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Why Do Birds Fly Into Windows?

Cats kill billions of birds every year in the United States and Canada, but there's another culprit in as many as 1 billion additional bird deaths each year: windows.

An indigo bunting. (Photo via Shutterstock)

The number of birds that die because they fly into windows is hard to determine and varies from year to year. A study in 2014 concluded that between 365 million and 988 million birds die each year in the United States from flying into buildings.

The birds don't always die on impact. Some may die later of their injuries. In addition, some birds are more vulnerable to predators after being injured from flying into windows.

Birds fly into windows at all times of year, but it is most common during the spring and fall migration seasons and when young birds are just learning to fly. Birds fly into buildings at all times of day, but the reason behind it is different during the day than it is at night.

During the day, birds can see through the glass and what is reflected on the glass, but they can’t see the glass itself. What is reflected back at birds often looks like an open area that looks easy to fly through. We see the same reflections in the glass that birds do, but we understand that glass is present because of the other visual cues, such as window frames and nearby structures like doors. Birds do not understand these cues as we do.

At night, birds often fly into windows because light from buildings attracts them. Many migratory birds, including many songbirds, fly at night. For reasons scientists do not yet fully understand, the presence of lights at night can cause birds to deviate from their path.

In urban areas, artificial light is particularly dangerous to migrating birds. The light can confuse birds, drawing them toward the city as they confuse the city lights for natural nighttime light sources from the moon and stars.

If birds flying into your house's windows is a problem, there are a few things you can do to help prevent it. The best thing to do is install bird-friendly glass, which the Forest Preserve District recently did at Plum Creek Nature Center in Crete Township. The windows look normal to people and make it easy to view the birds outdoors, but the glass has fiber in it that the birds can see, helping prevent collisions.

This glass is effective, but it's more expensive than a normal window. Luckily, there are some less expensive ways to make your windows more visible to birds. Even something as simple as closing your blinds or curtains will prevent birds from flying into windows. And if your windows have screens on them, they will help prevent birds strikes.

Windows without screens can be made more visible to birds by adding some sort of vertical markings on the outside of them. You can do this with soap or tempera paint, both of which are inexpensive and won't damage windows, or with a more long-lasting solution like the American Bird Conservatory's BirdTape, a translucent tape that can last for up to four years on the outside of windows. Even decals, stickers and masking tape affixed to the exterior of a window can help. Be sure to ask your parents for permission before you begin marking the windows in any way. They may even offer to lend a hand.

The key to marking your windows is to use a grid-like pattern. Vertical markings — whether soap, paint, tape or stickers — should be no more than 4 inches apart, and horizontal markings should be no more than 2 inches apart. If your windows are frequently hit by hummingbirds, a smaller grid is necessary; use a 2-inch-by-2-inch grid pattern instead.


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