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Wisconsin Natural Resources magazine

Wisconsin Natural Resources magazine

In animal intelligence tests crows consistently outscore rats and often dogs. © Jack R. Bartholmai
© Jack R. Bartholmai

October 2007

Cunning corvids

Learn a thing or two about the Einsteins of the bird world.

Kathryn A. Kahler

In animal intelligence tests crows consistently outscore rats and often dogs.
A premonition when West Nile strikes

Their attributes read like an ad in a singles column: Intelligent. Communicative. Good sense of humor. Family-oriented and looking for long-term relationship. Community-minded and unhesitant in sticking up for their friends.

With such endearing qualities, one has to wonder why the crow doesn't get a little more respect. The species has been associated with darkness and death throughout history. Mark Twain described it as "a low comedian, a fussy woman, a scoffer, a practicer and propagator of irreverence and a busybody." Instead of letting myth or legend jade our opinion of crows, let's take a look at some of the facts about this much maligned bird.

A member of the Corvidae family, which includes ravens, magpies, jays and nutcrackers, the American or common crow (Corvus brachyrhynchos) is one of the most common North American birds. It's found across a wide range of habitats from coast to coast and from the northern reaches of Canada throughout the United States, except for a thin strip near our southwestern border. Other members of the Corvus genus include the northwestern crow, the fish crow found mostly in southeastern coastal states, and the common raven. One occasionally sees ravens – distinguishable by their larger size and shaggy throat feathers – in northern Wisconsin.

Aside from some parrots, the American crow and its cousins worldwide are considered the most intelligent birds, close to humans in their brain-to-body ratio. Betty, a New Caledonian crow, attained celebrity status a few years ago when she was videotaped shaping straight pieces of wire into hooks to fetch out-of-reach food in a laboratory setting, not just once, but nine out of 10 times. Wild crows of that species use their beaks to fashion twigs and leaves into tools to poke and prod into crevices in search of grubs and insects. Crows have been observed flying higher and higher then dropping mussels and walnuts onto hard surfaces to crack them open. Crows in Japan were seen setting nuts in roads then hopping up on the curb and waiting until cars drove over the nuts before hopping back to retrieve their dinner.

Crows also take part in cooperative feeding, a kind of tag-team effort used to steal food from larger birds, or even mammals. Reportedly, one crow will sneak up behind a bird with food in its mouth and pull a tail feather. When the bird turns to defend its meal and drops its food, another crow swoops in and recovers it. Similar encounters involve crows pulling otters by the tail to nab their meals. Even ice-anglers are the victims of crows' craftiness when they steal bait from unattended fishing lines.

Crows are extremely social and join together for cooperative feeding. © Jack R. Bartholmai
Crows are extremely social and join together for cooperative feeding.

© Jack R. Bartholmai

Caching food is another crafty crow tactic. Crows, like other corvids, stash excess food in hiding places, usually in holes in the ground that they cover with dirt, leaves or stones. If a crow thinks it's being watched, it will recognize the risk of giving up the location of its nest and hide a morsel of food, rather than take it directly to the nest. If a raven thinks another bird has spied its cache, it will fly back after the other bird is gone and move, or re-cache the food to prevent pilfering.

Nathan Emery and Nicola Clayton reported in the December 2004 issue of Science magazine that caching and re-caching food is evidence that corvids are equal to apes in their ability to reason from a cause-and-effect perspective. Like apes, corvids also show flexibility, imagination and prospection, or future-thinking.

"Cache recovery may require more than simply remembering where their caches are hidden, for species that store many types of food," they observed. "These species may need to process information about the location of the cache site, the type and perishability of the cached item, and the social context of caching."

Other corvids may worry about perishability, but crows seem to relish all food in varying degrees of decay. A better question than "What do crows eat?" would be "What don't they eat?" Studies show that crows eat over 600 different food items, one-third animal matter – like insects, worms, larvae, fish, frogs, snakes and roadkill – and two-thirds vegetable matter. Their vegetable of choice is corn, but crow fanciers contend they do more good than harm to cornfields, consuming thousands of cutworms, grasshoppers and harmful weeds, and gleaning kernels from picked fields to prevent volunteer plants next spring. Urban crows add all manner of garbage – bread, spaghetti, even French fries – to their varied bill of fare.

Scientists may speculate or disagree about its purpose, but of this they are certain – crows play. How else could one describe this behavior reported by a birder in Springfield, Ohio? "I spotted three crows on a short grass lawn alongside the road on the outskirts of town. Two of the birds were standing and walking around in the normal fashion but the third was lying on its back, moving its legs in a 'bicycling' motion reminiscent of one of the exercises I was forced to do in physical education class in school."

Young birds drop sticks from their bills in flight and catch them before they hit the ground, or use them to play tug-of-war. They perform aerial acrobatics in gusty winds and swing upside down from swaying branches or vines. They dance to the beat of an imaginary drummer as they bounce and jump with outspread wings.

Communication is another characteristic that sets crows apart from other less outgoing species. Crows have about two dozen calls, ranging from their distinctive caw-caw to clacking, rattling or gurgling sounds. They mimic all sorts of other beings, such as humans, cats, dogs, geese and their most hated predators, barred and great horned owls. Some of their varied calls are a means of keeping in contact with mates and offspring, and another acts as a call-to-arms for any crow within hearing distance to drop everything and join the mobbing of a predator.

The crow chorus: Loud, raucous cawing alerts other crows to danger and scares away predators. © Jack R. Bartholmai
The crow chorus: Loud, raucous cawing alerts other crows to danger and scares away predators.

© Jack R. Bartholmai

Mobbing behavior is initiated when a crow observes a predator near its nest or anywhere else, for that matter. The crow begins its loud, raucous cawing and keeps it up until joined by others in the vicinity. The din reaches a riotous level. The behavior serves the two-fold purpose of alerting others to danger and sometimes driving off the offending predator.

Stan Temple, retired UW-Madison wildlife ecologist and falconer by avocation, described an encounter with one of his hawks. His red-tailed hawk caught a crow and brought it to the ground. The screaming of the captured crow sounded the alarm for the crows in a huge nearby roost and "They descended like a black tornado on the hawk," and the captured crow got away.

When it comes to family life, crows are exemplary in many respects. They mate for life and provide for their young longer than any other bird. The lovefest begins in March or early April when both male and female pick a nest site, usually high in the crotch of a tree, and get to work constructing their home. They always start fresh with a new nest each year. The male does the heavy construction, forming a base of large sticks. His mate provides a feminine touch, shaping an inner bowl of twigs, bark and mud, lined with soft grasses, feathers and fur. Four or five brown-speckled aqua eggs are incubated by the female and hatch in 19 days, usually by early May. The males don't share incubation responsibility, but bring food to their mates while they are on the nest. Young stay in the nest for 35 days and fledge in six to eight weeks. They are usually food-independent by early August. With that long a nesting period, crows usually have only one successful brood a year.

That's where similarities with other species end. Unlike most other birds, crows don't rush matters when it comes to rearing their young. After fledging, young birds stay with their parents for one to three years, sometimes as long as five years. These adolescents help their parents raise subsequent broods by bringing food to the incubating female and chicks still in the nest and chasing away predators. Scientists call this behavior cooperative breeding. These extended families of parents and siblings of differing age groups, commonly 10 to 15 birds, stay on the breeding territory, forage and roost together throughout the summer.

In fall and winter, crow families may leave their home territories and assemble in small neighborhood groups at designated staging areas late in the afternoon. At dark, the flock takes off and moves to a nighttime roost, joined by other neighborhood groups from miles in every direction. These communal roosts can attract hundreds, thousands, even hundreds of thousands of birds and are usually located near a reliable food source, like a landfill or cornfield. Come morning, the roost empties quickly as the smaller flocks find their way back to individual territories. In spring the roosts break up when the nesting cycle begins anew.

Crows are considered partially migratory, depending on where they live within their range. Birds in the northernmost parts of Canada, where minimum temperatures average 0 F, will migrate to Nebraska and Oklahoma, where roosts have been known to hold half a million birds. Wisconsin crows probably stay put most of the year, especially in mild winters like those of late. During the winter roosting period, individuals may spend time on their breeding territory during the day and join the communal roost at night. Other non-breeding juveniles may leave home for the winter and return to the breeding territory in spring to help raise their new brothers and sisters.

So come garbage day if you find that crows have had their way with your pizza boxes and cold French fries, consider this – they've got a family to feed. Get a heavier lid for the garbage can or you might just find them picking their way through the grease-stained newspapers to the personals column, to find the perfect mate and take up residence over your driveway.

Writer Kathryn A. Kahler recently retired from her position as circulation, promotions and production manager of Wisconsin Natural Resources magazine.

A premonition when West Nile strikes
Though there isn't much published research on the subject, American crow populations seem drastically affected by West Nile virus (WNV), both in virulence and numbers of birds killed.

Crows' intelligence makes them extremely difficult to study by traditional trapping/marking techniques. Most studies rely on testing collected dead birds or infecting laboratory birds with the virus. This research shows this extremely virulent virus introduced to the United States in 1999 hits crows harder than other wildlife species. In the lab, 97 percent of the crows experimentally infected with the virus died. Another study found only three percent of a sample of over 100 live wild crows in Illinois had WNV antibodies in their blood. That means most don't survive infection.

Once infected, most crows die within one season, as many as 72 percent in a Stillwater, Oklahoma study done in the summer of 2002. Field studies in Kentucky and New York, where bird carcasses were collected and tested, showed that crows had the highest infection rate of all species tested – 44 percent of 12,500 birds in New York and 87 percent of 1,549 birds in Kentucky. In one of the few population studies of its kind, a 2002 University of Illinois banding study found WNV responsible for 90 percent of American crow deaths during the study period.

These statistics might suggest that crow populations nationwide would decrease as WNV moved westward, but monitoring efforts such the Christmas Bird Counts don't back up that notion. While some states have seen crow populations decline over the past 10 years, others have seen substantial increases, even in years with significant incidence of WNV. Nationwide, these annual crow counts in 2006 were 13 percent lower than in 1999, with peaks and valleys in between. The crow count in Wisconsin registered 14,000 in 1999, peaked at 22,000 in 2002, and declined slightly to 17,000 this past December.