By now you probably know about the risks facing California Condors. Following a remarkable captive breeding and reintroduction program, the magnificent birds continue to suffer because they gobble up too much lead. Condors eat carrion, and the dead animals often have lead inside of them, from bullets. That lead winds up inside the condors, where it slowly poisons them. In the most severe cases, lead poisoning can kill the birds; in other cases, it can affect them indirectly by modifying reproductive behavior or their immune function. But condors aren’t the only ones suffering. Griffon vultures in Spain are also suffering from the effects of heavy metals like lead.

That’s the conclusion of a group of Spanish and Portuguese researchers led by Manuela Carneiro, a graduate student at the University of Trás-os-Montes and Alto Douro in Portugal.

Griffon vultures feed nearly exclusively on animals that have already died: they’re scavengers. Because they feed at the top of their food chain, the species risks the slow accumulation of toxins over time from the meat that they eat. That’s especially true because the vultures have come to rely on human activities for their food. In modern Spain and Portugal, most of the carcasses they feast upon came through livestock management or hunting practices.

Not only is their suffering due to heavy metal contamination—worrying itself from both conservation and animal welfare perspectives—but the vultures also serve as indicators of the level of environmental contamination more broadly. If vultures are being poisoned by lead, they’re surely not the only ones, and their decline could represent broader problems within their ecosystems.

While Griffon vultures are classified as a species of “least concern” by the IUCN, they are classified as “near threatened” within Portugal. That’s because there was a widespread decline in their numbers throughout the Iberian Peninsula in the 20th century, mainly because they fed on poisoned bait intended for carnivores, but also due to shifting livestock management practices. For example, the EU legislated in 2001 against the disposal of livestock carcasses in the field as a means of curbing the spread of bovine spongiform encephalopathy, or mad cow disease, which meant that the vultures had to work harder to find food. That reduction in food availability has forced some vultures to seek food in garbage dumps, where they risk ingesting heavy metals as well. In addition, they ingest lead from bullet fragments embedded in the meat they consume. Last, intentional (and illegal) poisoning is the most significant non-natural cause of mortality for large birds of prey, including Griffon vultures.

To evaluate the health of Spain and Portugal’s Griffon vultures, the researchers sampled the blood of 121 Griffon vultures (Gyps fulvus) on the Iberian Peninsula and assessed their samples for mercury, cadmium, and lead. They compared the concentration of those heavy metals in the blood of wild vultures and in individuals living in avian rehabilitation centers. (The birds were admitted to those centers either due to malnutrition or because they were wounded.)

While the majority of the birds were free of cadmium (98.3%) and mercury (95%), every wild vulture had at least some lead in its blood, ranging from 4.97 to 300.23 μg/dl (micrograms per deciliter).

On average, those vultures caught in the wild had more lead in their blood than those admitted to wildlife rehabilitation centers. That’s perhaps surprising, since the wild birds appeared externally healthy, at least in comparison to those brought for care to veterinary facilities. That’s even true for the one individual with more than 300 μg/dl of lead in its blood, a level at which it is possible to clinically diagnose lead toxicity. Remarkably, that individual did not show any behavioral differences from those with subclinical levels of lead in their blood. On the other hand, three quarters of the vultures admitted to rehab centers were malnourished; if they were eating less food, they ostensibly would have had fewer opportunities to accumulate the heavy metals lurking within that food. Either way, it’s bad news for the vultures.

What does it mean that the wild vultures appeared healthy, despite their lead accumulation? The researchers point out that even at sub-clinical levels, most of the vultures in the study had higher concentrations that the minimum threshold at which their antioxidant systems can become affected. “As a result, [the] birds may become increasingly susceptible to starvation and infection by disease,” they write, “increasing the probability of death from other causes.” And even sub-clinical lead toxicity can disrupt their reproductive success.

Given that, Carneiro and her colleagues urge researchers to more closely evaluate the effects of sub-clinical lead accumulation in vultures: they may be sub-lethal, but still toxic. In addition, they advocate for the prohibition of lead-based ammunition in big-game hunting activities, or perhaps for an incentive-based program to motivate hunters to switch voluntarily from lead-based ammunition to other varieties. Third, they suggest that new livestock management regulations by implemented that would allow parts of dead animals to be left in natural areas where they can be eaten by Griffon vultures and other scavenger species. That, they think, could reduce the rate at which juvenile vultures become malnourished, and may also reduce the vultures’ motivation to scavenge in garbage dumps. – Jason G. Goldman | 03 June 2015

Source: Carneiro, M., Colaço, B., Brandão, R., Azorín, B., Nicolas, O., Colaço, J., Pires, M.J., Agusti, S., Casas-Diaz, E., Lavin, S., & Oliveira, P.A. (2015). Assessment of the exposure to heavy metals in Griffon vultures (Gyps fulvus) from the Iberian Peninsula. Ecotoxicology and Environmental Safety, 113, 295-301. DOI: 10.1016/j.ecoenv.2014.12.016.

Header image: Griffon vultures admitted into a wildlife rehabilitation center. Copyright Carles Martorell, used with permission.