From the American Bird Conservancy https://abcbirds.org/threat/bird-strikes/
Windows: The Invisible Killer
Each year up to 1 billion birds die after hitting glass surfaces in the United States. Why do birds hit windows? Birds collisions occur for two primary reasons:
- They perceive glass as reflections of vegetation, landscapes, or sky to be real.
- They attempt to reach habitat, open spaces, or other attractive features visible through either glass surfaces or free-standing glass.
Can Birds See Glass?
There are many questions about birds’ relationship with glass and windows. Several of the most common are:
Can birds see glass?
Can birds see glass windows?
Can birds see through glass?
Despite claims that humans — unlike birds — can see glass, the truth is that glass presents problems to both people and birds.
When we bump into unmarked doors and windows, minor bruises and embarrassment are the typical results. For birds, however, the impact can result in instant death or serious, often fatal, injuries. Why? Birds tend to collide with glass at high speeds and their small bodies, composed of hollow bones, leave them particularly vulnerable to injury.
Even though people cannot see glass, from a young age we are able to grasp the concept of glass as a solid, transparent material. Detecting and avoiding glass then becomes an exercise in learning the cues that signal its presence, including window frames, door handles, and our location relative to building features.
So why can’t birds see glass? The reason is that they do not learn the same visual cues as humans. As a result, glass is undetectable for them. Birds may learn about individual pieces of glass in an area where they live, through repeated interactions — if they survive the initial collision — but they remain unable to generalize from the experience and remain susceptible to collisions with other structures.
Collisions Happen Year-Round
Collisions in the U.S. are most frequent during spring and fall migration, as enormous numbers of birds move between northern breeding grounds and southern wintering grounds. The largest number of collisions occur during the fall, when migrants include both adult birds and their offspring.
However, because birds can’t see glass, dangers exist for birds year-round, especially glass near bird feeders and bird baths. Resident birds, particularly juveniles, tend to collide more often with glass in summer as they explore local environments. Collisions also happen all winter long.