Birds and Glass

October 18, 2007 by Russell Greenberg


One hundred million to one billion birds die each year in North America after colliding with window glass. The Smithsonian Migratory Bird Center has been carefully monitoring this issue at the National Zoo, and is committed to finding safe solutions to protect our migratory birds.

At the Zoo

A bird's dented skull, the result of the bird colliding with  window glass
A bird's dented skull, the result of the bird colliding with window glass

In an effort to enhance the experience of zoo visitors, outdoor animal enclosures are often lined by clear glass panels that offer an unobstructed view of the exotic species within. This clear view also, unfortunately, appears as open space to birds often with the result that they fly into the glass and are killed on impact. Methods used to eliminate or reduce window-strike fatalities among birds include strategically placed plants and window appliqués.

Evidence suggest that visual barriers such as appliqués, placed no more than two inches apart horizontally and four inches apart vertically are effective deterrents. Using window appliqués appeared to dramatically decrease the incidence of fatal window strikes at the Zoo.

Frosted glass with clear wedges for viewing
Lemur Island Exhibit

For example, in March 2007, all 25 glass panels at Lemur Island were treated with sheets of semi-opaque reflectant material out of which were cut large triangles for visitor viewing (see picture below). From March to October 2007, there was one window-strike fatality, compared to six fatalities during the same time span the year before.

In the first three weeks following the construction of a new sloth bear habitat surrounded by 19 glass panels, five birds died after striking the windows. Application of UV reflectant stickers in the shape of maple leaves in November 2006 (see picture below), eliminated window-strike fatalities in the ensuing migratory season, spring.

Visitors, however, gradually removed stickers and exposed more glass, and there were fatalities again in this area in June and July 2007. There are now semi-opaque bamboo designs on the glass.

Glass with closely-spaced maple leaf decorations
Sloth Bear Exhibit

We found that an acceptable balance can be achieved between prevention of avian window strikes and maintenance of a quality visitor viewing experience with the use of reflectant-treated glass panels in zoo exhibits.

Helpful Tips at Home

Birds often see a reflection of woods in windows, or can see through one window to the other side of the house. The tips below may help reduce the view of enticing habitat.

  • Use one-way glass on windows.
  • Use shades on windows.
  • If you use decals on windows, use many of them and place them fairly close together.
  • Do not put out bird feeders, or if you do, place them very close to windows.