Midmorning at the river and the air is still. The heat is building towards another 100º plus day. A tall cottonwood, its canopy brilliant green against a blazing blue sky, is decorated with large, dark shapes. One by one, the dark shapes shake and unfold long wings held up to the sun’s warming rays. The songs and calls of birds fill the air, but the dark shapes themselves are silent. The only sound is that of feathers rustling against leaves. Suddenly, with a crashing of wings, the forms fall from the cottonwood, great long wings flapping mightily to gain the sky. Within minutes the tree empties, and the dark forms effortlessly climb high into the sky in a spiraling circle, their wings held outstretched and their bodies gently rocking on invisible pockets of warming air. Thus begins another day for the turkey vultures of Big Bend National Park.
Seen on the wing, the turkey vulture is an awe-inspiring and graceful bird. Designed for soaring flight, turkey vultures rule the skies of Big Bend from March through early October. With a nearly six-foot wing span and a light body weight (3 1/2 -5 lbs.), turkey vultures have a very low wing loading ratio (ratio of body weight to wing area). This enables them to buoyantly ride rising columns of warm air to heights of almost 5,000 feet and to travel up to 40 miles per hour with almost no flapping of the wings. The closely related black vulture, though capable of sustained soaring flight, is no match for its cousin. With a heavier body and shorter wings and tail, the black vulture must laboriously flap its wings often to gain and maintain altitude.
Seen in soaring flight, the turkey vulture is identified by its wings held in a dihedral or "v" pattern, the slight rocking motion of the body, and the two-toned underwing; a dark leading edge and a lighter trailing edge. The black vulture in contrast soars with its wings held nearly flat or a shallow dihedral, and has a completely black underwing with a large silvery-white patch at the base of the primaries.
With its greater powers of flight, the turkey vulture has a much larger range than the black; found from southern Canada to the southern tip of South America. Populations in the north and west of North America are migratory, including the turkey vulture of Big Bend. They arrive in early March, some members of the population nest, then they all depart to Central and South America in the autumn. Black vultures range from the eastern and south central U.S., down through Mexico and South America. They are mostly non-migratory across their range and here are resident year-round, but never common. In Big Bend, turkey vultures are found from the mountains, through the desert, and down to the Rio Grande. Black vultures are seldom found away from the river.
Seen up close, neither the turkey nor the black vulture inspire words of praise. Both have un-feathered heads, the adult turkey vulture displaying the characteristic red-skinned head, while the black vulture has a wrinkled, black-skinned head. Both have long bare legs, often discolored by their habit of defecating on themselves for cooling. The talons are weak and the hind toe small and nonfunctional. Because the talons are almost useless for defense, both will, if threatened while nesting or roosting, either fall over and play dead, or vomit. Anyone who has had the misfortune to hit a vulture while driving can attest to the extremely foul odor of the vomit that is rendered in a final act of defiance.
Fortunately this is rare, and most people see vultures on the wing or in their large social roosts. A common behavior at the roost is the spread wing posture in the early morning. In the case of the turkey vulture, this activity serves to aid thermoregulation. In the evening a turkey vulture’s body temperature drops to conserve energy. In the morning, spreading the wings to the sun’s heat brings the body temperature back up quickly and allows the bird to prepare for flight. On rainy or cloudy, cool days, vultures seldom take to the air.
Both vultures are predominately carrion eaters. The family name for the New World vultures (Cathartidae) is from the Greek word meaning "cleanser," alluding to the tremendous service vultures and their kin provide. They patrol the air, finding and ridding the land of carcasses that, if left on the ground, could become harbors of disease. At one time in Texas and other areas where ranching occurs, vultures were killed because of the belief that they carried and transmitted anthrax and hog cholera. Recent studies have shown that these disease organisms do not survive passage through a vulture’s digestive system and it has also been shown that vultures are immune to botulism. Vultures will on occasion eat rotting fruit and other plant material and are known to take small live prey, most often young, defenseless animals.
Because of similarities to true birds of prey, New World vultures were long classified as close relatives of hawks and eagles. Recent genetics studies however, strongly suggest that vultures are actually more related to storks.
One adaptation that sets the turkey vulture apart from most other members of the family is its well developed sense of smell. It was thought that all vultures found their food strictly by keen eyesight. In experiments using hidden carcasses, turkey vultures successfully found the carrion every time, always approaching into the wind on the scent of the food. In areas where turkey and black vultures coexist, usually the turkey vultures will locate carrion first by scent and sight. Black vultures, lacking a sense of smell, watch their cousins converge on a carcass, then descend and drive off the turkey vultures to take over the meal. It may not seem fair, but the end result is the same; a potential harbor of disease is removed and recycled once again.
Call them buzzards, carrion crows, zopilotes, auras, buitres, whatever name you choose. But always enjoy their amazing feats of flight, and be thankful for the service they render in cleansing the land.
Did You Know?
Big Bend National Park contains the most representative example of the Chihuahuan Desert ecosystem in the United States. The park’s river, desert, and mountain environments support an extraordinary richness of biological diversity, including endemic plants and animals. More...