by David Manuwal

In the early evening of September 24, 1962 near South Bend, northern Indiana, there was a brief thunder shower with some wind gusts from the northwest. During the following two days, I found 77 dead birds under a 1100 foot tall television transmitter that were killed during that single storm event. The most common species killed was the Blackpoll Warbler (30). This is one of my early experiences with documenting bird collisions. Since those days, ornithologists have learned a great deal more about bird migration and sources of mortality.

It is difficult to estimate avian mortality from the many possible sources. The largest threat to birds is habitat loss but this has not been quantified. Remember this when reviewing the following information on quantified mortality factors. A recent compilation by the U.S. Fish and Wildlife Service (as of 2017) estimates that annual bird mortality in the U.S. from all sources is around 3.3 billion birds. The top threats and the estimated percentage of each are shown below (U.S. Fish & Wildlife Service website):

ThreatPercent of Total
Cat predation72.2
Collisions – building glass18
Collisions – vehicles6.5
Collisions – electric lines0.6
Collisions – communication towers0.1
All other sources0.3

In 2014, Smithsonian researchers attempted to estimate bird losses due to collisions with buildings of various heights. Homes and other buildings one to three stories tall accounted for 44 percent of all bird fatalities, about 253 million bird deaths annually. Larger, low-rise buildings four to 11 stories high caused 339 million deaths. And high-rise buildings, 11 floors and higher, kill 508,000 total birds annually. Individual skyscrapers can be quite deadly for birds, but they kill fewer birds overall because there are relatively few of them. By combining these numbers, the Smithsonian reported that collisions likely kill between 365 million and 1 billion birds annually in the United States, with a median estimate of 599 million.(Christine Sheppard and Bryan Lenz, American Bird Conservancy).

What can we do about lowering bird mortality at our home? The first thing is to identify windows that have caused bird mortality. Windows near bird feeders are very likely to cause bird deaths. The most inexpensive remedies include:

  • Apply visible patterns to the outside of your windows. Birds tend to avoid glass with vertical or horizontal stripes at least 1/8 inch wide and spaced 2 inches apart . Patterns of dots at least ¼ inch in diameters also work.
  • View problematic windows from the outside to see what may be causing the strikes such as reflectivity or a view through the house and another window. (sometimes closing blinds at various times of day or keeping them open may fix the problem)
  • Hanging strands of cord across the window.

There are many commercial products available. See the American Bird Conservancy website ( ) for a more complete treatment of this topic.