why do birds hit buildings?

We take glass for granted — most of the time– so it can be surprising to us that birds would run into windows.  But birds don’t see glass, and neither do we.  But people have learned to adapt to a world of glass-enclosed spaces.  Most birds haven’t experienced glass in their own habitats, so they don’t recognize it when they encounter during migration.

Birds hit buildings for three main reasons:

  • reflections
  • see-through buildings
  • artificial lights


During the day, birds can fly head-on into windows, confused by the reflection of trees, clouds, buildings, or skyline in building facades.

We can be confused by reflections too, but we read other cues, such as window frames, to help us distinguish reflection from reality.

bird collision reflection hazard tree animalia project
Is this tree real or a reflection? You probably guessed it’s a reflection, because you recognized the window frames. Birds often confuse this reflection for a real tree, then strike the window when they fly toward the ‘tree’. {photo by susan ask / animalia project}

see-through buildings

Birds also collide with glass when they attempt to fly towards something, like the tree in the lobby pictured below, that is visible– but inaccessible– because of a glass barrier.

It’s become popular to plant trees in large glass atria.  But these lobbies are a real hazard to migrating birds searching for food and shelter when they stopover in cities.

bird collision hazard: trees seen through windows. animalia project.
These trees, planted inside a building, pose a hazard to birds. When birds fly towards the trees, they can collide with the windows, because they don’t see the glass in front of the trees. (animalia project photo by susan ask).

artificial lights

At night, artificial lighting can confuse migrating birds, causing them to fly until they are exhausted.

Some birds use the stars to navigate their routes.  Light pollution from cities interferes with their ability to navigate.

bird collision hazard lights animalia project susan ask
Artificial lights can confuse migrating birds. (animalia photo by susan ask)

Learn more about solutions to bird collisions here.