The Raven Returns

Evermore than Before

by on July 01, 2002

 
Painting by Maryjo Koch.
 

 

My first raven flew out of the fog along the road to Clingmans Dome in the Great Smokies. For years, that encounter formed my image of the bird: a creature of the dark forests, numinous, unknowable.

That changed when I moved to California, and the ravens moved into my neighborhood.

On my way to the Ashby BART station one morning, I watched two ravens being chased by a pair of crows, in a cacophony of caws and grawks. The ravens were the bigger ones—the megacrows—with the deeper, guttural voices. Although the ravens were more powerful than their pursuers, the crows appeared to enjoy a home court advantage.

Not long ago, ravens would have been an exceptional find in the Berkeley flatlands where I live, or in San Francisco’s Financial District, where I’ve seen them negotiating the high-rise canyons. How did these spirits of the wilderness become urbanites, our everyday neighbors?

Corvus corax, the common raven, is a bird of superlatives and paradoxes. With its 2.5-pound bulk and 53-inch wingspan, it’s the biggest North American member of the order Passeriformes, the perching birds. It’s also the world’s most widespread passerine species, ranging from the high Arctic to Nicaragua, North Africa, and India. Once rare over much of their California range, these adaptable birds are now abundant enough to be a relatively common sight throughout the Bay Area and even to pose a threat to the survival of some rarer species.

Raven

Ravens have shaggy throat feathers and heavier bills than crows. Illustration by Giovanni Maki.

The common raven weighs twice as much as its closest California relative, the American crow (Corvus brachyrhynchos). Besides size and voice, there are other ways of distinguishing between these two large, black birds. Ravens have heavier bills and shaggy throat feathers and, in flight, display more pointed wings and a wedge-shaped tail. Flight style is also a clue. “Ravens often soar, crows never do,” says birding authority David Sibley. And if the bird you’re watching does a barrel roll, it’s definitely a raven.

It’s long-lived for a bird; one captive in the Tower of London died at 44, although 13 years is a more typical life span in the wild. With the highest brain volume relative to body mass, it may also be the brightest of all birds. Ravens have demonstrated apparently insightful behavior and a capacity for abstract problem solving…

The modern raven is known to have been present in the North American Pleistocene (1.8 million to 10,000 years ago). Ravens are among the most common bird species in the Rancho La Brea fossil deposits in Southern California, which range from 40,000 to 10,000 years in age. They shadowed wolves, bears, and other predators and followed the bison herds. When human hunters made their first kill on North American soil, ravens were waiting for their share.

Raven nest sites
Ravens use a variety of nest sites, from coastal cliffs to the NASA wind tunnel in the South Bay. Photo by Jeffery Rich.

California’s ravens are enjoying prosperous times now, but they’ve had their ups and downs. During the Gold Rush era, ravens were a common sight along the coast road between San Francisco and San Mateo. But by the 1920s, W. Leon Dawson described them as “almost disappearing from the more thickly settled regions,” and Joseph Grinnell said they were rare in the Bay Area except for Point Reyes and the Sonoma coast. Lacking hard data, we can only speculate as to causes for this decline. Ravens may have been shot as vermin, as crows and jays were; and the decimation of Native Californian hunting cultures, the extinction of the California grizzly, and the overexploitation of marine mammals may have reduced the availability of carcasses for scavenging.

It’s a different story today. Using data from the Audubon Society’s annual Christmas Bird Counts, Charles Coston of the San Francisco Bay Bird Observatory in Alviso looked at population trends for ravens, crows, and jays from 1966 through 1997 in four Bay Area count circles: Oakland, Palo Alto, San Jose, and Crystal Springs Reservoir. While jay populations appeared stable, those of ravens and crows were different. “Since the early 1980s the raven population has exploded,” Coston concluded. “The most telling trend is for ravens in Oakland and Palo Alto. Rare before the middle 1970s, they are now routine. Something changed.”…

That jibes with my own review of Christmas Count results for a shorter period, with 1983—the year the San Francisco count was revived—as a baseline. Point Reyes and western Sonoma County, the raven’s historic strongholds, did not show major fluctuations. But San Francisco went from 14 ravens in 1983 to 239 in 1999, while Oakland went from 5 to 101. Areas reporting no ravens, or only one or two, at the beginning of the period had them in double digits toward the end.

Coston also reported a dramatic jump in Bay Area crow populations within the last decade, and the most recent Point Reyes count had record highs for both crows and ravens. If these close relatives are competing for the same resources, so far there seems to be enough to go around. Ornithologist William Boarman, who studies ravens in Southern California, says the raven boom there doesn’t appear to be adversely affecting either crows or hawks and other raptors.

The local ravens are not just passing through; they’ve become year-round residents. Nesting has been confirmed in all Bay Area counties, including San Francisco. Breeding Bird Survey data parallels Christmas Count trends, with increases of more than 10 percent per year for some roadside survey routes. Bay Area ravens use varied nest sites, from coastal cliffs and sea stacks to the NASA wind tunnel structure and the Shoreline Amphitheatre in the South Bay. Reporting on a 1999 survey, John P. Kelly, director of research and resource management at Audubon Canyon Ranch in Bolinas, concluded: “Concentrated raven use of coastal and agricultural areas is matched by their ability to exploit the most urbanized habitats surrounding San Francisco Bay.”

What’s fueling the boom? Garbage, in part. Ravens, with their taste for carrion, learned to take advantage of human-related food sources, from battlefields to slaughterhouses, in medieval Europe, becoming common urban scavengers. In North America, where urbanization occurred much later, ravens have recently made the adaptive shift from wilderness to a landscape transformed by human activity. Modern landfills have been a bonanza. Boarman says these cornucopias of garbage, along with roadside rest areas, farm fields, and sewage ponds, provide year-round food and water sources for desert ravens. In cities, they’ve discovered Dumpsters, open trash cans, and school yards.

“Ravens will go a long way for human foods,” ornithologist John Marzluff told me, “and I think this primarily increases their numbers by increasing the survival of young birds.” In Washington, he has documented survival rates that are higher close to towns and campsites than in wilderness areas. “Ravens can . . . pump out four to seven young per pair per year,” Marzluff continued. And young birds wander widely, pioneering new areas. Ravens banded by Bernd Heinrich in Maine were found later as far away as New Brunswick, New Jersey, and Buffalo, New York.

Garbage-fed ravens don’t abandon their predatory ways. They’re formidable birds, capable of killing seal pups, reindeer calves, and lambs (Sonoma County sheep ranchers once lobbied—unsuccessfully—to have the local ravens exterminated). And they’ll take a wide range of prey: mammals, reptiles, other birds. Kelly watched one raven tackle a running ground squirrel, and Robin Smith of the Sequoia Audubon Society saw a pair try to force a great horned owl off its nest to get at the owlets.

Where they’ve come to rely on landfills and similar sources, ravens are what ecologist Michael Soule calls “subsidized predators,” with populations far exceeding the normal carrying capacity of their habitat. This multiplies their pressure on the prey population base; and when the prey is an endangered species, the raven boom becomes a dilemma for wildlife managers.

Boarman’s Mojave ravens feed on juvenile desert tortoises and may be threatening that species’ survival. In the Pacific Northwest, raven predation on marbled murrelet chicks is a growing concern. Bay Area ravens have raided the sea cliff colonies of the common murre and the treetop egret nests at Audubon Canyon Ranch; and San Francisco birder Dan Murphy says they’ve been checking out the bank swallow colony at Fort Funston. Most vulnerable in our region, though, is the endangered western snowy plover, a small beach-nesting shorebird.

Raven predation on snowy plovers isn’t new, but the long-term increase in raven numbers in western Marin County—combined with the loss of snowy plover habitat in other locations—has put the shorebirds there at serious risk. At Point Reyes National Seashore, snowy plovers lost more than two-thirds of their eggs and nestlings to ravens in the 1995 season. Beginning the following year, Point Reyes Bird Observatory volunteers and National Park Service staff placed protective enclosures around plover nests, and the shorebirds began to rebound. PRBO then launched an intensive raven study.

In her Point Reyes study area, PRBO biologist Jennifer Roth has equipped 16 ravens with lightweight transmitters. Tracking their movements has revealed two discrete groups with different behavioral patterns. While breeding pairs stick to home ranges of a few square kilometers, flocks of younger nonbreeders patrol larger areas on both sides of Tomales Bay. Nonbreeders may pair off, but stay with the flock until a nesting territory opens up. Once the pair bond forms, it’s for life, although a lost partner is quickly replaced. Roth estimated the Point Reyes raven population to be about 285, including 13 nesting pairs. The distinction between territory-holding breeding pairs and roving nonbreeders has also been observed by Heinrich in the Maine wilderness and Murphy in San Francisco, where the nonbreeders hang out along Ocean Beach.

Social relationships among ravens are complex and imperfectly documented. In a recent summary of research, Heinrich and Boarman described our current understanding of ravens’ life history and behavior, including how they claim and defend territory, as “woefully inadequate.” Studies in Europe relate the size and spacing of territories to food resources. Interactions between holders of adjacent territories range from talon-grappling boundary clashes to cordial neighborly visits. Heinrich found in his Maine studies that nonbreeders ganged up to gain access to food in the territories of breeding pairs, with a dominant flock member recruiting other ravens to the effort via a distinctive “yell.”

Sharing a long-term communal roost near Drake’s Estero, the more numerous nonbreeders at Point Reyes also establish temporary bases near major food sources. In the National Seashore, that means dairy ranches. “They’ll get down in the trough and eat cracked corn with the cattle,” Roth said. The ranches also provide carrion: “Calving areas are really popular.”

Reducing raven impact on snowy plovers will be a tricky business, Roth conceded. The removal of individual predators would only be a temporary solution. To keep raven numbers in check in the National Seashore, ranchers will need to end or minimize the free lunch.

At the Audubon Canyon Ranch heronry on Bolinas Lagoon, John Kelly deals with a pair of known offenders. They moved in eight years ago, scavenging around the colony. “Then in 1998 we suffered heavy predation by the resident pair,” Kelly said. “All the great egret nests in the colony failed.” Although the last few years have not been as bad, there’s still concern that the egrets might abandon their nest sites. The ravens don’t go after eggs unless another predator—an eagle or raccoon—rousts the adult egrets off their nests. Instead, they kill chicks when, about three weeks after hatching, both parents leave them un-attended to forage. The great blue herons, “bigger and meaner,” are less vulnerable. Kelly also mentioned that ravens have taken adult snowy egrets at another heronry on the Marin Islands.

Both Roth and Kelly have learned to respect raven intelligence. “We have to bait for quite a while before we trap them,” Roth told me. “We go to great lengths: hiding the traps, hiding ourselves, starting before dawn. It’s difficult to outsmart them. They really key into the landscape.” That ability may explain their success in colonizing diverse habitats, including farmland and cities, and exploiting novel food sources.

Just how sophisticated is raven cognition? In a famous experiment with captives in Maine, Heinrich demonstrated that ravens can size up a problem—how to get at a tasty slice of salami suspended from a perch by a string—and solve it by apparent insight rather than trial and error. Heinrich’s crow subjects never got it. Ravens’ food-caching behavior shows an ability to anticipate the moves of other individuals. “Ravens hide food purposefully out of sight of others,” he explained. “They appear to know other ravens will raid their caches.”

When Heinrich spoke of ravens having local dialects and traditions, I asked if it would be a stretch to think of them as having culture, which can be broadly defined as a nongenetic way of instilling behavior. “I don’t think it’s a stretch at all,” he replied. Young ravens stay with their parents for up to three months, plenty of time to learn foraging techniques by observation. Innovations could be passed from one generation to the next. This may sound far-fetched, but then ravens are no ordinary birds.

Their huge vocabulary, from croaks and screams to a semimusical warble, suggests they have a great deal to say to each other. And they’re uncanny mimics (Heinrich trained one to say “Nevermore”). Play may also correlate with intelligence. It’s hard to watch a raven pair’s tandem aerobatics without sensing an exuberance, a delight in flying for flight’s sake. Ravens have been observed playing catch, tobogganing down snow-covered hills, and pulling the tails of wolves and dogs. As adolescents they’re insatiably curious, deconstructing anything that might conceal food, including parked cars and grounded aircraft.

It all adds up to a behavioral flexibility rare in birds. Ravens are what evolutionary biologist Ernst Mayr calls “open-program” animals. Much of what they do is not hardwired; they learn it by exploring their environment and watching elders and peers. And an important part of that environment is social: the flock’s hierarchy, the pair next door, even the predators who lead them to food. Social complexity may drive the evolution of big brains; and as Heinrich writes in his book Mind of the Raven, “Social complexity increases inordinately when individual recognition becomes possible and the animal tracks not just others, but myriad specific others.”

Is this beginning to sound familiar? “Several features of the raven’s life history and ecology are comparable to those of the hominids’,” Heinrich writes. To some degree the human mind and the mind of the raven may be convergent, shaped by similar selective forces. It may just be that ravens thrive in the world we’ve made because, cognitively speaking, they’re a bit like us. California’s ravens may have lost the first round in their encounter with modern civilization, but these intelligent, opportunistic birds are back—and it looks as if they’re here to stay, adding a touch of wildness to our urban lives.

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one comment:

Sherry on July 4th, 2013 at 2:53 pm

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