The Cape Vultures of Marakele National Park

An adult Cape Vulture weighs up to 11kg with a wing span of about 2.5 metres. Adults lose the vertical streaking seen on the plumage of young birds.

Cape Vultures are a Red Data Species, classified as “threatened” and “vulnerable”. Reduced food availability, poisoning, human disturbance at nesting colonies, electrocution on power lines and drowning in farm dams are all considered threats to Cape Vultures.

Vultures are a specialised group of raptors (birds of prey) that feed mainly on carcasses of animals that they do not kill. Within the broad “old world vulture” category there are groups of related species that specialise on certain food types like large carcasses vs. small carcasses, soft tissues vs. harder tissues, etc. The Cape Vulture (Kransaasvoël) is one of eight species of the scientific genus Gyps. Gyps vultures feed mainly on soft tissues which includes muscle and internal organs of animals like Wildebeest, Springbok and domestic livestock. Gyps vultures have features to facilitate feeding on these tissues. All have a long bare neck to reach deep inside ungulate carcasses through holes in the hide. A very sharp beak quickly cuts through the flesh and a tongue with backward pointing barbs moves rapidly back and forth, pushing food down the bird’s throat, often filling the bird’s crop in a minute or so. A large group of Gyps vultures can strip a carcass in a few minutes, leaving only skin and bones.

The Cape Vulture is the second largest of the Gyps vultures, weighing up to 11 kg, with a wingspan of about 2.5 metres. By weight, it is the largest African Vulture.

Though people often believe vultures are dependent on lions and hyeanas for food, very little food is gained by vultures from these predators. Mammalian carnivores are competitors with vultures. Gyps vultures feed mainly on carcasses of animals that die of old age, disease or accidents. Vultures are excellent at finding and exploiting this resource, often using cues from crows, other scavenging animals or even humans to locate them. Their ability to travel large distances in a short period of time and excellent eyesight give them an advantage over large mammalian carnivores in finding and utilising the carcasses of the migratory ungulates, with which they evolved.

As wild ungulate herds dwindled, the birds have switched to carcasses of the domestic livestock that replaced them. Today, Cape Vulture colonies are found near traditional communal grazing areas, where stocking rates are greater, veldt conditions poorer and livestock mortality higher and therefore more available carcasses than in commercial farming areas, where livestock is more closely managed and mortality lower. As people have migrated to the cities from communal grazing areas, resulting in less farming, or when these areas are converted to commercial farms, livestock numbers and carcasses for vultures have declined. This accounts for a general drop in the number of breeding pairs of vultures, over much of their range. Though Marakele National Park has provided protection for the breeding sites of the Cape Vultures, the birds still mainly forage to the west of the park, and the neighbouring country of Botswana, traveling up to 180 km from the colony for food.

Breeding Cape Vultures are constrained by the presence of the cliffs where they make there nests. Cape Vultures are limited to southern Africa. Historically, Cape Vultures bred throughout most of southern Africa, where suitable cliffs were present. They are absent, as breeding birds, from areas where there are no suitable cliffs like central Kalahari and the Kruger National Park.

Study of the Kransberg Cape Vulture colony at Marakele National Park began in 1981. In the early 1980’s almost a thousand pairs of Cape Vultures nested on a 5.1 kilometre section of cliff on the farm Groothoek at the south western corner of the Waterberg Mountain range, 20 km northeast of Thabazimbi (north of the Alma road). Breeding numbers at this colony have declined since then, as has occurred at most of South Africa’s Cape Vulture colonies.  In the 1980’s the Kransberg was the largest colony with the Blouberg, 217 km northeast of the Kransberg and west of the Soutpansberg Mountains, near Vivo, and Manutsa, in the northern Drakensberg, near the J. G. Strydom Tunnel and Hoedspruit, colonies, being the second and third largest. These three colonies, all in Limpopo Province, remain the three largest Cape Vulture breeding sites; though by 2000 the Blouberg was slightly larger than the Kransberg. The peak number of breeding pairs, known to occur during the study of the Kransberg Cape Vulture Colony was in 1984, when at least 961 active nests, where an egg was laid, were present. A gradual decline in the number of breeding pairs occurred until 2003, when there were 561 active nests. From the 2003 low, numbers have increased to the 2007 season when 697 active nests were observed. The number of fledged young has also varied over the years, with high numbers produced in the early 1980’s when drought conditions occurred. Presumably the dry conditions provided large amounts of food due to higher ungulate mortality. Higher fledging success rates have occurred in other drought years. The negative impact of drought on ungulate mortality has a positive effect for vulture food availability with  more ungulate carcasses. This suggests that food is a limiting factor in the reproductive success of vultures at the Kransberg. Monitoring of this colony is ongoing.

Other factors, natural and man-made, affect the Marakele Cape Vultures. A lone male baboon, in 1982, was observed stealing and eating eggs from nests. Baboon troops, searching for food, normally avoid vulture nests when climbing up and down the cliff and are generally not a threat to the birds. Black Eagles (Witkruisarend) occasionally take nestlings and also scavenge carcasses of young vultures which fall from the nest.

Extended periods (3-4 days) of overcast and mist on the cliff can prevent adult vultures from foraging and therefore providing food for their young. When young vultures are hungry they will eat nesting material. If the young birds eat too much of this un-digestible material, they cannot regurgitate it and their stomachs and crops become impacted, preventing proper feeding and eventual death. Young birds also get stepped on during squabbles between their parents and other intruding vultures and the nestlings’ growing, naturally softer bones are easily damaged. Many young birds also perish when, after fledgling, they cannot get back to their nest site to be fed by their parents, on whom they are dependent for food for several months. These factors are normal and generally have little impact on the number of birds at a colony. Poisoning is the main man-caused mortality factor, and the leading cause of death of free-flying adult and sub-adult Cape Vultures, at the colony in Marakele National Park. This happens when vultures feed on livestock carcasses laced with poison, by farmers trying to control jackals, which may have killed these stock animals. This technique is not very effective in the control of predators, but has killed many vultures. Fortunately, poisoning incidents have greatly declined during the period of monitoring at the Kransberg Cape Vulture colony. Some fledglings die when they collide with guy wires of the Sentech (SABC) tower on the top of the mountain above the colony. Preventative measures like the orange balls or flappers on the guy wires, have been taken to prevent this; however young birds are still occasionally killed. In the past, recreational mountaineers climbing on the cliffs have disturbed incubating birds, resulting in reduced hatching rates of vulture eggs.

Reproduction of Cape Vultures is a long process. In early April birds start gathering nesting materials (grass and small sticks), carrying it in their beaks to a ledge on the breeding cliff. The materials are arranged into a small nest, where a single egg is laid. One parent bird will go to forage while the other incubates the egg or guards their nestling. During incubation the foraging bird may spend several days away from the breeding colony while its mate sits on the egg. They then reverse roles. It takes about 57 days for the egg to hatch. Once the egg hatches and the vulture nestlings need feeding, each parent spends one day at the nest and the next foraging. As the nestling grows it needs more and more food. At the peak growth period, a nestling will eat as much as a kilogram of food per day. This period is short, but is important as it strains the parent birds that have less food than they need to maintain their own body weight. If parent birds are not in very good body condition at the beginning of the breeding season it is likely nestlings will not get enough food at this time and will not survive. Generally, young birds stay in the nest for 4.5-5 months, before fledging. Once fledged a young bird may be physically capable of searching for food for itself, but must learn how and where to look. The fledgling vulture is dependent on its parents as a backup for several months and will regularly return to its nest to be fed.

Material prepared by Patrick C. Benson, Ph. D.
School of Animal, Plant & Environmental Sciences University of the Witwatersrand

Funds for its development were provided by the South African National Lottery Distribution Fund.