COLUMBIA — For years, John George has been keeping an almost daily eye on the turkey vultures that congregate at Stadium Boulevard and Rock Quarry Road.
Although he doesn't recall an "ah-ha" moment, George and others have seen more of the migratory birds during the winter in recent years.
George, regional wildlife supervisor for the Missouri Department of Conservation, said that during the past decade, turkey vultures have been spotted in Columbia more often during colder months. He's heard the same from birders in online forums and during conversations at conservation areas.
"There's a lot of birders out there, and they document what they see," George said. "I've just heard them talk that there's more in the area recently."
Andrew Farnsworth, research associate at the Cornell Lab of Ornithology, agreed there have been increased numbers of turkey vultures spending winter in states like Missouri rather than flying south. "Turkey vultures are definitely spending winters farther north than ever," he said.
Climate change is one factor that could be changing the migration patterns of turkey vultures, he said. Other potential factors include urbanization, which creates more roads and, consequently, more roadkill for vultures to feed on.
Farnsworth said he "absolutely" expects turkey vulture migration patterns to change along with changes in the distribution of the food turkey vultures depend on to survive. "Migration represents a dynamic suite of processes, and there are many, many changes likely," he said.
Tess Rogers, a naturalist at the World Bird Sanctuary, said the departure of turkey vultures from their summer haunts depends on weather. “If there’s going to be a lot of frost or snow, they’re probably not going to hang out,” she said.
Columbia bird enthusiast Bill Clark said the cold itself isn't necessarily the determining factor in whether turkey vultures stick around for the winter or leave but rather the availability of viable food sources.
When temperatures drop enough, he said, dead animals and carcasses freeze, rendering the turkey vultures' only source of food — they don't hunt their own prey — inaccessible.
"As soon as the ground thaws out, you can bet on them being back," Clark said.
In the summer, Clark said, it is common to see a couple hundred turkey vultures at roosting locations around Columbia, including trees and transmission lines that take electricity across the Missouri River.
George said vultures prefer to roost in sites near bluffs because of how the sheer rock faces are oriented. Bluffs that face south or west, such as those at Stadium Boulevard and Rock Quarry Road, are especially attractive to turkey vultures because of the increased exposure to the sun.
"I would not be surprised if there were a dozen or more roost locations around the overall Columbia area," George said.
Turkey vultures also tend to roost near highways and major roads where roadkill is common, Rogers said.
Because they clean off rotting carcasses, Rogers said they are a valuable resource to prevent the spread of disease, since their extremely potent stomach acid can kill off anything known to man.
“They help keep malaria, anthrax, and the bubonic plague in check by getting rid of rotting meat,” Rogers said.
To give an idea of the strength of a vulture's stomach acid, Rogers compared it to battery acid and said it falls between a 0 and 1 on the pH scale, making it acidic enough to strip paint from a car door or burn through clothing.
The high acidity paired with a diet of rotting meat combines for an “awful” smell, she said. “I don’t even have words to describe it."
In addition to killing diseases, a turkey vulture’s stomach acid aids in defense against predators. Because turkey vultures are scavengers and have to wait for animals to die and begin to rot, they might not eat for days, so when they find a carcass, Rogers said they will “gorge themselves.”
Sometimes, they eat so much the extra weight makes it difficult to fly. If a predator approaches and they can’t escape, Rogers said, vultures will projectile vomit on whatever is scaring them, effectively burning predators — blinding them, in many cases — and lightening their body weight, enabling them to fly away unharmed.
“They have a lot of epic adaptations,” Rogers said.
Clark said turkey vultures are one of his favorite birds despite their reputation.
"They're maligned because of what they do and how they look," Clark said. "They are a tremendous asset to mankind."