Why Birds Hit Glass

Birds can’t see glass. As a result, up to one billion birds hit glass and are killed each year in the U.S. alone as they try to navigate around houses, office buildings, and other obstacles.

Wood Thrush, a frequent window collision victim, Greg Homel

Windows: The Invisible Killer 

There is a myth that people can see glass, but birds can’t. In fact, neither birds nor people can see glass.

Many people are injured every year by colliding with unmarked doors and windows; embarrassment is the usual result. But because of their small size and high-speed flight, birds hit glass and are usually killed or sustain injuries that will likely kill them.

Birds can learn to avoid glass: for example, birds in zoo exhibits learn to avoid exhibit walls if the glass is marked for the first few days of their residence. Wild birds can learn about specific pieces of glass.

But overall, birds don’t seem to be able generalize clues that windows are present, and frequently don’t survive the first impact.

Showing why birds hit glass, vegetation reflected by windows, Christine Sheppard

Triple Threat to Birds

Birds hit glass because it presents a triple threat:

  • Reflections of vegetation or landscape attract birds to collide with glass.
  • Green habitat inside buildings with see-through glass can lure birds to their deaths.
  • Glass corners or narrow passages can allow birds to see through to habitat on the other side of a building, and they die trying to fly through.


Understanding Why Birds Hit Glass

As researchers have begun to understand collisions they are creating better strategies to reduce impacts on birds.

For example, some have documented mortality patterns and how they are influenced by lighting, the amount of glass present, the distribution of nearby vegetation, and other variables. Others have looked at the type of structure to identify which pose the most risk.

This science has come a long way, but many questions remain unanswered. Our efforts on the testing of materials are helping to expand understanding of how birds see and respond to their environment, and will lead to more effective solutions.

And now, our Bird-Smart Glass Program provides proven effective solutions for every conceivable situation where birds hit glass.