Analysis

Winnipeg Free Press - PRINT EDITION

Clever crows increasingly plague cities

By: Robert Alison

Posted: 12/14/2008 1:00 AM | Comments: 0

Resident urban crows are increasingly plaguing North American cities. Victoria, B.C., has the dubious distinction of being the "crow capital" of North America, with Vancouver and Seattle not far behind.

Winnipeg merits honourable mention because of the total number of permanent-resident crows on the Prairies, almost 60 per cent reside in Winnipeg.

According to the Center for Wildlife Damage Management at the University of Nebraska, excessive urban crow numbers cause ecological and other problems and the birds are widely considered to be urban pests.

Urban crow numbers are skyrocketing, up thirtyfold since the 1970s in some cities. Zoologist John Marzluff of the University of Washington calls the phenomenon an "urban invasion." Crow numbers have escalated more than 500 per cent in Victoria in the past 40 years, but other cities have experienced large increases, too: Albuquerque (New Mexico) 425 per cent, Hartford (Connecticut) 187 per cent and Sacramento (California) 122 per cent. Greater Victoria hosts close to 10,000 crows.

Several crow species are experiencing urban upsurges. Along the Pacific coast, the northwestern crow is mainly involved. Elsewhere, it is the American crow and fish crow. Some urban crow populations are increasing at the annual rate of 15 to 32 per cent.

Researchers attribute rapid growth in urban crow numbers to abundant food sources, which the intelligent birds readily exploit by means of inventive foraging behaviour. Stan Temple, a researcher at the University of Wisconsin, says the handy availability of "Dumpster-style" food has made urban crows up to 100 times more abundant per unit area compared with their rural counterparts.

Marzluff says discarded human food and garbage increase urban crow numbers. Urban food sources "fuel" crow population growth, he and his colleague, Eric Neatherlin, have concluded on the basis of their extensive studies of urban crows in Seattle, where Dumpsters, especially at fast-food outlets, are crow restaurants.

"Downtown crows are much bolder than rural crows," the researchers have concluded, adding that they easily find food through co-operative searching strategies in which groups of crows forage as units.

Urban-crow nuisance behaviour, and ecological problems stemming from excessive crow densities, are exacerbated because urban crows have longer lifespans and produce more offspring compared with rural crows.

The wildlife damage centre confirms that crows "hunt, pirate and scavenge" caterpillars, fish, frogs, salamanders, snakes, eggs and young of birds, garbage, fruit, grain and vegetables.

According to the Washington Department of Fish and Game, crows pull nestling birds from nest boxes.

Crows are extraordinarily clever, an attribute that helps them thrive in downtown areas. Nicola Clayton and Nathan Emery of Cambridge University have suggested that crows are as clever as non-human primates.

So far, there have been no comprehensive studies of the ecological impact of high urban crow numbers in Canada, but research elsewhere shows excessive urban numbers can diminish urban songbird numbers and reduce densities of snakes and amphibians.

Crow intellectual capabilities make the birds very difficult to control. They are not easily live-trapped. There are no effective crow repellents or toxicants. The birds are not easily frightened by exploders, clapper devices, pyrotechnics or alarms. Recorded crow distress calls are only temporarily effective.

Human aversion to crows has been persistent and pervasive throughout history. The birds have been consistently reviled. Consequently, the birds have been singularly excluded from protection throughout the 4,000-year development of wildlife conservation laws worldwide.

Crows were excluded from Britain's wildlife conservation laws for 1,000 years. Canadian wildlife protection laws, which were originally modelled on British laws, specifically exclude crows from protection, the only native North American birds not protected by federal or provincial legislation.

Organized crow shoots, which occur annually in Manitoba, are not believed to have any impact on urban crow numbers, Manitoba Conservation says.

Crows, as well as ravens and jays, are classified as corvids, a group of birds notorious for preying on birds' eggs and young.

Recently, West Nile virus has substantially reduced some rural crow populations, but urban crows have not experienced similar population losses.

Robert Alison is a Victoria wildlife biologist and writer with a PhD in zoology.

Republished from the Winnipeg Free Press print edition December 14, 2008 b5

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