scavengers and raptors (n = 10; Aegypius monachus, Gyps fulvus, Circaetus gallicus, Aquila chrysaetos, Bubo bubo), storks (n = 28; Ciconia ciconia) and waterbirds (n = 5; Cygnus olor, Pelicanus onocrotalus). Although most of the specimens in this study were admitted during May and July—outside the active hunting season—a clear link to hunting activities cannot be established. This is due to the fact that our sample does not represent the full range of gunshot cases, which limits the ability to identify clear seasonal patterns. Moreover, lead accumulation in bone tissue is a chronic process, and its effects may manifest months after the initial exposure.

For the purpose of the study, we listed the etiological factors that led to the patients’ injuries as natural causes (juvenile, fallen from the nest), anthropogenic causes (collision, entangled with a cord wire, poisoned) and a group with unidentified causes. The patients were also distributed into categories related to their age characteristics (juvenile and adult individuals). We also determined the primary institution/person that found the victim and forwarded it to the WRBC for treatment or necropsy—zoos, NGOs, regional environmental departments, and private persons. It should be clarified that all wild animal species, including protected, rare, and vulnerable ones, fall under the authority of the Ministry of Environment and Water (MEW) as the competent authority in the Republic of Bulgaria for the protection of wildlife. Its territorial units, the Regional Inspectorates of Environment and Water (RIEW), are the local structures responsible for wildlife conservation, including the reception and referral of injured or distressed animals for therapy.

Sample Collection for Lead Detection

Our dataset consisted of 43 wild birds (10 of which were admitted dead to the WRBC for necropsy; the rest were admitted in very bad overall condition—15 of them were subjected to euthanasia, and the remaining 18 birds died due to the fatal traumas). From each patient, a bone sample was taken in order to measure the lead concentration. We used preferably the femur, while for the smaller birds, bone samples were taken from the humerus. All samples afterward were transported to an accredited national laboratory, where the lead concentration in bone tissue was measured through mass-spectrophotometry. The laboratory employed the standardized method BDS EN 15763:2010—Determination of elements by inductively coupled plasma mass spectrometry (ICP-MS) [27]. The protocols from the laboratory analysis indicated the levels of lead in the samples, and, as interpreted by [28], we considered bone lead concentration with values more than 10 mg/kg dry weight as elevated, while levels of 20 mg/kg dry weight and higher were associated with lead toxicity with clinical manifestation.

All data were statistically processed (IBM SPSS SPSS-Inc., 2019, version 26.0, Chicago, IL, USA). The investigated parameters were analyzed through descriptive statistics (frequency distribution) and an ANOVA test. The significance of the results was represented by the exact p-value (2-tailed), known in SPSS 26.0 as Exact Sig (2-sided). The significance was interpreted at p < 0.05. The final results were presented in tables.

3. Results

The frequency distribution of lead concentrations in bones showed a significant difference among species groups (

Table 1. Lead bone concentrations from samples of wild-bird patients of WRBC.

	Bird Species/Lead Concentration	N of Birds	Lead: Mean ± SE (mg/kg)	Group Lead: Mean ± SE (mg/kg)
Scavenger and Raptors	Aegypius monachus	2	0.55 ± 0.39	8.05 ± 3.16
	Aquila chrysaetos	1	31.04
	Bubo bubo	2	10.13 ± 8.89
	Circaetus gallicus	1	0.96
	Gyps fulvus	4	6.79 ± 1.71
Storks	Ciconia ciconia	28	1.88 ± 0.54	1.88 ± 0.54
Waterbirds	Cygnus olor	4	1.41 ± 0.41	1.14 ± 0.41
	Pelicanus onocrotalus	1	0.07
Total		43		3.23 ± 0.89
Significance		F = 5.41; p = 0.008

). Only 1 of the 43 samples studied shows a lead concentration higher than 20 mg/kg dry weight, which is an indicator of lead poisoning with clinical signs. This result was observed in a Golden Eagle (Aquila chrysaetos), where two embedded lead pellets were found upon X-ray examination. Results from two other samples showed bone lead concentrations between 10 and 20 mg/kg dry weight, which is considered sublethal poisoning. These concentrations were found in Bubo bubo and Ciconia ciconia. Three other samples had lead concentrations between 5 and 10 mg/kg dry weight, all from scavenger birds (Gyps fulvus). The statistical (ANOVA) analysis of the results for lead levels in bones showed a significant relationship between species (F = 5.41; p = 0.008) and the elevated concentrations of the heavy metal.

The reasons for admission to WRBC for 9 of the total 43 studied specimens were successfully identified, and the main etiological causes that led to the incidents were grouped into four categories—juvenile, fallen from nest; entangled in cord; poisoning; collision (

Table 2. Etiological factors as causes of incidents and bone lead concentrations detected in the injured birds.

Etiological Factors—Causes of Incidents		N of Birds	Lead: Mean ± SE (mg/kg)	Group Lead: Mean ± SE (mg/kg)
Identified causes	Juvenile, fallen from the nest	1	1.99	8.27 ± 3.53
	Entangled in cord	1	0.68
	Poisoning	2	9.61 ± 0.04
	Collision	5	10.50 ± 6.23
Unknown causes		34	1.90 ± 0.46	1.90 ± 0.46
Significance		F = 10.34; p = 0.003

). Within these groups, two young individuals of the species Ciconia ciconia (n = 2) had an accident in the nest (fallen from the nest and entangled in cord at the nest), and the levels of lead measured in their bones were low (below 2 mg/kg dry weight). For five of the examined patients (n = 5), the main cause of death was collision with a power line. For these patients, the highest amounts of lead content in the bones were also found—over 10 mg/kg dry weight. The last two patients (n = 2) had an etiology of poisoning, and both were griffon vultures. One of them was detected with an increased bone lead concentration (9.65 mg/kg dry weight), while the other was found with elevated levels of lead (9.57 mg/kg dry weight) and residues from another substance—pesticide carbofuran (5.92 mg/kg).

A large portion of the samples examined were from patients who arrived at WRBC with an unknown cause of injury (n = 34) (they were found in poor condition, without any assumption of the cause). For them, the average bone concentration of lead was also low (1.90 mg/kg dry weight). Due to the relatively small number of individuals with clearly identified causes of admission (n = 9), we grouped them together under the category “Identified causes” for the purpose of statistical comparison with the much larger group of birds with “Unknown causes” (n = 34). While we initially presented the identified causes in four subcategories (juvenile, entangled, poisoning, collision) for descriptive purposes, the statistical analysis compared two broader groups: “Identified causes” (all nine individuals combined) versus “Unknown causes”. There was a significant difference found between the Pb values measured in bones and the etiological factors that caused the incidents, i.e., traumas and/or death to the patients admitted to WRBC (F = 10.34; p = 0.003).

Nearly three-fourths of the wild birds were adult individuals (n = 32; 74.42%), and the lead levels in bone samples from these patients were higher compared to younger birds (n = 11; 25.58%) (juveniles—under one year or before/during their first flight). Although the fact that the highest bone lead concentration was found in the Golden Eagle (Aquila chrysaetos) upon reaching adult age, with the inclusion of this specimen within the group of “adult” birds whose bone lead concentrations were not significantly elevated, there was no statistically significant difference between individuals in different age categories (F = 1.22; p = 0.275) with regard to the Pb bone levels (

Table 3. Age groups of the wild birds and the corresponding bone lead concentration in the injured/dead patients from the same category.

Age Group	Adults	Juveniles-0	Total	Significance
N of birds	32	11	43	F = 1.22 p = 0.275
Lead: Mean ± SE (mg/kg)	3.81 ± 1.17	1.56 ± 0.33	3.23 ± 0.89

).

4. Discussion

Understanding the source and nature of lead exposure is essential when interpreting its effects on wildlife. Acute and chronic lead poisoning may result from different exposure pathways, with some species, such as waterfowl and game birds, accidentally ingesting lead pellets, mistaking them for food items like grains [5]. In contrast, our study found that these species generally exhibited low lead concentrations. The highest lead levels were observed in raptors and scavengers, which are more likely to be exposed to embedded lead fragments retained in tissues, rather than through ingestion of pellets with food, as reported by [29]. This highlights the importance of species-specific exposure routes and their implications for lead accumulation and toxicity.

The highest lead concentrations in bone samples detected in our survey were found in a Golden eagle (Aquila chrysaetos), which is a bird of prey. Similarly, very high levels of lead are reported in the same species by [30,31]. The authors suggest that such values correspond with substantial lethal contamination and acute poisoning [6]. Given the history of the golden eagle in our study—being shot and surviving, and afterward dying from electrocution—we argue that these extreme bone concentrations are due to the recent lead exposure from the embedded ammunition fragments, which is consistent with the findings of [28]. However, we suspect that this particular golden eagle suffered from some neurological or behavioral disorders due to lead intoxication effects, which led to its entanglement in power lines and subsequent death from electrocution.

Investigating the link between the lead concentrations in bone samples from our specimens and the causes that led to their admission as patients to the WRBC, we speculate that levels of 1.99 mg/kg in the juvenile individuals fallen from the nest, appeared not to be a result from ingested pellets delivered with food by their parents, as [32] also reported significantly lower heavy metals concentrations, including Pb, in juveniles.

Similar values of 1.90 mg/kg were detected in the majority of our study specimens with unknown reasons for their traumas or accidents, on which basis we rejected the hypothesis of lead intoxication as a cause [28]. Supposedly, some explanations could be related to the animal species characteristics, as Ref. [33] reported that higher values of lead were detected in carnivorous birds, including scavengers, when compared to other non-carnivorous species like insectivores. However, for those birds with etiology of poisoning or collision and high levels of Pb in their bone samples, we assumed that the elevated lead values were the reasons for behavioral disorders in the patients that led to subsequent secondary incidents [9].

According to the regulations for the functioning of the established rescue centers [34], when animals are admitted for treatment and/or rehabilitation, the Center’s specialists should coordinate their activities with the Regional Inspectorate for Environment and Water. Actually, half of our patients (n = 22) in the lead bone investigation were reported by the Regional Inspectorates for Environment and Water (RIEW), which are the local governmental structures responsible for wildlife protection in Bulgaria. At the same time, another one-fourth (n = 10) of the sampled birds were found and forwarded to the WRBC for treatment by various private entities and citizens. We argue that the discovery or collection of injured birds by private individuals in the absence of specialists at the scene of the incident appears to be the reason for obstacles in the proper collection and interpretation of information and physical evidence. This in turn leads to difficulties in determining the main cause of the incidents, as for 34 of all 43 of the patients in our study, the etiological factor remained “unknown”. The indicator regarding the type of the sending institutions is close to the findings of [35] on the Western European centers, where most of the animals are transported by competent authorities, such as forest rangers (75%), other police authorities (9%) and private individuals (12%). We consider that these organizations play a crucial role in the protection of biodiversity and raising awareness of wildlife incidents and crimes, among which rescue and rehabilitation centers are a key conservational unit and tool for monitoring and detecting environmental threats [36].

Some authors suggest that there is a connection between the age of the birds and the levels of lead detected in their organisms [37], especially when feeding habits are considered as a possibility for poisoning [38]. Although the present study did not find any statistical significance for these parameters, we detected values of 1.56 mg/kg Pb in the bone lead samples in the group of young patients—juveniles (n = 11)—which is in line with the findings of [31] for scavenger birds in their first months of life. Compared to non-scavenging birds, the results are also similar to those for Ciconiiformes, for which lead levels in juvenile storks were also reported to be below the values at which harmful effects appear [39].

Age is considered to be a factor related to higher poison-related mortality rates in older birds of prey [40]; however, the causes for incidents identified in our survey encompassed many more factors. When distributed within the group of adults (n = 32), our specimens also showed low lead concentration levels of 3.81 mg/kg. Ref. [31] determined a significant difference between the juveniles and adult categories when bone lead values were analyzed, and while in our adult patients the Pb concentrations are higher than those in the younger ones, still, the lead concentrations are below the threshold.

5. Conclusions

Despite the strict restrictions on the use of lead ammunition in hunting, environmental contamination and the associated risks to animal and human health posed by lead continue to be a significant problem. The present study found that although the concentration of lead in the bones of some protected bird species admitted to the national wildlife rescue center with various injuries remains below the established threshold values for clinical and subclinical intoxication, its presence is undeniable. Furthermore, we found that the problem is particularly significant in predatory and scavenger bird species. We further determined that illegal practices associated with the unregulated shooting of protected species could result in embedded lead pellets in bone tissue and subsequent prolonged release of the heavy metal, which can cause lead poisoning. This poisoning, in turn, has the potential to affect the behavior of the affected wildlife, increasing the likelihood of secondary incidents, including collisions with the power grid.