Vultures

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Vultures

Vultures are large birds of prey specialized to scavenge the bodies of dead animals. Species of vultures are assigned to two families in the order Falconiformes. The vultures of the Americas include seven species in the family Cathartidae. The vultures of Eurasia and Africa , numbering 14 species, are specialized members of the Accipitridae, a family that also includes hawks and eagles . The Cathartidae and the Accipitridae are not closely related.

Biology of vultures

Although the vultures of the Cathartidae and Accipitridae have evolved from different ancestral stock, the two groups occupy broadly similar ecological niches. As a result, the vultures in these families are highly convergent in many aspects of their biology and ecology . All vultures have very broad wings with marginated primary feathers at the tips that allow them to soar at great heights and position themselves in thermals, thus flying for hours with little effort. This is a very useful ability for animals with excellent eyesight, because they can scan for carrion over great expanses of terrain. Once most vultures become airborne, it is unusual to see them flap their wings. In fact, most species are rather weak at active flying, and they sometimes appear to be straining their capabilities when they are taking flight from the ground.

All vultures have a hooked beak. The neck muscles and beak of vultures are too weak to tear the tough skin of recently dead, large animals, but they are able to deal with carcasses that have had some time to decompose. Until this happens, only the eyes of the dead animal , apparently a delicacy among vultures, can be eaten. Vultures have long, clawed toes, but their feet cannot grasp with much strength. Because of their relatively weak beaks and feet, these birds survive almost entirely by scavenging dead animals, and increasingly in many areas, the refuse of human towns and habitations.

Vultures rarely kill anything. They are rather timid when confronted by a living adult animal, even if the creature is obviously unwell, and therefore a potential meal. In such a situation, vultures will typically wait patiently nearby until the animal dies before they begin to eat. However, vultures have been known to attack relatively helpless, recently born, young animals.

Vultures are binge eaters of the first order. Their prey of dead animals is not always plentiful, and these birds sometimes must pass a rather long time between meals. However, when a large corpse is available, vultures will avail themselves of it to the fullest possible degree. Under such conditions, vultures can sometimes become so engorged with ingested carrion they are unable to fly. If they are disturbed when thus grounded, vultures must lighten their load so that they can become airborne. They do this by regurgitating their food, representing a case of reluctant avian bulimia.

New World vultures

The vultures of the Cathartidae have perforate nasal septums, which means that when one is looking at a sideways profile of the head of one of these birds, daylight can be clearly seen through their paired nostrils.

It appears that at least some of the American vultures have an excellent sense of smell , a very rare and unusual trait among birds. The turkey vulture (Cathartes aura), for example, has the largest olfactory system of any bird. There is also a great deal of behavioral evidence that a sense of smell is a significant aid to vultures in finding their food of dead animals. This ability is especially useful for vultures that find some of their fragrant food of carrion in relatively closed-canopied ecosystems, such as shrublands and forests .

Most species of vultures have a featherless, naked head. This is likely an adaptation that facilitates sanitation, because these birds often have to reach far into a decomposing carcass in order to feed. Most species with naked heads have brightly colored and patterned, warty skin on their neck and head, which is important in species recognition and in courting and aggressive interactions. The male Andean condor (Vultur gryphus) has a large, fleshy structure known as a caruncle on the top of its naked head, seemingly bizarre to humans, but undoubtedly most alluring to females of the species.

The courtship rituals of American vultures generally involve open-winged strutting on the ground, followed by impressive displays of aerial soaring and gliding. Vultures are monogamous, and both sexes brood the eggs and care for the young. Vulture chicks mature slowly, taking up to six months to leave the nest, in the case of the Andean condor.

Species of New World vultures

The most widespread vulture in the Americas is the turkey vulture (Cathartes aura), which ranges from southern Canada to South America . This species is migratory in the northern part of its range, often travelling in flocks of several hundred birds. The black vulture (Coragyps atratus) is another common species that breeds from southern North America to South America. The only other species of vulture in North America is the California condor (Gymnogyps californianus), an endangered species on the edge of extinction .

Other vultures occur in South America. The largest species is the increasingly rare Andean condor, which has a wingspread of 9 ft (3m), the largest of any living bird. This species, which can weigh more than 14 lb (6.5 kg), is also the heaviest flying bird.

A now-extinct American vulture was the very impressive Terratornis mirabilis, which had an enormous wing span of at least 16 ft (5 m). This species is known from fossils collected at the Rancho La Brea tar pits of southern California. Terratornis mirabilis may have still been around as recently as the end of the most recent glacial epoch, about 10,000-15,000 years ago. This enormous vulture may have become extinct as an indirect consequence of the disappearance at that time of many large species of mammals , an event that was likely due to the first colonization of the Americas by very effective human hunters.

New World vultures and man

As scavengers of dead animals, vultures provide an important ecological service. This has long been recognized by some human societies, which have viewed vultures as useful birds because they contribute to cleaning up some types of unpleasant refuse around habitations. However, in some places these views have recently changed and the presence of these scavengers is no longer encouraged. This has happened, for example, in parts of the Andes, where vulture dropping has been shown to contribute to the contamination of open reservoirs of drinking water with microbial pathogens .

Species of vultures have had great cultural significance to various groups of people. The Maya of Central America commonly used a hieroglyphic associated with the king vulture (Sarcoramphus papa) to signify the thirteenth day of each month, and this species also appears to have been a religious symbol. The Andean condor was also culturally important to indigenous peoples over large parts of its range.

Unfortunately, because of their feeding habits vultures are regarded with great distaste in some other human cultures. Until rather recently, vultures were erroneously thought to be responsible for spreading some important, contagious diseases of livestock , because they fed on the bodies of animals that had died of those maladies. Vultures have sometimes been shot or poisoned
in large numbers for this reason. One American farmer claimed to have shot 3,500 black vultures during a single winter, in an attempt to relieve himself and his ranch of this harmless bird, which he perceived as a pest. Fortunately, vultures are rarely treated this way in North America any more, although they are still persecuted in some other areas, usually by a few misguided people.

The California condor is a critically endangered species of vulture. This species was formerly widespread in North America, especially in the west. However, this slowly reproducing species has declined enormously in abundance because of hunting and poisoning, and until recently it only survived in a critically endangered population of a few birds in the San Joachin Valley of southern California. In 1984, only 15 birds survived, and by 1986 there were only three adult male California condors . The few surviving wild condors are still under threat from shooting, lead poisoning following ingestion of lead bullets in scavenged carrion, and habitat degradation.

In 1987, no California condors remained in the wild. The handful of surviving wild condors were caught for use in a captive-breeding program, with the hope that enough birds could be produced to allow the eventual reintroduction of this endangered species into the wild. At that time the total population of the species was only 27 individuals, all in captivity. These last California condors are being used in a program of captive breeding supported by the United States Fish and Wildlife Service, and located at the San Diego Wild Animal Park and the Los Angeles Zoo. Fortunately, this breeding program is showing encouraging signs of success, and in 1991 the first, careful releases of California condors were made to a large ecological reserve created and managed for their benefit in southern California.

The California condor remains a critically endangered species, but there is now guarded optimism for its longer-term prospects of avoiding extinction.

Status

Andean condor (Vultur gryphus). The largest flying bird. Resident of the Andes.
Turkey vulture (Cathartes aura). Highly developed sense of smell. Ranges from southern Canada to Tierra del Fuego. Reportedly declining population on the southern plains. Eggshell thinning has been a widespread problem. Today the populations appear stable.
Yellow-headed vulture (Cathartes burrovianus). Resident of Mexico, Panama, and the lowland areas of South America.
Greater yellow-headed vulture (Cathartes melambrotus). Found in several regions of northern South America.
Black vulture (Coragyps atratus). Resident of middle North America to South America. Winters in North America. Loss of suitable tree cavities due to fire control has been a problem, as has eggshell thinning as a result of pesticide use. The population has expanded in the northeastern United States, but declined in the Southeast (possibly due to a loss of nesting sites in hollow trees).
King vulture (Sarcorhamphus papa). Striking bird with pinkish-white plumage and black flight feathers. Some records suggest this bird may once have been a resident of Florida. Today it is a rare resident of tropical America.
California condor (Gymnogyps californianus). Resident of North America. On the verge of extinction, now an endangered species. A captive breeding program introduced in 1987 has met with some success, and hopes now exist that a wild population may again be re-established.

Old World vultures

Some Old World vultures have evolved unusual behaviors that help them with feeding. The lammergeier or bearded vulture (Gypaetus barbatus) of Eurasia and Africa is known to drop large bones onto rocks in order to break them open so that they can eat the nutritious, internal marrow. The Egyptian vulture (Neophron percnopterus) is known to pick up stones in its beak, and throw them at ostrich eggs to break them, again so that the contents can be eaten.

Unlike the New World vultures, the Old World vultures lack a perforate nasal septum, and they are generally heavier billed. The Old World vultures have an ancient evolutionary lineage, extending to at least 30 million years, whereas it appears that the American vultures evolved much more recently.

In regions of India, vultures are given access to human corpses within secluded, walled facilities. This custom would seem to be a cultural acknowledgement of the continuity of life and death, and of the relationships of humans with other species and ecosystems.

Old World vultures, unlike their New World counterparts, lack a highly developed sense of smell, and are therefore entirely dependent on their vision to find food. Their dependence on their sense of sight has restricted their range to the open country, and has kept them from making their homes in rainforests like those inhabited by the New World vultures. Representative Old World vultures include the following:

Lappet-faced vulture (Torgos tracheliotus). Largest of the African vultures, and second in size only to the European Black Vulture among Old World vultures.
Griffon vultures (Gyps fulvus; also G. bengalensis, G. indicus, G. himaleyensis, G. africanus, G. rueppellii, G. coprotheres). Medium to large-billed resident of Eurasia. Griffins are found chiefly in Spain and the Balkans; in Asia , Iran, the Hindu Kush, and parts of the Himalayas.
Lammergeir or bearded vulture (Gypaetus barbatus). A handsome bird, rather unlike many of the vultures that many consider repellant. A threatened species in Europe (1989), this vulture maintains healthy populations in the Middle East, Central Asia, and the African highlands. Strychnine used in bait has led to its decline in Europe.
Egyptian vulture (Neophron pecnopterus). Small vulture. Resident of Africa, southern Europe, the Middle East, southwestern and central Asia, the Himalayas, and India.
Hooded vulture (Necrosyrtes monachus). Small vulture. Resident of Africa, south of the Sahara.
Palm-nut vulture (Gypohierax angolensis). Feeds on crabs , molluscs, fish, and the husks of the Oil Palm. Resident of Africa.
Indian black vulture (Sarcogyps calvus). Resident of India, the Himalayas, Burma, Thailand, the Malay Peninsula , and southern Vietnam.
European black vulture (Aegypius monachus). Resident of southern Europe, Afghanistan, Tibet, the Himalayas, and China. Small numbers.
White-headed vulture (Trigonoceps occipitalis). Resident of southern Africa, south of the Sahara. Scarce.