Birds and Buildings, Part 1: The Problem

The nation is waking up to the incredible threat that buildings pose to flying birds. Buildings are not a natural part of the environment, and humans have disguised our buildings in a number of ways to make them more deceiving, and deadly, to birds. The result is that billions of North American birds are killed each year in building collisions, another danger added to the growing list of threats to bird populations worldwide.

Maine Audubon, working with the University of Southern Maine, the Portland Society for Architecture, and Avian Haven, has recently launched an effort called BirdSafe Maine, which seeks to better understand the problem of bird collisions in Maine and provide solutions for municipalities and homeowners. (Read about it in this Sept. 23 article from the Portland Press Herald). 

In Part 1 of this series, we’ll lay out the science behind the problem of bird collisions, discussing what we know about why birds die, and what architectural features are most dangerous to birds. In Part 2 we’ll talk about solutions.

Part 1: The Problem

The U.S. Fish and Wildlife Service estimates that somewhere between 388 million and 988 million birds die from building collisions each year. Using the absolute lowest estimate, that means 1 million birds are dying every day from building collisions. Except for feral cats and habitat loss, building collisions kill more birds than any other human-induced cause of bird death, including wind turbines (an estimated 200,000 per year), poison (72 million), and oil pits (750,000).

Birds fly into glass windows because they don’t know what glass windows are, and because the nature of glass tricks them. It’s as simple as that.

Glass can be reflective, which tricks birds into thinking they’re flying into open sky or landing in vegetation. Glass can also be transparent, invisible to birds that attempt to fly through it. For birds weighing just a few ounces, a full-speed crash into a glass window (plus a potential fall to the ground below) is enough to break necks and cause additional internal trauma.


Some glass reflects trees and other habitat.


Other glass buildings reflect the sky.


Lit from inside, and with inviting indoor habitat, glass can be invisible and deadly to birds (The atrium at the Thurgood Marshall Federal Judiciary Center in Washington, DC).

Birds simply don’t know what glass is, and the bodies of many birds are ill-equipped to easily pick out glass from habitat. Unlike humans, many small birds have their eyes mounted on either side of their head rather than facing forward. This adaptation gives them better peripheral vision, making it easier to see predators sneaking up from the sides but more difficult to judge depth and see what’s right in front of them. It’s much more common to find small birds as the victims of bird strikes–warblers and sparrows, for example–than larger predators birds like hawks, which have forward-facing eyes and better depth perception.

Scarlet Tanager found in Portland, Maine, Sept. 2020

The problem is evident in cities across America during spring and fall migrations, when billions of birds fly between their wintering grounds and their breeding grounds. Birds migrate at night but come out of the sky in the mornings to rest and refuel. If they find themselves in a city full of glass windows they are more likely to collide with windows. Similarly, migrating birds flying lower than usual due to fog or rain more frequently find themselves close to buildings.

Though cities often get most of the attention for bird strikes and for the bodies of birds lying on sidewalks, residential windows are just as dangerous. Birds flying near houses, whether migrating, breeding, or wintering, may strike windows or glass doors on your home for the same reasons they hit larger buildings.

Thankfully, there are solutions for both large buildings and family homes that can reduce the likelihood of bird collisions, and we’ll cover them in our next post. In the meantime, if you find a bird that has collided with a building, please take a photograph and send it to

Coming next: Part 2–solutions.