The major, non-natural factors of mortality that limit or cause vulture population declines are described below.
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.
Bird mortality by electrocution on power poles is a global problem that has become more prevalent in recent years as energy demand increases, resulting in infrastructure growth often in previously undeveloped areas. Electrocution associated with powerlines occurs when a bird comes into contact with two wires, one of which is live, or when it perches on a conductive pylon (for example, a metal structure) and comes into simultaneous contact with a live wire. Large species such as vultures, eagles and storks are particularly vulnerable. Eelectrocution risk can be very significant in old, badly designed and insulated poles and poorly sited power lines. Effective planning, design and mitigating measures can dramatically reduce the impact of energy infrastructure on avian populations.
Electrocution from powerlines is one of the key threats for Cape Vultures in South Africa with data suggesting that this cause of mortality makes a significant contribution to low juvenile and immature survival rates. Despite this, in certain situations, vultures might derive some benefit from the presence of power lines in relation to increased nesting, roosting sites and nursery areas, which may allow them to expand their range, especially if suitable mitigation measures can be taken to lessen the risk of electrocution.
Each year millions of birds die worldwide as a result of collisions with above ground power lines, and the impact on populations is likely to increase as energy infrastructure continues to grow, especially in developing countries. As for electrocution, the risks can be very significant in old, poorly sited power lines. Under the current commitments to reduce carbon emissions, signatories to the United Nations Framework Convention on Climate Change (UNFCCC) are increasing their investments in renewable energy, particularly large wind farms. However, any other renewable energy installations, e.g. solar and geothermal generation facilities will inevitably lead to an expansion of the powerline network which will likely increase the risk of collisions and electrocutions for vultures in certain areas. Despite their acute vision, vultures’ field of view and normal head position when foraging can make them unaware of obstructions in their direction of travel, so they may be particularly vulnerable to collisions with infrastructure such as wind turbines and powerlines. The proliferation of renewable energy initiatives can therefore be detrimental to vultures if the location of turbines and associated infrastructure are in areas favoured by these birds.
An estimated minimum number of 80 vultures (Cape and White-backed Vultures) are killed annually by collision with powerlines in Eastern Cape Province.
Decline of food availability
As obligate scavengers feeding on carcasses of various sizes, vultures are susceptible to declines in the availability of carcasses, especially of ungulates, to feed on. Four main factors could reduce food (carcass) availability for vultures. First, a reduction in the numbers of dead livestock could result from carcasses being buried or burned, or dumping sites for carcasses being closed entirely. These measures could be prompted by concerns over smell or public health campaigns to reduce the number of rotting carcasses. Second, competition for food with feral dogs and other scavengers may reduce food available to vultures. Third, reduced wild ungulate populations would diminish food availability for vultures where these are more important than livestock. The fourth is the impact of improved animal husbandry which results in fewer carcasses being available for vultures to feed on.
Livestock populations have increased significantly since the 1960s, and vultures would probably feed on livestock carcasses if local practices were to allow them to be available to scavengers. However, use of domestic ungulate carcasses for food by humans, changes in practices in butchering animals, changes in livestock management and improved sanitation at slaughterhouses may offset the increased numbers of livestock as a food source for vultures, either partly or completely. Hence, although not fully established, declines in abundance of wild ungulates are likely to have impacted vulture populations, especially where the ungulate declines have been most severe.
Provision of food at supplementary feeding sites for vultures has the potential to guarantee poison-free food, and can modify the birds’ behaviour, encouraging them to forage only in safe areas and minimising their foraging movements in areas where poisoned baits may be used.
Habitat loss, degradation and fragmentation
The impact of habitat change on vulture populations is complex although it is often cited as a contributing factor to vulture declines. This may concern large scale modification affecting food supply (considered above) or other ecological factors. More specifically, cliff or tree-nesting vultures have specific breeding site requirements, which are easily affected by human activities such as: quarrying; building of tourist or leisure facilities near breeding cliffs; widening of roads and highways; logging, other forms of deforestation and clearance of large trees in agricultural areas.
Habitat loss and degradation are suspected to have played roles in the dramatic declines of large vultures with the root cause being the rapid increase, and associated development, in the human population and loss of suitable habitat as a result of settlement expansion. Land use changes in southern Africa are varied and include degradation by intensive agriculture, cultivation, urbanisation, roads, dams, mines, desertification, afforestation and alien vegetation.
A wide range of human activities can cause disturbance, such as construction of infrastructure, agriculture, aviation, mining, blasting and quarrying. A range of human activities in close proximity to nesting sites may have an impact on breeding success and may cause abandonment of previously successful nests. These include livestock farming activities and possibly recreational activities such as mountaineering, climbing, photography and recreational aviation . A range of developments and construction could have a similar effect. Eggs and nestlings have been stolen and young birds have been attacked by vandals.
Tree nesting species are vulnerable to nest harvesting or disturbance by humans, especially outside protected areas. Cliff nesting species suffer from disturbance, especially from climbers or aviation activities close to breeding cliffs.
Climate change affects birds in different ways, altering distribution, abundance, behaviour, genetic composition, and timing of events like migration or breeding. Direct effects of climate change such as changes in temperature and rainfall patterns can also impact birds due to increased pressure from competitors, predators, parasites, diseases and disturbances such as fires or storms.
Very little work has been done or published to illustrate the impact of climate change on vultures. It is, however, speculated that the species breeding at higher altitudes (Bearded and Cape Vulture) in southern Africa may experience range contractions due to increased temperatures. There are concerns that Cape Vulture breeding colonies in the north of the species’ range are at greater risk from the effects of climate change than those in the south and that areas currently containing the bulk of the breeding population may become unsuitable for breeding. The overall impact of climate change can be more severe when it occurs with other major threats such as habitat loss and reduction in available food sources.
A range of additional threats affects vulture populations throughout their ranges, but these are often more species-specific, with more localised effects than the threats discussed above. However, particularly at breeding sites, these can have locally significant impacts on productivity, the importance of which is likely to increase if vultures continue to decline and populations become more fragmented.
- Drowning – Historically Cape Vultures were susceptible to drowning with records of at least 120 individuals (21 incidents) being killed in small farm reservoirs in southern Africa between the early 1970s and late 1990s.
- Illegal killing, taking and trade in various forms not covered above can be directly targeted at vultures. In some cases, this can be purely because of a dislike of or superstition against vultures and may involve poison, shooting or capture.
- Sport hunters may occasionally shoot at vultures as novel targets.
- Other collisions (in addition to those with energy infrastructure) such as those with aircraft, trains and motor vehicles (especially individuals feed on dead animals along the roads).
- Disease- the extent of this threat is unknown.
- Genetic bottlenecks- Small, isolated vulture populations could in the long term suffer a reduction in genetic diversity which could influence breeding success and their ability to adapt to global change, and ultimately reduce the probability of persistence of these populations.