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On panes of death

Window strikes kill billions of birds in North America

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As many as two billion birds die in North America every year as a result of smashing into windows, according to a recent re-analysis of window-kill avian losses. Much of the mortality goes unnoticed because many of the victims' carcasses are rapidly removed by crows, cats, raccoons, rats, gulls and other scavengers before they are seen by human passersby.

A preliminary 1990 analysis of the number of birds killed by flying into window panes, by Daniel Klem of Muhlenberg College, suggested the North American toll probably tops one billion birds. That figure was based on per-building avian deaths of up to 10 per year. But recent studies indicate actual per-building avian losses are four to six times greater than that.

Klem, who has studied bird-window mishaps for almost four decades, concludes his earlier estimated bird loss figure was "highly conservative."

"Birds do not perceive clear glass as a solid object," confirms Toronto's official Bird-Friendly Development Guidelines, which were developed by ornithologists and municipal officials.

Scientists agree that birds are not capable of perceiving images reflected in glass as reflections of the natural world, and they fly toward windows that they think are gardens, trees or sky.

Dozens of recent studies suggest the per-building avian toll is 29 to 60 per year. Although there are no concise figures for the Prairies, data from the Fatal Light Awareness Program suggests up to 10 million birds strike buildings in the Metropolitan Toronto area every year.

Some 10,000 are known to die in the Toronto downtown core annually. FLAP estimates that 50 per cent of strikes are fatal, which would mean about five million birds die of strikes in Toronto annually.

Current mortality statistics are based on the actual number of birds found dead, but researchers suggest that the death toll could be much higher because at least 50 per cent of the birds that strike windows sustain serious brain injuries that are fatal soon afterwards.

Even so, published bird-loss figures confirm the enormous magnitude of mortality. The New York City Project Safe Flight confirms some downtown buildings kill more than 250 birds each year. In one year, 650 birds died after flying into Toronto's Consilium Place. According to ecologist Leslie Ogden, 16 Toronto buildings kill on average 30 birds per building per year.

One study estimates one million of the eight million birds that migrate annually across the Chicago area are killed as a result of smashing into buildings in that city.

Urban buildings are not the only culprits. A 20-year study confirms that on average 393 birds die each year by striking a single Lake Erie lighthouse; under foggy conditions, up to 2,000 die per night.

Klem's research indicates a significant window-kill loss occurs at private residences, especially those with bird feeders located close to deadly window panes. A new analysis indicated that feeders located more than two feet, but less than 30 feet, from windows are extraordinarily deadly to birds.

Most avian window strikes occur during the day. But, additional losses take place at night because migrating birds are attracted to lighted buildings. Disorientation occurs if they encounter "light pollution" in the form of "artificial sky glow, light trespass or glare," according to FLAP.

"Migrating birds are attracted to sources of artificial light," confirms a recent FLAP report. "Once inside a beam of light, they are reluctant to fly out (and consequently smash into buildings or other structures)".

A 2008 study by researchers at Max Planck Institute for Ornithology concluded that artificial white light sources interfere with avian visual orientation. Birds mistake lights for celestial cues, such as stars, by which they navigate.

Prairie Canada is an avian hotspot; 137 official "Important Bird Areas" occur there, according to BirdLife International.

There are 379 bird species in Manitoba. In 1979, 21 of them were involved in an incident at St. Agathe in which 220 birds fatally crashed into a TV tower. A 1990 study shows a four-month avian death toll of 29 birds that died as a result of flying into windows and walls at a single house September to December.

The most common avian victims of window or building strikes are: robins, dark-eyed juncos, cedar waxwings, ovenbirds, Swainson's thrusts, northern flickers, hermit thrushes, yellow-rumped warblers, northern cardinals, evening grosbeaks and white-throated sparrows. Of the 150 bird species most commonly killed by striking buildings, 64 are in decline continentally.

Remedial strategies have had mixed success. Several Canadian municipalities (including Richmond Hill, Ont., and Saanich, B.C.) have species bylaws limiting night-light pollution. Since 2006, Toronto has had a "lights out Toronto" program.

Private citizens can play a major role in preventing birds from striking windows by taking steps to help birds recognize windows as barriers. Tests show an effective approach is to place narrow cloth strips, either horizontally or vertically, across window panes about three inches apart. Screens or netting over windows entirely eliminate bird strikes.

On the other hand, Klem's research shows that silhouettes of falcons or owls placed on window panes are not a deterrent, nor are objects dangled on strings in windows.


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

Republished from the Winnipeg Free Press print edition January 8, 2009 A11

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