| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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Feature The Accidental Reaper
Technically speaking, the word "ravenous" does not come from the word "raven," but it might as well. Ravens— big, shaggy cousins to the crow—have always had an appetite for trouble. The Haida of the Pacific Northwest say the raven once stole their salmon by rolling up the landscape in its beak. As the bird flew away, the salmon fell out and stocked the rivers of the world. Ravens still look at most of the world as if it is on the menu. They follow wolf packs through snowy backwoods, scavenging from their kills. Where wolves are scarce they follow roads, eating roadkill. They pick at mussels and dead seals on the high-tide line, or gather at riverbanks where crayfish or salmon wash up. They eat corn and berries, ants, maggots, pistachios, and French fries. They pick apart the droppings of wolf packs and sled-dog teams. Confronted with an especially big hunk of food, such as a moose, they stash scraps under dirt or snow for later. Ravens are also hunters, and their ingenuity can be a little spooky. Ravens wait in trees above birthing ewes and peck the lambs to death as they're born. They distract nesting seagulls by dropping tufts of grass on them, then swoop in and steal the eggs. They have even been caught sneaking into henhouses like foxes. Today, ravens see our sprawling landfills and overloaded dumpsters as so many carcasses where the meat is never finished. Well-fed ravens fatten up, pair off, and make more ravens. Still hungry, or perhaps just bored, they drift through redwood forests and steal threatened murrelet eggs. They wheel over the Mojave Desert, bringing bite-sized baby tortoises back to nests built on power lines. Hordes of ravens on city horizons are taking a toll on other rare species, too. People have suggested shooting them, or using foul-tasting dummy food to make them regret their dinner choices. But ravens are native to the West, leading some people to argue they should be left alone. And anyway, the big black birds are tough to outwit. The most promising answer may be a new idea some are calling "crumb management." It rests mainly on persuading the public that there is something they can do to help. Ravens live all over the western United States, in the Northeast, along the Appalachian Mountains, and throughout Canada. Almost everywhere, their numbers are rising. In California, the Audubon Society's annual Christmas Bird Count records a ninefold increase in the number of ravens since 1970. Correcting those numbers for the growing number of count participants, there are still at least five times as many ravens in California now as 35 years ago. In Santa Cruz County, ravens are booming. Christmas counts here logged no more than two ravens per year during the 1970s. Back then, the mountains had fewer roads, and fewer houses left fewer trash cans unattended. There was more forest and less pasture, scattered with less horse feed. But as development crept in from all sides, 1980s counts crept into the dozens. The 2002 count was 356 ravens. The term "crumb management" was coined in Santa Cruz County, at Big Basin Redwoods State Park. Here, sunbeams drift between massive redwood trunks and head-high ferns. In summer, the picnic spots are perfect: cool, quiet, and dim. But those picnics could spell disaster for the marbled murrelet, a small oceangoing bird that nests on stout branches high in the ancient trees. Recent work by ecologist Zach Peery and others at the University of California, Berkeley, has determined that ravens and jays raid as many as three-quarters of all murrelet nests in the area. It's a piecemeal offensive. Ravens don't search out murrelet nests amid the mossy branches; picnic tables and potato chips are much easier to find. But ravens are sharp-eyed, restless, and seldom full. They find the nests anyway. Murrelets lay only one egg per year, and forgo nesting in many years if fish stocks are low. Slow reproduction plus intense predation means that the murrelets in these mountains are unlikely to survive the next 40 years. Ravens that hang around picnic areas are driving murrelets extinct without even trying. To protect the murrelets, park managers have decided to get serious about stray crumbs. Starting this summer, rangers in four Bay Area redwood parks will give out brochures, screw placards onto picnic tables, and lead campfire talks to highlight the connection between discarded food, curious ravens, and struggling murrelets. "We can hope that if we take away the major food source they might go elsewhere looking for food," says crumb project coordinator Portia Halbert, an ecologist at Big Basin State Park. Supporting this idea is work by biologist David Suddjian, who found that raven numbers in redwood campgrounds were five to ten times higher than in unbroken forest just a half-mile away. Peery thinks it's a tall order. "If you've got 1,000 people camping up in the woods every summer weekend, people just aren't on the ball enough to keep their food cleaned up all the time," he says. But even though crumb management is unlikely to keep ravens out of redwood forests entirely, it may make them less likely to run across murrelet nests. In Southern California's Mojave Desert, the threatened desert tortoise moseys around more than ten million acres of creosote bush and cactus nibbling on grasses and wildflowers. It's an unforgiving landscape with little water and few trees. Pickings here have always been slim for ravens. But the arrival of landfills, livestock watering troughs, and power lines—perfect places for raven nests—has spurred more than a 1,000 percent increase in ravens over the past 30 years. Desert tortoises are California's state reptile and one of the largest tortoises in North America. Adults have high, boxy shells up to 15 inches long, flecked with gray and yellow. Mature tortoises are large enough, and their shells hard enough, not to suffer from raven attacks. But ravens can lift entire young tortoises back to their nests, puncture their soft shells, and feed the threatened reptiles to their nestlings. The threat from ravens depends on where a tortoise lives, according to a study led by William Boarman of the United States Geological Survey in San Diego, California. Ravens congregate around landfills, putting nearby tortoises in much the same kind of danger as murrelets nesting around picnic tables, he says. If not for the food and water humans unwittingly provide, Boarman's research indicates, Mojave ravens would be declining instead of booming. But ravens that nest far from landfills have to hunt for wild food, jeopardizing tortoises there as well. Boarman found, paradoxically, that tortoises are best off if they are near a landfill but on the territory of a nesting pair of ravens. "That's because the other ravens keep the breeding adults too busy defending their territory, and they can't hunt for hard-to-find food," he says. In the redwoods, the raven problem centers around picnic tables in state parks. But ravens are eating tortoises across five California counties and two other states. Amy Fesnock, a biologist at Joshua Tree National Park, says her park can't fix the problem on its own. "We're completely surrounded by cities and we don't have any jurisdiction over what they do," she says. "Until cities start passing ordinances and enforcing them, there's not much we could do that would make sense." Possible solutions to the raven problem divide into three categories: quick fixes, good ideas that don't work, and ways to address the root problem. Quick fixes, such as shooting ravens en masse, tend to focus on the short term. Because there are so many ravens, killing them is "like stepping on ants," says Mark Pavelka, a U.S. Fish and Wildlife Service biologist in Carlsbad, California. "As long as we think we've got to go out and shoot ravens we're going to be on the losing end of the stick," he says. An Icelandic study backs Pavelka's argument. In Iceland, people shoot ravens to protect eider ducks and their valuable down feathers. But raven numbers remained essentially constant there for a decade despite an average of 4,000 birds being shot each year. Some scientists have actually trained wild ravens not to eat bird eggs through a process called conditioned taste aversion. In two separate experiments, one working with sandhill cranes in Oregon and the other with endangered least terns near San Diego, biologists allowed ravens to steal dummy eggs they had treated with a chemical that made them sick but didn't kill them. The ravens stopped raiding nests but stayed on their territory, defending it against intruding ravens and protecting least terns in the process. Unfortunately, taste aversion only works at colonies, where nests are numerous enough for ravens to associate dummy eggs with real eggs. And even so, breaking ravens of their egg habit doesn't stop them from stealing young birds. So most biologists recommend dealing with the root of the problem. The solution is easy in concept: just clean up the extra food and water. The hard part is getting thousands of people to realize that ravens are a problem worth fixing. In the Mojave, an educational program similar to crumb management is set to begin in 2006. Sponsored by the nonprofit Defenders of Wildlife and a consortium of agencies and county governments, the program will encourage people to adopt raven-proof trash cans via radio spots, brochures, and recommendations printed on grocery bags, says Defenders ecologist Cynthia Wilkerson. The group also plans to lobby landfill owners to cover their trash, and to persuade local governments to pass laws governing trash disposal. The issue has even brought environmentalists and off-highway-vehicle clubs onto the same side, because off-highway drivers are eager to see tortoises protected without closing desert trails. "We need to work on the public to make sure the tortoise stays in its environment, so the people can come see it there and not in a pen somewhere," says John Stewart, director of environmental affairs for the United Four Wheel Drive Associations. Stewart's association has educated its members about desert tortoises for the last two years. Despite the consensus on education and tidier trash as long-term solutions, no one knows yet how effective crumb management will be. "I don't know whether to be cautiously optimistic or cautiously pessimistic," says Boarman. "It's just too hard to control human behavior as much as we might need." As the message trickles out to the public, ravens will continue to snack at dumpsters and explore their surroundings. Although no one thinks wholesale raven shooting is the answer, most biologists agree that shooting may be necessary as a stopgap. Removing specific ravens could buy time and keep species like the marbled murrelet from winking out. This idea has gained support because ravens have individual habits. Some discover that baby tortoises make a great dinner for their nestlings, while others content themselves with roadkill. The few that make a habit of eating tortoises should be shot, says land manager John Hamil. "They're a big, beautiful bird," Hamil says. "But when you find a nest with 10, 20, 30, 40 baby tortoises under it, it's hard to say that's not a problem." Opponents argue that ravens are native species, and it's not our place to take them out. In the early nineties, the courts agreed. A plan to shoot 1,500 ravens to protect desert tortoises had won approval. But the courts granted a restraining order to the Humane Society of the United States, stopping the shooting after 120 ravens had been killed. The reprieve may have been just as well, says Boarman. The clouds of ravens that congregate at dumps are mostly young birds. Breeding adults forage elsewhere, and will have little trouble restocking the landfill populations. The plights of murrelets and desert tortoises are just the starkest examples of a trend that's happening nearly everywhere. Ravens attack seabirds on cliffs, yanking them off their nests to get at the chicks. Around San Francisco, some ravens watch heron colonies, waiting for adults to leave their spindly chicks undefended. Others stumble over snowy plover eggs nestled in the sand at public beaches, or raid shrike nests out on the Channel Islands. With such widespread mayhem, it's easy to think of ravens as belonging in nature's "bad ideas" pile, along with rats, rattlesnakes, and mosquitoes. But ravens are only booming because they are better at uncovering food than we are at covering it up. If crumb management works, it will work by fixing the human side of that equation. After all, ravens can't see where their appetites are taking them. We can. Hugh Powell is a science writer in northern California who always finishes his fries. |
