Archives of The Cold Blooded News

The Newsletter of the Colorado Herpetological Society

Volume 31, Number 8;   August, 2004

Preying Ravens' Toll on Tortoises Studied

Scientists Work to Save Imperiled Desert Species

by Lisa Petrillo

With development of the desert comes garbage, and garbage attracts ravens that have learned to love landfills and dumpsters. So much so, that the raven population has increased an estimated 1,500 percent in the western Mojave during the last 25 years.

Yet ravens still take time out from garbage buffets to do what comes naturally -- to hunt, including dive-bombing fist-sized young desert tortoises. "They are just slow walking packets of meat," said Boarnlan, a biologist with the U.S. Geological Survey who has researched the food-chain problem with Kristan, a science instructor at California State University, San Marcos.

Still, emotion is hard to avoid out there in the high desert, acknowledged Boarman, who kept a pet turtle as a kid. "When you're out there, you're going to nest after nest after nest, it's really boring and when you get your first sighting of a shell (pecked open and empty), you start to get excited," he said. "Then you see a pile of shells and your heart starts sinking."

Boarman's Kearny Mesa office contains boxes of samples of such shells, all bearing the violent marks of mangling and tearing by ravens. A pile of them under a raven's nest indicates an especially aggressive bird devoured most of the local turtles under 6 years old, when their shells were still soft.

So cunning and aggressive are some ravens, researchers say, that they have found as many as 250 mangled shells under a single nest. Boamlan and Kristan calculated the probability that ravens could wipe out the dwindling number of young tortoises, which are on the federally threatened species list.

Their research, published in the September issue of the journal Ecology, aims to provide proof of the problem and potential solutions. "What we came up with was a measurement of what people suspected for a long time," Kristan said. Researchers have long believed that the decrease in tortoises was linked to the increase in ravens. To provide stronger evidence, they used simulated tortoise shells made of styrofoam as bait. They theorized that soft fake shell would, like a real shell, show piercing by a raven beak and therefore prove that it was the big bird and not another predator, like a snake or fox that ate the tortoise.

They placed 100 fake shells in both isolated and developed areas of the Mojave, within a 300-square mile area in and around Edwards Air Force Base, with government cooperation. They extrapolated the results over a larger area, using a computer model. Around landfills, the researchers calculated a 100 percent tortoise death rate. In developed areas with a less-available food source, they calculated raven predation of the baby tortoises as high as 59 percent. The only place the tortoise was safe, they calculated, was far, far from humans. While Boarman and Kristan aim to save the prey, the scientists express admiration for the predator.

Ravens, the largest of all songbirds, are often mistaken for crows -- though they are bigger and smarter. They live throughout the Northern Hemisphere, includ.ing terrain as tough as Alaska's Mount McKinley and western United States' deserts. Man has long held a special relationship with the big bird, and ravens figure into the mythology of major cultures as widespread as the ancient Babylonians, American Indians and early European tribes. The Bible's Noah used ravens, as did the Norse god Odin. And who could forget the menacing raven of Edgar Allan Poe's classic book.

In their research, Boarman and Kristan suggest how to better balance the desert food chain. They propose covering landfills and garbage dumpsters to keep ravens away from an easy food supply, thereby dispersing them and potentially lowering their population. They urge stronger efforts to "manage" the raven, but do not endorse doing it with bullets, as some have proposed in the past.

As Boarman notes, open hunting of ravens presents danger to other wildlife. The researchers suggest the weeding out, perhaps by poisoning, of highly aggressive ravens. But Boardman said past proposals for more aggressive action against the birds have prompted legal challenges from animal-rights groups. Bette Stallman, a wildlife scientist with the nonprofit Washington-based Humane Society of the United States, said she would oppose indiscriminate hunting or poisoning of the desert raven population. However, she said if there was proof of specific super-aggressive ravens preying on young tortoises, she would consider some control measures.

Still, Stallman said she would need more proof of a direct link that ravens are the reason for the decline. "You can't just pin it on the ravens," she said. "If they can prove that the raven population is artificially inflated, (increased because of man) our point of view is to get rid of what's artificially inflating the population," said Stallman.