The Danger to Birds Caused by the Reflections


By Benjamin P. Burtt

TOPIC: The dangers to birds caused by the reflections from glass windows.
Some are killed by colliding at high speed with the glass while others spend fruitless hours attacking their own reflection visible in a window near their nest.
One Billion Birds are Killed each Year when they Collide with Windows in the United States.


This column is divided into two sections here

Section 1 contains a copy of the column as it appeared in the newspaper on the date above.

Section 2 contains additional information for the reader who is interested in learning more about the subject of this column


The fall migration has startedSparrows that nested further north will pass through during September and October. Even now the white-throated sparrow is migrating.

Question: Mr. Burtt: Recently, a bird hit my window and died. Is there some way to prevent this? M.S. Liverpool, NY.

Dear M.S.: This happens to a lot of birds as they fly away from a feeder towards the reflection of the sky or the reflection of the plants nearby.

In addition during the next two months, migrating birds will also collide with windows on homes that are situated on a migration route. They collide with the glass on buildings in our big cities. Birds do not recognize glass as a solid object.

Professor Daniel Klem of Muhlenberg College has studied bird collisions with windows in commercial buildings and homes. It is a very serious environmental problem and windows kill more birds than any other thing that humans do except our destruction of their habitat.

A conservative estimate indicates that almost a 1 billion birds are killed by glass each year in the U.S. This is about 10 birds per building. Before the World Trade Center towers were destroyed, about 32 birds were found dead there per year by volunteers from the NYC Audubon Society. This is only a fraction of the casualties for many carcasses probably were not found, or were cleaned up by maintenance. Some undoubtedly were eaten by gulls or rats.

CAPTION: Thousands of migrating birds are killed by colliding with windows in the buildings of New York City. The white-throated sparrow is the most frequent victim found in downtown Manhatten. The arrows in this painting from Peterson's "Birds of Eastern and Central North America" call attention to important field marks for identification, the white throat and the white stripes on the crown. ( Courtesy of Houghton Mifflin Co.)

A number of things have been tried in an effort to decrease the reflection or to frighten the birds away.

There is a new simple method that I think we should try at our homes. It was invented by Stiles Thomas of New Jersey.

One or more lengths of monofilament fishing line are fastened above the window and extend to the bottom. Attach a six to seven inch white or colored feather about every 7 inches. Keep the line loose so the feathers can blow back and forth across the window. I am not sure why it frightens birds, but would you give it a try and let me know whether it works? You can get feathers at a craft shop or perhaps from a chicken or turkey farm.

Many other methods are worth trying and are discussed in Section 2 that follows just below.


A. Bird Collisions with windows and additional methods for preventing these accidents.

B. The problem of birds attacking their own reflection that they see in a window near where they are nesting.

One of the hazards of any window is that it reflects the garden and sky and birds in flight mistake the reflection as part of the scenery and fly headlong into the window.

Professor Daniel Klem of Muhlenberg College was mentioned above. Here is more of what he discovered over a 20 year period as he studied the accidents that befall birds that collide with windows. From his studies he came to the conclusion that each year, between 100 million birds and 900 million birds are killed by colliding with windows in homes and buildings, particularly with tall ones in the cities. This is where I obtained the estimate of the annual window caused deaths of nearly a billion birds.

Often at our homes, a bird that has been at a feeder will leave it and crash into a window as it heads towards the reflection of the open sky. Klem has shown that the number of fatalities increases as the feeder is placed further from the window glass. There were no fatalities if the feeder was located 3 feet or less from the window. However, birds that hit the window after leaving a more distant feeder were much more likely to die.

If the feeder was 15 feet or more from the glass, over half of the collisions were instantly fatal. So you can save the lives of some birds by having your feeder close to the window. You can also better see the birds when they are close to you!

Home owners often report that a particular window kills many, many birds each year. Such a window usually faces north or south in an open area where migrating birds are already moving at high speeds.

Our large window faces the area where the birds feed. The house and window form the southern boundary of a clearing in the trees and shrubs.

Our window is not protected by an overhanging roof, so the room inside is bright and well illuminated. This reduces the intensity of the reflection. Windows that are underneath the shelter of a roof generally kill more birds since the reflection is more obvious when the room behind the window is dark.

If you are building a house with large windows facing the garden, you can have the glass tilted downward, that is have the glass lean outward from the top. Sometimes a building will have large windows designed this way. A bird flying towards the window does not see a reflection of the sky and garden, but only the image of the ground below the window and will veer away.

Attacking the window.

Here is another question about birds and windows.
Question: Dear Mr. Burtt: A cardinal was flying up against my window last spring for hours at a time. Is there some way to stop this if it happens again? J.B., Hastings, NY

Dear J.B.:This question is also related to the fact that birds do not understand the nature of reflections they see in a nearby window. The cardinal was attacking the image that it saw of itself that was reflected by the glass. It instinctively tries to drive away any other cardinal that comes near to where it plans to nest. To the annoyance of the homeowner, this thumping against the glass may begin as early as sunrise. The bird often carries on this activity until it is almost exhausted.

Each bird tries to keep the area around its nest clear of birds of the same kind. This area, called the territory, varies in size with different species. With robins for example, it may be from 70 by 70 feet up to an area 100 feet on a side.

If a male bird of the same species enters into that territory, the resident threatens the intruder. Sometimes there is physical contact, but more often a threatening rush will send the visitor flying.

Having a territory to itself is advantageous to the resident. It reduces interference by others, it may insure a better food supply, it spreads birds out and reduces losses due to predation and disease.

If a window happens to be within the territory and the bird sees its own reflection in that window, an attempt will be made to drive out the apparent intruder. Since the reflection does not go away, the local bird fights its image for one fruitless hour after another.

Birds have even fluttered at their image in the shiny hubcap of a car parked in the territory. Sometimes the outside rear view mirror gets the birds attention.


If a bird fights its image, it will exhaust itself in this hopeless task. The useless hours spent in fighting the reflection very often cause the nest to fail. The nest may not be properly constructed or the bird will fail to keep the eggs warm because it spent so much time fighting the intruder.

To stop this activity, something must be installed to frighten the bird away or somehow the reflection must be reduced.

Frightening the bird.

It has been recommended that a silhouette of a hawk be placed on the outside of the window. It is hoped that this will frighten the bird and cause it to veer away from the "hawk" before it gets close to the window.

Some people say that it has no effect what so ever while others think that it has decreased the number of hits on the window. I suspect that any piece of cardboard or paper fastened to the outside of the glass will break up the reflection enough to be helpful.

As indicated above in Section 1, there is a new method using feathers that you should try. It was first described in Bird Watchers Digest a few years ago. It involves the use of feathers dangling outside the window on one or more strands of monofilament fishing line. The details were given above.

Some people have tied the shafts of several feathers together loosely so that there is a cluster of feathers together that stick out in all directions.

Why does this frighten birds? The most likely suggestion is that loose feathers or some blowing about suggest that a bird has been killed by a predator and perhaps birds instinctively stay away from an area where there are loose feathers and possible danger.

Bill Thompson III, the editor of Bird Watchers Digest and his wife Julie have tested this method at their rural home in Ohio that has many reflecting windows. Now the Bird Watchers Digest store on the web is selling these "Feather Guards" ready made. Their unit has a plastic suction cup on each end of the fishing line to hold the string of feathers loosely in front of the glass.

Check their web site at If you wish to order one, click on "Order Now"( it works better than the"Buy Now" button.) If you want to sell some in your store, click on "Wholesale Information".

Some Stores that specialize in items for attracting birds already have them in stock. Ask around.

I hope that you will try this feathers idea and let me know how it works.

Reducing the Reflection

A white material placed against the glass on the inside usually will reduce the reflection enough to stop the attacks. Avoid pulling a dark colored drape across for this will enhance the reflection. Similarly, a darkened room with a clear glass window makes the reflection more pronounced.

One of simplest things to do is to temporarily tape up pieces of newspapers on the inside of the window. Another treatment is to spread a coating of white window cleaner on the inside of the glass. This could be in large blotches so that your own view out the window is not completely obstructed.

At the present time efforts are being made to manufacture a window glass that will appear to be frosted when viewed from the outside and yet appear transparent like normal glass from the inside.

If nothing else works for either the "attacks" against the window or to prevent high speed collisions, you may need to cover the outside of the window with a thin netting which decreases the reflection, without seriously interfering with the view out the window.