Scavengers play an important role in an ecosystem, and the population of one species in particular has been growing for decades, proving a resiliency that could also characterize its stomach.
According to the U.S. Department of Agriculture, black vultures have been increasing in density and expanding their range in the eastern U.S. since at least the 1960s. In Oklahoma, they're beginning to compete with the indigenous turkey vulture; it's just taken some time for people to notice.
Mark Howery, a wildlife diversity biologist for the Oklahoma Department of Wildlife Conservation, said no one quite knows why black vultures have been multiplying, but there are a couple of possibilities.
“One common reason is that they’ve adapted to a new food supply, or a different food supply,” said Howery. “Certainly, in Oklahoma, we’ve seen a tendency for black vultures in the wintertime to be closely tied to water.”
Black vultures tend to roost in trees along hillsides, and drivers along State Highway 10 in Tahlequah might notice them hovering above the bluffs that line the Illinois River. They are scavengers around rivers and lakes, where they can eat dead fish, or those that fishermen have thrown up on the banks after they’ve cleaned them.
Roadkill was also not as common a century ago, so they’ve taken advantage of the increase in that food source. However, wildlife’s response to changes such as this can take several decades.
“In the case of a bird like a black vulture, they’re very long-lived and have kind of a low reproductive range,” said Howery. “So when these changes take place, we’re noticing it recently, but the increase has actually been going on for 40 or 50 years."
The black vulture routinely lives around 30 years. They don’t become reproductively mature until they’re 6 to 8 years old, while many animals don’t even live that long. They also only lay two eggs a year, so their rate of reproduction is considered fairly low.
Black vultures can stomach just about anything. Howery said in the past 15 years, experts have noticed the birds can digest soft plastics.
“I’m not aware of any cases in Oklahoma, but in Virginia, North Carolina, and some of the eastern states, they’ve had problems with black vultures eating windshield wipers, plastic boat cushions, and just kind of odd, rubberized material,” he said. “They don’t seem to eat it as a preferred food source, but it’s something they’ll turn to in the wintertime when food is scarce.”
The black vulture can ingest animal carcasses, a typical place for disease to spread. And they can withstand material that would make a human sick.
“They don’t get botulism the way we do. They have a resistance to salmonella that we don’t have,” Howery said. “So there’s a lot to be learned from studying vultures and scavengers to get a better handle on fighting diseases.”
And while black vultures play their part in a healthy ecosystem, they can be a nuisance for ranchers and other humans. Around calving season – which for many ranchers is February and March, or in late fall – black vultures have been seen feeding on the afterbirths of newborn calves. In some areas, they’ve learned to peck at the newborn calves’ eyes, ears or mouth, and sometimes kill them.
Curt Allen, senior wildlife biologist for the ODWC, said it’s becoming more of an problem lately.
“The actual evidence – the sure-fire smoking gun – that they’re out there attacking newborn calves and killing them is a little bit lacking,” said Allen. “Everybody who's got a cow-calf operation pretty well swears by it, but nobody sits out there with a camera long enough to see it taking place.”
Not long ago, Allen said, he stopped by a pasture near Eldon Hill, where he noticed black vultures were following a new calf.
“They weren’t necessarily after the calf itself,” he said. “As soon as that calf had a bowel movement, one of the vultures was right there, pecking on it in the grass. As soon as it started pecking, some from the other areas of the field ran over, too. So it's a lot of passed-on nutrients.”
Following calves is not something most vultures do. If producers calved all year long, perhaps black vultures would take advantage of that as a potential food source more often.
“It seems to be a learned behavior that a black vulture will figure out, and that behavior will spread to other black vultures that witness it,” he said. “So you’ll have lots of places where black vultures don’t pose a threat to cows and other places where they do.”
And as the vultures have learned, so have the cows, apparently. Typically, cattle do not perceive vultures to be threats, so they often sit by and watch as a bird pecks at their calves. But in places like Georgia and Florida, where black vulture populations are much higher, they’re now beginning to see cows react aggressively.
“In Oklahoma, Missouri, and Kentucky, where this behavior is less common and a lot more recent, those cows tend not to pay attention,” said Howery. “You’ll find that milk cows tend to completely ignore black vultures, whereas a lot of the meat cattle that are a little more free-ranging will attack black vultures.”
In some areas, black vultures can cost ranchers thousands of dollars in damage to livestock. But there is a solution.
“Just like the majority of birds are, they’re protected under the Migratory Bird Treaty Act,” said Cody Youngblood, Cherokee County game warden. “Because of the damage and killing livestock, the USDA Wildlife Services will issue depredation permits to farmers to shoot them. There is a report they have to do at the end of every year afterward, and if they don’t fill it out, they pretty much won’t ever get it renewed.”
Like most creatures, the black vultures have adapted. They do not migrate during the winter like the turkey vulture, so they can be seen all year in Oklahoma. Winter is usually the toughest time for any wildlife to survive, but according to Howery, the cold does not impact black vultures too much.
“The black vultures have certainly benefited from the all the recent warm winters that we’ve had, but even in the 70s when we had those really cold winters, their trajectory was still on an upward trend,” he said.