There are three vultures in the United States, the California Condor, the Turkey Vulture, which has the widest range, extending from Canada down into South America, and the Black Vulture, the smallest of the three. If I wanted to observe black vultures twenty five years ago, I went to the Everglades or the Texas Gulf Coast. About fifteen years ago, I began seeing black vultures in swampy areas of northern New Jersey, and, with warming climate, they’re now showing up in the southern Adirondacks.
The black vulture probably evolved out of a relative from the Pleistocene, which was about 15% larger, while in their current form, black vultures are no different from the fossilized remains of those from at least ten thousand years ago. Black vultures are stubbier than turkey vultures, two to two and a half feet in length, have a wing span of up to five and a half feet, and weigh in between four and a half to six and a half pounds. Following Bergman’s rule, larger specimens are more likely to survive and breed in cooler climates, so northerly vultures are larger than their tropical counterparts.
Black vultures have grayish heads and black feathers, long weak feet with blunt talons, and somewhat flattened bills for tearing at decaying flesh. The word vulture is from the Latin word vulturus, which translates as “tearer”, while the latin name for black vulture is Coragyps atratus, which means “raven-vulture clothed in black”. Since turkey vultures are Cathartes aura, or “golden purifiers”, there was recognition by our predecessors that vultures, gross as their behavior may seem to us in many respects, were important as recyclers of the animal kingdoms waste.
Both black and turkey vultures have slightly webbed feet, and black vultures will actually wade into water to harvest decaying fish. Since old world vultures have no sense of smell, and locate food visually, it appears that new world vultures are more closely related to storks than old world vultures, and the similarity between new and old world vultures is the result of convergent evolution, a process by which animals performing similar roles in different habitats, come to resemble one another by the natural selection of traits which allow them to survive and flourish.
Black vultures are basically carrion eaters, but they will grab new born animals and eggs as opportunity presents itself. Black vultures often hunt together, and may mob victims, even going after new born calves and other relatively defenseless animals, pecking at the eyes, and other extremities, causing the prey to collapse. Black vultures seem to be less discriminating in their ingestion of decaying flesh, with turkey vultures preferring carrion not more than four days dead. The featherless heads appear to be an adaptation to prevent the buildup of bacteria on vulture heads, which are frequently inside the body of a decaying animal.
The liquid and solid wastes of birds form uric acid, and both turkey and black vultures defecate on their legs, as a way of cooling off the blood vessels in the unfeathered tarsi and feet, through the evaporation of the liquid portion of the uric acid, a process called urohidrosis. Neither vulture has a syrinx, so the only vocalizations are grunts and defensive hisses.
Few predators go after these relatively imposing looking birds, and even vulture chicks regurgitate a noxious, stinging vomit on attackers, providing another applicatrion for the strong stomach acids used in digestion, as a defense against the pathogens in rotting flesh. Both vultures are awkward hoppers on the ground, and vomiting on a would be, grossed out predator buys the vulture extra time to get off the ground.
While the turkey vulture roosts in groups, they tend to be individually solitary hunters, though other turkey vultures may be drawn to a carcass where an individual turkey vulture is already feeding. There is a dead tree on the western bank of the West Branch of the Ausable, maybe 100 yards north of the Adirondack Wildlife Refuge, which flanks the east side of the river, where kayakers may observe turkey vultures roosting in groups.
Both vultures are frequently seen roosting in sunny weather in the horaltic pose, with wings spread, probably absorbing warmth, drying their wings or baking off bacteria accumulated while feeding. At night, black and turkey vulture body temperatures drop by 9 degrees F, a hypothermic adaptation, which reduces calorie burning. Since we believe birds are descended from warm blooded bipedal dinosaurs, and since the latter are descended from cold blooded reptiles, this adaptation is like an atavistic evolutionary throwback.
Both vultures ride rising thermals to get to heights where they can glide to other thermals, the lazy birds way of getting from one area to another with minimal wing flaps. Circling vultures are called a kettle, and may be riding a rising thermal, or in the case of turkey vultures, dropping to locate the source of ethyl mercaptan, an odor arising from decaying flesh. Vultures on the ground are called a venue. Turkey vultures are generally larger, have longer wings, and are more efficient gliders than the stubbier-winged black vulture, whose gliding is more frequently interrupted by vigorous wing flaping.
Black vulture mating depends on climate and latititude, so black vultures in the northern range mate around March, while those in southern ranges many months earlier. There is a courtship ritual involving males strutting and bobbing heads around females, and sometimes flights by vulture pairs over prospective nesting sites, which may be on the ground, in a log or a cavity no more than ten feet up above the ground. The vultures may decorate a nesting site with a ring of colorful manmade products, bottle caps, as well as shards of plastic and glass, but no special gathering of nesting materials. What did they do before we showed up, and is this sort of decoration not a sign of intelligence?
Both parents sit on one to three eggs, and the hatching period is between 28 and 40 days. The chicks leave the nest and can fly after about two and a half months. Eggs and young may be scavenged by raccoons, foxes and other small mammals. In more tropical climates, coatis will go after vulture eggs. Like turkey vultures, black vulture migration only occurs in colder northern climates.
The turkey vulture and the smaller lesser and greater yellow headed vultures of South America, are the only vultures with a sense of smell, have relatively large nostrils, as well as an enlarged olfactory lobe in the brain, dedicated to detecting ethyl mercapatan. Both turkey vultures and black vultures, which have much smaller nostrils, have perforate nostrils, lacking a septum, meaning you can see right through the two nostrils. Turkey vultures can be fooled by leaking propane tanks, as ethyl mercaptan is that offensive odor mixed in with propane, which has no odor, as a warning signal to home owners of a leaky tank or propane line.
One advantage of roosting with turkey vultures, or following their flight, is that black vultures may be able to steal carcasses by mobbing or bullying outnumbered turkey vultures. Turkey vultures tend to eat solo, with the dominant male feeding before its waiting companions. Black vultures have stronger bills, and feed together, with the effect they are better at tearing apart carcasses which the turkey vulkture can not open. Alex and I have observed turkey vultures and black vultures roosting in the same tree in the Santee Wildlife Refuge in South Carolina.
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