Preserving Wildlife

Bird Window Collision Prevention


Most of us have had the unpleasant experience of having a wild bird collide with a window at our home or workplace, but most of us don’t appreciate the magnitude of the problem.

In order to grasp it, we need to begin with an understanding of bird migration—one of the greatest natural wonders on Earth. Most of the birds that breed in North America are migratory. They spend their breeding season in the United States and Canada, but in autumn, they begin a long and treacherous migration to warmer places as distant as southern South America. Some of these birds will fly thousands of miles nonstop, while others need to make more frequent stopovers for refueling.

Night Shift

Many of us are aware of the migratory habits of birds, but even those who live along major migratory corridors rarely notice the millions of birds flying directly over us in the fall and spring. One of the reasons for this is that most songbirds migrate only at night. They do this to avoid the heat and higher winds that occur during daylight hours, as well as hazards such as avian predators.

Amazingly enough, these night-migrating birds occur in such dense flocks that they can be detected by weather radar as they pass overhead. Interested individuals can actually monitor their progress on websites. Radar images show heavy migratory bird passage over San Diego early in the mornings. After flying all night long, these songbirds will begin dropping to the ground by daybreak, looking for water, food, and the protective cover of trees and shrubs. Images taken after sunrise show a much lower bird density as most birds have already come to the ground looking for food, water, and cover.

Window Widows

Unfortunately, these birds all too often mistake the reflections of trees and shrubs on windows for the real thing and fly directly into the glass. Or in the case of our facilities at the Zoo and Safari Park, they often see attractive vegetation and water features through large panels of transparent glass and fly into the glass trying to reach the other side. We have been mapping bird window collisions in our facilities for several years to identify problem areas, so we know this is a significant issue we need to address.

Careful monitoring below buildings across the country has revealed that anywhere from 100 million to 1billion birds die from collisions each year in the continental United States alone. This is now thought to be one of the leading causes of the gradual decline of migratory birds in North America.

Possible Solutions

Fortunately, there are some simple things that can be done at home to prevent bird window collisions. Temporary solutions that can be used during seasonal migrations in the spring and fall include taping streamers to windows or applying thin vertical stripes of tape or water-soluble paint 3-4 inches apart.

More permanent solutions include external window screens or shade structures that prevent the formation of reflections on the glass. Window stickers or decals can also be used, but they must be placed no more than 3-4 inches apart.

In Our Own Backyards

Solutions for our facilities at the Zoo and Park are a bit more challenging because of the size of the glass panels, the need for extreme durability, and a requirement for good visibility for our guests.

We are about to begin our first trial of a custom-designed transparent window film that contains thin vertical gray-frosted stripes. The expectation is that birds will see the stripes and not try to fly through them, while our guests will continue to have unobstructed views of our amazing animals through the intervening clear spaces.

We are also developing an architectural toolkit for use in future exhibit design so that we can choose the most appropriate window collision prevention strategy for each location. Our goal is to ensure that our facilities become a safe stopover point for our native birds as they complete their amazing migratory journeys.


Bruce Rideout, D.V.M., Ph.D., Dipl. ACVP, Director, Wildlife Disease Laboratories, Institute for Conservation Research.
Photos by Trent Stanley.