Lead (Pb) is a highly toxic heavy metal and lead poisoning manifests itself as a cumulative, multi-systemic condition affecting all body systems (Locke and Thomas 1996, Pain 1996, Redig and Cruz-Martínez 2009). Lead poisoning is the most frequent form of heavy metal poisoning in birds of prey and is one of the most important causes of mortality of raptors worldwide (Franson 1996, Mautino 1997, Pain et al. 2005, Fisher et al. 2006). Lead poisoning has been implicated in the decline of a number of critically endangered raptors (Kramer and Redig 1997, Kenntner et al. 2001, Clark and Scheuhammer 2003, Pain et al. 2005) including the California Condor (Gymnogyps californianus; Green et al. 2008) and a number of Old World vultures such as the Griffon Vulture (Gyps fulvus), Pyrenean Bearded Vulture (Gypaetus barbatus), Cinereous Vulture (Aegypius monachus) and Egyptian Vulture (Neophron percnopterus; Hernández and Margalida 2009, Nam and Lee 2009, Rodriguez-Ramos et al. 2009, Kelly et al. 2011, Lambertucci et al. 2011, Stringfield 2012).

Consumption of prey containing lead shot or bullet fragments is generally perceived as the primary source of lead poisoning in raptors (Kendall et al. 1996, Mateo et al. 1999, Clark and Scheuhammer 2003, Fisher et al. 2006), and for a number of species this has been confirmed through isotopic analysis (Finkelstein et al. 2014, Madry et al. 2015). The high velocities of modern firearms cause lead-based bullets to fragment widely along the wound tract (Stroud and Hunt 2009). The level of fragmentation makes scavengers, such as vultures, highly susceptible to lead poisoning from carcasses or gut piles left in the field (Clark and Scheuhammer 2003, Green et al. 2008, Hunt et al. 2009). Vultures dissolve and absorb lead soon after ingestion due to their low stomach pH, often resulting in acute death (García-Fernández et al. 2005, Fisher et al. 2006). Sublethal or chronic exposure (prolonged exposure at lower concentrations) is also of concern because it can make birds less fit and predispose them to other causes of death (Mateo et al. 2003, Pain et al. 2007, Berny et al. 2015) or may affect reproductive success (Buerger et al. 1986, Gil-Sánchez et al. 2018), behavior (Scheuhammer 1987), or physiology (Burger 1995, Fair and Ricklefs 2002, Naidoo et al. 2012).

Blood lead levels are a good indicator of recent exposure and the degree of lead poisoning over short periods of time (Tirelli et al. 1996), because the half-life of lead in blood is 14 d (Frey et al. 2003, Pain et al. 2009). Bone lead levels represent repeated exposure and absorption of lead, because bones are long-term repositories of lead (Pain et al. 1993, Franson 1996, Clark and Scheuhammer 2003, Gangoso et al. 2009). Long-lived species, such as vultures, are particularly susceptible to the bioaccumulation of lead in bone tissue (Gangoso et al. 2009, Behmke et al. 2015).

Vulture populations across Africa have undergone massive declines, with seven species declining at a rate of >80% over three or more generations. The declines have been attributed mainly to poisoning and trade in traditional medicines (Ogada et al. 2016). Similarly in southern Africa, the Bearded Vulture population has declined by >30% during recent decades and has been listed as regionally critically endangered with only approximately 100 pairs remaining (Krüger et al. 2014a, Krüger 2015). The influence of lead poisoning on the decline of the vulture populations in Africa has been relatively poorly explored. However, recent research has suggested that it may be a more significant issue than previously appreciated (Naidoo et al. 2017, Garbett et al. 2018). We examined the blood lead levels of captured individuals and bone lead levels of Bearded Vultures found dead throughout their range in southern Africa to determine the level of exposure, and to ascertain whether lead poisoning may pose an important threat to this small and isolated population.

Methods

Study Area and Data Collection

Lead levels of Bearded Vultures were determined from blood samples (11) and bone samples (8) collected across the species' southern African range in the Maloti Mountains of Lesotho (n = 1) and in the Drakensberg Mountains of the KwaZulu-Natal (n = 11), Free State (n = 5) and Eastern Cape (n = 2) provinces of South Africa (Fig. 1).

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Blood was collected from individuals that were captured at supplementary feeding sites between 2009 and 2012 when birds were fitted with satellite transmitters (n = 10), and marked as a nestling in the nest (n = 1). Blood was taken from the brachial vein using a sterile hypodermic needle and syringe, and samples were immediately transferred into EDTA coated tubes. Five of the blood samples, collected in 2009, were submitted to a commercial laboratory (Ampath Pty. Ltd., Pretoria, Gauteng, South Africa) and whole blood lead concentrations were determined using graphite furnace atomic absorption spectrometry (GFAAS) following a preparation step with 0.1% Triton X-100. Results from these five samples were reported previously (Naidoo et al. 2017). The remaining six samples were analyzed by V&M Analytical Toxicology Laboratory Services Pty. Ltd. (George, Western Cape, South Africa), using the Inductively Coupled Plasma Mass Spectrometer (ICP-MS) method.

Bone samples were collected from seven individuals that were tracked with GPS tags (see Krüger et al. 2014b for more details) and found dead during the study period (2009–2017), and one non-tagged individual that was opportunistically found dead. Four of these individuals had been tested for blood lead levels when they were captured in 2009 (n = 2) and 2012 (n = 2). The bone lead concentrations were determined by (1) Biochemical and Scientific Consultants Pty. Ltd. (Hilton, KwaZulu-Natal, South Africa), using the GFAAS method (n = 6) and the ICP-MS method (n = 1), and (2) V&M Analytical Toxicology Laboratory Services Pty. Ltd. using the ICP-MS method (n = 1). The quantitative determination of lead in blood/bone by the ICP-MS method is a more modern technique with wider application scope and was used on the more recent samples because it is a more accurate and sensitive method.

Results and Discussion

A total of 19 samples from 15 individuals were tested for lead levels (Table 1). The average bone lead concentration in our study was 11.79 ± 8.34 μg g⁻¹ (0.50–20.92 μg g⁻¹, n = 8), which suggests that the individuals were exposed to lead. Two non-adults had concentrations suggestive of pathological exposure and absorption of lead (19.96 and 20.92 μg g⁻¹), which may well have been the cause of the birds' deaths, as levels >20 μg g⁻¹ indicate chronic poisoning (Gangoso et al. 2009) and concentrations >20–30 μg g⁻¹ were found in birds that had died of lead poisoning (Pain et al. 2007).

Table 1.  Bone (μg g⁻¹) and blood (μg dL⁻¹) lead concentrations in the southern African Bearded Vulture population obtained from birds across the Maloti-Drakensberg Mountain region. EC is Eastern Cape province, KZN is KwaZulu-Natal province, FS is Free State province, and LES is Lesotho. Letters in parentheses after Pb levels indicate whether the individual was an adult (A) or non-adult (N) at the time the sample was taken.

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The average blood lead concentration using the more sensitive analysis (ICP-MS) was 0.62 ± 0.81 μg dL⁻¹ (0.00–2.08 μg dL⁻¹, n = 6); however, concentrations measured using the less sensitive GFAAS analysis were all classifed as <3 μg dL⁻¹, the limit of quantification for this method (n = 5) and therefore not included in the graphical representation of our results. All our blood samples had lead levels far below those indicative of lead exposure (20–40 μg dL⁻¹), suggesting only background levels of exposure to lead.

For two birds we had blood samples collected when they were alive and bone samples collected following their deaths 2 yr and 4 yr later. In both cases their blood levels were relatively low (<3.0 μg dL⁻¹), but their bone lead levels showed higher levels of exposure (19.96 μg g⁻¹ and 16.60 μg dL⁻¹ respectively). These results are suggestive of a temporal change in exposure and an accumulation of lead in the bone tissue over time. Although samples sizes are small, it would appear that both males and females of all ages may be at risk (Table 1). Our results suggest that while the southern African Bearded Vulture population does not show signs of recent or short-term exposure to lead, it does show exposure to lead over the long term.

Based on average blood lead concentrations, the lead exposure of the southern African Bearded Vulture population (<3 μg dL⁻¹) is similar to that of Bearded Vultures in the Pyrenees (3.6 μg dL⁻¹; Hernández and Margalida 2009). It also appears somewhat lower than that of Cape Vultures (Gyps coprotheres) in southern Africa (12.5 μg dL⁻¹; Naidoo et al. 2017), African White-backed Vultures (Gyps africanus) in South Africa (7.81 ug dL⁻¹; Van Wyk et al. 2001 and 15.5 μg dL⁻¹; Naidoo et al. 2017) and Botswana (10.7 μg dL⁻¹; Garbett et al. 2018), but well below that found in Griffon Vultures exposed to lead bullets in Spain (43.07 ± 3.96 μg dL⁻¹; García-Fernández et al. 2005; Fig. 2).

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However, bone lead concentrations in this study (11.8 μg g⁻¹), were higher than those found in Pyrenean Bearded Vultures (2.5 μg g⁻¹; Hernández and Margalida 2009) and Egyptian Vultures on the Iberian Peninsula (6.2 μg g⁻¹) and Canary Islands (7.4 μg g⁻¹; Gangoso et al. 2009) but were lower than those of White-backed Vultures in South Africa (27.1 μg g⁻¹; van Wyk et al. 2001; Fig. 3).

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Lead toxicity in avian species manifests itself as a cumulative, multi-systemic syndrome affecting the liver, kidney, heart, gastrointestinal, hematopoietic, reproductive, and nervous systems (Locke and Thomas 1996, Redig and Cruz-Martínez 2009). The ultimate consequences of poisoning by lead can be readily underestimated, given that the sublethal effects are difficult to detect and may only show over a very long time period and in combination with other factors (Franson et al. 1983, Mateo et al. 1997, Smits et al. 2007). For example, the accumulation of lead in bones from lead ammunition affects the degree of bone mineralization in Egyptian Vultures, which could mean an increase in bone fragility (Fleming et al. 2000, Whitehead and Fleming 2000). The accumulation of lead over time is an additional threat to the species because it can affect the fitness of a population, including reproduction (Gil-Sánchez et al. 2018), thus affecting the long-term survival of long-lived species. The effects of the cumulative exposure to lead on this population are difficult to assess with the low sample sizes in this study. However, lead exposure may render individuals more susceptible to other agents of mortality, such as poisoning and collisions with power lines or cables (Berny et al. 2015), which may in turn affect population dynamics.

Lead is a highly toxic heavy metal, which can be released into the environment mainly by industrial, mining, and hunting activities (Fisher et al. 2006). South Africa is known to have high soil and air lead concentrations linked to mining activities (Herselman 2007, Mathee 2014). Although there are no major industries within the species' distribution range in southern Africa, some localized mining does occur. However, Naidoo et al. (2017) found no relationship between blood lead levels in vultures in southern Africa and vicinity to mines or soil lead concentrations, and three soil samples collected from within our study area had background lead levels (<10 μg g⁻¹, S. Krüger unpubl. data). Naidoo et al. (2017) concluded that vultures in southern Africa are exposed to another source of lead, with lead bullets being the most likely source. In Botswana, elevated blood lead levels in White-backed Vultures were closely associated with hunting seasons and hunting areas, providing strong support for the idea that spent lead ammunition is the major source of lead exposure in southern African vultures (Garbett et al. 2018).

Lead poisoning, resulting from feeding on carcasses shot with lead ammunition, is one of the main threats to the species in the Pyrenees (Hernández and Margalida 2009, Berny et al. 2015) and vultures elsewhere (Gongoso et al. 2009, Mateo 2009, Nam and Lee 2009, Rodriguez-Ramos et al. 2009, Kelly et al. 2011, Lambertucci et al. 2011, Stringfield 2012). Although hunting activities within the species' southern African range are limited, there are numerous supplementary feeding sites where animals are placed after they have died or been shot with lead ammunition. Lead presents an unnecessary additional threat to the species; therefore, we recommend the ban of hunting or culling with lead within the species' distribution range and the replacement of lead-based ammunition with nontoxic alternatives. We also advocate an education program aimed at those managing feeding sites, to ensure that animals killed using lead ammunition are properly processed or not placed out at all.

Our results provide a preliminary indication that lead poisoning is an additional threat to the southern African Bearded Vulture population that may predispose individuals to other threats (Berny et al. 2015). Further studies are required to ascertain whether lead poses a significant threat to populations of this threatened species in the long term by affecting reproductive success or survival (Gil-Sanchez et al. 2018). The potential risk of lead poisoning must also be considered in future reintroduction programs, which are actively being considered for the region (Brink 2016).