Homeowners can minimize bird window kills

Copyright © 2005 Blethen Maine Newspapers Inc.

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I expect that you have had a similar experience to one I had a few years ago. I was standing at our kitchen sink when I saw a blur out of the corner of my eye and heard a crash. A sharp-shinned hawk had attacked the birds at one of our feeders, missed its intended prey and then crashed into a window.

Although the window had a screen, the momentum of the hawk was strong enough to shatter the storm-window beneath. Fortunately, the hawk was only briefly stunned and quickly flew away. Barring a cerebral hemorrhage or other internal injuries, the sharpie lived to hunt another day.

Many birds are not so lucky.

The importance of window kills as a source of bird mortality has been addressed in two separate studies. One study has been done by Project Feeder Watch, a program run by the Cornell Laboratory of Ornithology and the Long Point Bird Observatory in Ontario to monitor continent-wide abundance of birds at feeders. For the winter of 1989-1990, 5,500 participants throughout North America reported the number of birds, if any, killed by window strikes. 507 participants (9 percent of the total) reported at least one window kill over the winter; six reporters observed over 10 window-killed birds.

A second study, conducted by Professor Dan Klem of Muhlenberg College in Pennsylvania, has shown that between one and 10 birds are killed annually by each building in the United States.

That translates into a range of about 97 million to 975 million birds each year! The total bird population of North America has been estimated at 20 billion birds. Collisions with windows therefore kills somewhere been 0.5 and 5 percent of our birds each year.Ê

Why do so many birds die from flying into windows? There is little evidence to indicate that birds that hit windows have faulty eyesight. Reduced vision in fog and smoke also plays only a minor role in explaining window kills.

The chief explanation seems to be that birds perceive clear windows as open space and attempt to fly through. The likelihood of window kills is increased if there are windows on both sides of a building, producing a see-through effect. Windows with reflective surfaces act as mirrors, fooling birds into thinking that flight is possible through the window.

Bird feeding increases window kills by attracting birds to buildings. The high concentrations of birds at feeders, particularly in the winter, attract bird predators like the Sharp-shinned Hawk that broke our storm window. The appearance of such a predator, loud noises or sudden movements, may cause a panic flight of birds from a feeder with the possibility of fatal window strikes.

What can homeowners do to minimize window strikes by birds? The best solution -- to eliminate windows altogether -- is not feasible. Covering windows with netting can be expensive and affects the aesthetics of a building.

Making windows obvious as obstacles that should be avoided can reduce window kills. Spiders use this strategy to protect their webs. The webs of many spiders have thick, highly visible strands.

Flying birds easily see these strands and avoid flying through the webs. Klem's research has shown that placing vertical strips of tape on windows cuts down on window strikes dramatically, particularly if the strips are separated by four inches or less.

Horizontal strips are less effective. The black silhouettes of hawks and falcons that can be purchased for placement on windows are not effective.

These silhouettes do not provide enough contrast for birds to realize that windows are obstacles. In new or remodeled homes, architects are encouraged to install windows at a slight angle so that they reflect the ground rather than trees or the sky. Birds are not likely to try to fly into a reflection of the ground.

The placement of bird feeders can also curtail window kills. Klem's studies have shown that placing feeders very close to a window reduces fatal window strikes during panic flights.

Although birds may hit the window after they fly up in response to a loud noise or a predator, the birds will not be flying fast enough to suffer harm or death from the collision.

An alternative solution is to place your feeders well away from the nearest window.Ê

IVORY-BILLEDWOODPECKERS

Although birders everywhere are delighted to learn that ivory-billed woodpeckers are not extinct, the long-term prospects are rather grim. The recent sightings have been made in Cache River National Wildlife Refuge in Arkansas that is only 55,000 acres in extent. Such an area is equivalent to a circle with a 5.25 mile radius, clearly not a large enough area to harbor many Ivory-billed Woodpeckers.

A high-resolution satellite photo of the Cache River NWR can be downloaded at: http://earthobservatory.nasa.gov/Newsroom/ NewImages/images.php3?img_id=16896.

The image shows how the refuge is an island in a sea of agricultural land.

Herb Wilson teaches ornithology and other biology courses at Colby College.Ê He welcomes reader comments and questions at whwilson@colby.edu