Fatal Attraction

Ravens are flourishing in the Southwest, and that is bad news for beleaguered desert

04-01-1999 // Michael Tennesen

Dave Morafka remembers well the day the ravens attacked. The biologist had built an enclosure to study young desert tortoises at Fort Irwin, an army base in Southern California. Raven populations were skyrocketing in the Mojave Desert, and shells from tortoise hatchlings were appearing under raven nests. But whether the ravens were a serious problem for the declining tortoises was still under debate.

In May 1991, eight months into the study, the black birds swooped out of the sky and ended the argument. In little more than a week, they killed 18 of 24 hatchlings inside the study site and 8 more that ranged nearby, pecking through their soft young shells and feasting on their flesh.

Morafka, a biologist at California State University-Dominguez Hills, was horrified. "Once they discovered us, the result was quick, thorough and devastating," he says.

In the Southwest, the lives of the common raven and the tortoise are intertwining in a deadly way. Over the past 30 years, raven numbers in the Mojave Desert have risen more than 1,000 percent as people have introduced sources of food and water to this harsh environment. During the same period, tortoise numbers have dropped more than 90 percent in some areas. Ravens are not the only reason for the tortoise´s decline, but the quick-witted birds are a significant and growing threat. Ravens have caused more than 50 percent of juvenile desert tortoise deaths in some areas, according to biologist Kristin Berry of the U.S. Geological Survey in Riverside, California.

Biologists worry that, in places where tortoise numbers are already reduced by disease, habitat destruction and other problems, raven predation may drive these populations out of existence. "Normally if a predator eats a prey into oblivion, its own numbers decline," says Morafka. "But raven populations are subsidized by urban irrigation and garbage and can afford to hunt something to extinction."

The desert tortoise is one of four species of gopher tortoise native to North America, and its ancestors have been around for perhaps 80 million years. The creatures are found today in arid parts of California, Nevada, Arizona, Utah and northern Mexico. The lumbering reptiles are among the longest lived of all animals, surviving perhaps a century or longer.

To persist in such a harsh environment, desert tortoises may spend more than 90 percent of their time underground. They hibernate in burrows for several months in the winter, and spend days or weeks at a time below ground during hot summer months.

Much of the reptiles´ water intake comes from moisture in the grasses and wildflowers they consume in the spring. "Eating to drink," Morafka calls it. Tortoises store water in their bladders against drier seasons ahead. During very dry times they may give off waste as a white paste rather than a watery urine. Adult tortoises may survive a year or more without access to water.

Although desert tortoises are well-adapted to their arid surroundings, human-induced changes have proven problematic for the reptiles. Urban development and off-road vehicle traffic have destroyed much of their habitat and led to road kills. Cattle grazing has reduced the amount of food available for the tortoises in some areas. In recent years, a respiratory disease similar to some types of human pneumonia has also ravaged tortoise populations.

In 1973, Berry took state officials out for a 6-mile ride through what is now the protected Desert Tortoise Natural Area at the west edge of the Mojave Desert in California. "Every place we looked we saw tortoises," says the geological survey biologist. "We saw as many as 8 tortoises at one time, 50 for the whole day. Of course those days are gone." In the last 10 years, the tortoise population in this 38-square-mile area has declined 88 percent.

There are currently anywhere from 93,000 to several hundred thousand desert tortoises, biologists estimate. In 1990, desert tortoises in the Mojave Desert (which stretches over parts of California, Nevada, Utah and Arizona) were formally listed by the U.S. Fish and Wildlife Service as a threatened species.

The human-abetted ascent of common ravens is now the chief threat to young tortoises in some areas. The reptiles´ shells do not become hard enough to protect them from predators until about 5 to 7 years of age. "Up until then, they´re just soft little morsels, almost like walking raviolis," says Morafka.

Hungry ravens take full advantage of the young tortoises´ vulnerability, as Bill Boarman finds on a rare rainy afternoon in the Mojave Desert in eastern California. Boarman, a U.S. Geological Survey ornithologist, searches though the desert brush, rock and sands beneath a raven nest perched high on the cross bars of an electric tower. He spots a punctured tortoise shell--and another--and still more.

"You always get excited when you spot the first shell, but after that your heart begins to sink," says Boarman.

He spots 9 shells under one nest, 13 at the next tower, possibly from the same bird. In all, Boarman locates 28 shells this day under various nests and posts. The shells all feature large holes in the middle where the raven has pecked through to get at the tortoise.

It wasn´t until the late 1970s, when in-depth studies of desert tortoises began, that biologists discovered tortoise shells like these in the Mojave. Under one Southern California raven nest, scientists found 250 shells during a two-year period.

Most raven predation of tortoises takes place in spring when raven chicks hatch and raven pairs must feed their hungry, noisy broods. Spring is also the time when tortoise juveniles are most active. Though this predator-prey relationship is not new, prior to the advent of towns, roads, landfills, irrigation and farms in the desert, raven numbers had been so low that they were not a significant problem.

The raven is one of the most widespread birds in the world. Ravens gulp down their food in chunks, and then, like owls, cough up a pellet of all the indigestibles. Boarman displays a raven pellet that contains some aluminum foil, the shell from a beetle, bones from a rodent and a tortoise leg.

Humans do not generally regard ravens as beautiful. In the words of Edgar Allan Poe, the birds are "Grim, ungainly, ghastly, gaunt, and ominous." But they are clever. Says Boarman, "In grad school they always said never study an animal smarter than you, but I didn´t listen. Ravens are incredibly intelligent."

Bernd Heinrich, professor of biology at the University of Vermont and perhaps the world´s leading authority on ravens, demonstrated ravens´ intelligence in a classic experiment in 1990. Heinrich presented captured ravens with food dangling from a two-foot-long string. Several birds successfully got at the meat by pulling the string up and stepping on the pulled-up loops, until they grasped the food.

"It wasn´t programmed because they don´t do that in the wild, and it wasn´t learned because I was using young, naive birds," says Heinrich. "I thought it demonstrated that ravens could see through a situation and figure out the answer."

Ravens are also good mimics. They can imitate barking dogs, flowing creeks, even human speech. One correspondent to Heinrich claimed he had taught his raven to say: "Edgar, bad boy. Edgar, nevermore." Another reported a raven that heard repeated explosions set off by a highway crew. Afterward the bird shouted, "Three, two, one, kaboom!"

Heinrich reports raven play behavior can incorporate anything from sliding down snow banks, hanging upside down, or approaching dogs and large predatory birds from behind to yank on their tails.

In the Northeast, where common ravens were hunted almost to extinction, they are rare and skittish. In Tennessee they are listed as threatened by the state and currently being considered for reintroduction. But in California they are considered pests. Not without reason: Ravens take eggs and young from endangered least terns and California condors; they pull sparrow and finch chicks from their nests; and, in flocks, they may also take dogs and cats, according to Berry.

Boarman is working to find the animal´s Achilles heel--important resources such as water, food or habitat that can be removed to reduce raven numbers. He also hired marksmen to eliminate problem ravens in a controlled experiment in 1994. But it is not easy to shoot a raven, Boarman says. Though a marksman may kill one raven at a nest or out of a flock, the other ravens quickly move away. "If you shoot at a raven and miss, you never get another chance," Boarman says. Even if sharpshooters are successful, such raven-control methods can spark controversy--especially since the birds are a native species.

On the other side of the bird-reptile equation, Morafka and his colleagues are studying young tortoises so the creatures can be protected more effectively. After his 1991 experiment in which most of the hatchlings in open enclosures were killed by ravens, Morafka tried raising tortoises in covered enclosures with small openings on the side from which tortoises might naturally disperse. But ravens took to perching above the holes, waiting for the tortoises to come out.

Morafka´s most successful attempt to date is to take tortoise hatchlings and put them down random, unevenly spaced rodent holes under the cover of night. Ravens are not active at night, so they are unaware of the nocturnal translocations. And, since rodent holes are pre-formed, there are no telltale signs during the day of fresh tortoise activity. While these experiments are promising, it may be years before Morafka and other biologists have answered enough questions about tortoise biology to make a major difference in hatchling survival.

In the meantime, government officials have taken other steps to help protect the tortoise. Fences have been installed along highways in west Mojave to decrease the number of road-killed tortoises. (Annual surveys show one tortoise killed every mile.) Some landfills in California have been cleaned up, reducing the number of ravens at these sites, though there is no indication this has removed the raven predation problem.

Restocking tortoises may one day aid healthy populations, but where the animals are affected by respiratory disease, adding more juveniles may only fan the disease. And where common ravens are present, restocking may only serve to attract and concentrate the predator.

Any road to recovery for the tortoise is going to be long, experts say. The animals don´t reach sexual maturity until 15 to 20 years, and hatchlings from only a few eggs out of every hundred actually make it to adulthood.

"It´s not like deer management where you can recover from two deer to several thousand deer in a few decades," says Morafka. "With tortoises it might take 40 years just to double the population. It makes it hard to judge the long-term efficacy of our work. Only our grandchildren will know for sure."

Los Angeles writer Michael Tennesen spent two weeks with tortoise and raven researchers in the Mojave Desert last year.

Join today and get a 1 year subscription to National Wildlife magazine
     Flickr Icon           Find NWF on Facebook.           Follow NWF on Twitter.           YouTube Icon    
Connecting...
Certify your yard today!