New technique for vulture population estimates could aid control efforts, UF researcher says

Published: May 5th, 2009

Category: Environment, Research

GAINESVILLE, Fla. — For months, Erik Anderson tried to persuade a flock of vultures to stop roosting at Santa Fe College’s main campus in northwest Gainesville. In the end, the vultures won. Years later, he sees them as he motors down Interstate 75 to work.

“I don’t fight them anymore,” says Anderson, the college’s director of facilities operations. “It was a no-win situation for us.”

It’s a common story, says Michael Avery, a U.S. Department of Agriculture wildlife biologist affiliated with the University of Florida’s Institute of Food and Agricultural Sciences.

Avery is one of the nation’s top vulture management experts. He and U.S. Geological Survey and U.S. Fish and Wildlife Service colleagues just published a paper on a mathematical formula that can help determine how many vultures can be taken from a local population without jeopardizing its long-term viability. The article is in May’s issue of The Journal of Wildlife Management.

Though they perform a vital function by eating dead animals, vultures can create problems when they co-exist with people, something that’s happened more in recent decades as both populations have grown.

The birds habitually peck at soft objects, so in residential areas they destroy vinyl cushions and rubber seals around car doors and windows. When they roost on utility poles and cell phone towers, their waste makes climbing surfaces slippery. Sometimes flying vultures collide with aircraft, endangering pilots and passengers.

For Anderson, the birds were mainly an annoyance. They ripped up roof expansion joints and pipe insulation; occasionally they entered ventilation ducts and died, which caused odors in classrooms and left workers to disassemble exhaust systems and retrieve the bodies.

He tried several popular methods to chase them off — effigies, electrified wires and motion-activated sprinklers. The sprinklers irritated the vultures enough that they left, settling on another campus building. That’s when Anderson decided it was easier to live with the birds.

Killing nuisance vultures is a last resort, and is rarely used. Although it can drive a flock away in short order, lethal control is not a permanent solution, Avery says. Vultures are protected by federal law. Lethal control requires a U.S. Fish and Wildlife Service permit.

“When they issue permits USFWS need some assurance they’re not taking too many birds,” he said. “That’s when the question of how many birds are out there becomes relevant.”

Called Prescribed Take Level, the formula includes the size of the animal population, its maximum growth rate, and a variable determined by wildlife managers, based on how much they need to reduce population size.

The current study used the black vulture population in Virginia as a case study. Black vultures and turkey vultures are the dominant U.S. vulture species.

Precisely estimating local vulture populations is difficult, due to uncertainties about their lifespan and breeding habits. The researchers relied on annual bird-count data and studies of radio-tagged vultures.

Still, team members are confident they’ve gathered enough data to show the mathematical model can work, Avery says. He points out the study isn’t meant to advocate lethal control.

“Some people think there are too many vultures. Are there? I don’t know,” he says. “That’s more of a policy question, that’s not something we delve into.”

Michael Runge, a Maryland-based U.S. Geological Survey research ecologist and another author of the study, agrees. An objective permitting process will only improve inter-agency relations, he said.

Ranchers who lose livestock to black vultures will welcome the study, said Paul Rodgers, deputy policy director for the American Sheep Industry Association in Englewood, Co. The birds kill newborn lambs, sometimes attacking them as they’re born, he said.

“We certainly need a certain number of vultures,” Rodgers said. “But any population of any animal can get out of hand. And it seems like black vulture populations are getting out of hand in some areas.”

Credits

Writer
Tom Nordlie, tnordlie@ufl.edu, 352-392-0400
Contact
Michael Avery, michael.l.avery@aphis.usda.gov, 352-375-2229

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