Poisoning, in its various forms, is by far the most significant threat that impacts African vulture species. In the context of vultures there are two broad types of poisoning: unintentional (secondary) poisoning, where vultures are not the intended target; and targeted poisoning, where vultures are intentionally killed.
The use of poisons to kill wildlife intentionally has a long history worldwide. Both natural plant and animal-based toxins and synthetic pesticides have been used to kill wildlife, a method that is silent, cheap, easy and effective. Many classes of pesticides have been used to poison wildlife, including organochlorines, organophosphates, carbamates and pyrethroids.
Populations of scavengers have been decimated by feeding on poisoned carcasses. Vultures, for which the primary food source is meat, soft tissue and organs from naturally occurring carcasses, are obviously at risk. All vulture species are affected to varying degrees by unintentional (secondary) and intentional poisoning. Both south Asia and Africa have seen precipitous declines in vulture populations over the last 30 years due to poisoning. This has directly contributed to eight species in these regions currently being listed as Critically Endangered.
Unintentional poisoning occurs when vultures consume poisoned baits set out for other species or when they consume carcasses of animals that have died from poisoning. Pollution of the environment by a range of chemicals due to spills, dumping of chemical waste and other substances that can affect their food or water source can also have an unintended impact on vultures.
Farmers who experience frequent crop-raiding by elephants, buffalo and other herbivores and herders who lose livestock falling prey to predators, will occasionally resort to poisoning those animals to ‘take care’ of the problem. Synthetic pesticides are widely used as the poison of choice for killing these ‘problem’ animals such as lions, tigers, leopards, hyenas and jackal. Such use of pesticides is illegal in the vast majority of countries but implementation and enforcement of such regulations is often weak; consequently, poisoning has become the most widely used means of killing particular wildlife species. Poisoning using baited carcasses is indiscriminate and can affect a wide range of non-target species. Poisoning often does not affect the target individual or species, but instead kills a multitude of unintended species, including vultures.
NSAIDS and other veterinary medicines
In South Asia, unintentional poisoning by veterinary NSAIDs has caused catastrophic declines to vultures. The effects of poisoning with NSAIDs, and particularly diclofenac, has caused the largest population declines over the shortest timeframe of any known group of birds in history. Diclofenac was used extensively for domestic livestock and any animals that then died within two days of treatment had highly toxic levels in the tissues that would cause kidney failure and death of any vulture feeding on the carcass. Many Gyps vulture species worldwide rely on carrion from dead domestic ungulates as their traditional wild ungulate food sources have disappeared. This was the case over much of South Asia; after ingestion of livestock carcasses treated with diclofenac near to their death, vultures die as a result of visceral gout that is caused by kidney failure. Death of the vulture usually occurs within two days of exposure. Certain NSAIDs that are known to be highly toxic to vultures are becoming available elsewhere and are a significant cause for concern.
There is evidence that other NSAIDs in legal veterinary use are also toxic to vultures, as well as possibly to other scavenging birds, with just one safe alternative, meloxicam, identified so far. The clearest case concerns aceclofenac, which is a pro-drug of diclofenac, most of which is converted to diclofenac in treated cattle soon after it is administered. Hence, aceclofenac is expected to be as toxic to Gyps vultures as diclofenac is. Ketoprofen was identified as lethal to Gyps vulture species in 2009, and residues of this drug are found in ungulate carcasses in India at sufficient concentrations to cause mortality in vultures. Other NSAIDs thought to be toxic to vultures include nimesulide, carprofen and flunixin.
The impacts of lead poisoning through the ingestion of spent lead ammunition used by hunters and wildlife managers to kill game is well known for a wide range of bird species contributing to population declines as well as creating extensive avoidable deaths and sickness amongst waterbirds and scavengers. Elevated Blood Lead Levels have been found in White-backed and Cape Vultures in South Africa, Namibia and Botswana and elevated Bone Lead levels have been found in Bearded Vultures in South Africa and Lesotho. In areas where game-hunting is a significant activity the ingestion of lead fragments by vultures could have both lethal and sub-lethal effects. Elevated lead levels could have a detrimental impact on breeding productivity which is especially important for slow-reproducing species like vultures, the effects of which are compounded by small and rapidly declining populations. Sub-lethal lead poisoning also has a number of other secondary effects, such as reduced mobility or increased risk of collision.
Whilst direct mortality from poisoning is highly visible and newsworthy, all species of African Vultures are long lived and at a high trophic level (high up the food chain), which increases their vulnerability to bioaccumulation. Whilst most attention has been given to the lethal impacts of toxins on vultures, bioaccumulation may have sub-lethal but significant negative effects on reproductive success, immune response and behaviour. However, there is no robust evidence for such effects at present.
Belief-based use and the bushmeat trade
Pesticides are increasingly used to acquire wild animals or their body parts for consumption and commercial trade. Where vultures are concerned, a major driver of this trade is Traditional Medicine, in which wildlife parts and derivatives are used to treat a range of physical and mental diseases, or to bring good fortune. Vultures are sold alongside other species of birds, mammals, reptiles and other taxa at markets specialising in supplying Traditional Medicine. The traditional medicine trade associated with belief-based use has existed for many years in some areas (especially parts of west, central and southern Africa) and is accepted as cultural practice. However, not all of the uses for vultures have such a history: for example, those uses which supposedly increases a user’s chances of winning in recently introduced national lotteries and sport betting practices. With the rapid growth of human populations and more effective harvesting methods (through highly toxic poisons) the impact on vulture populations is becoming more apparent.
The other main driver of this trade is bushmeat. Many species are sold for their meat in the same markets as those sold for traditional medicine.
The recent increase in poaching of elephants has resulted in an increase in mass poisoning of vultures. Vultures are deliberately poisoned by poachers who may use large quantities of toxic pesticides on elephant carcasses because circling vultures signal potential illicit activities to those who are combatting poaching; vultures are killed because they play the role of sentinels. Between 2012 and 2014, 11 poaching-related incidents in seven (largely southern) African countries, resulted in the death of 155 elephants and 2,044 vultures. In at least two incidents, the harvesting of vulture body parts (seemingly for traditional use) may have provided an additional motive. Vulture mortality associated with ivory poaching has increased more rapidly than that associated with other types of poisoning, accounting for one third of all vulture poisonings recorded in Africa since 1970.
The scale of deaths at a single carcass can be significant, regularly exceeding 100 individuals. For example, at least 144 White-backed Vultures were killed after feeding on an elephant carcass in Gonarezhou National Park, Zimbabwe, in 2012, over 500 vultures were found dead in Bwabwata National Park, Namibia, in 2013 after feeding on a single poisoned elephant carcass, and 154 White-backed Vultures were killed in the Kruger National Park in South Africa in two incidents of feeding from poisoned elephant carcasses in 2017.