Migrating Birds and Buildings

Birds Fly Into Glass Because They Just Don’t See It

Oct 8, 2006 Rosemary Drisdelle

Millions of birds die each year when they crash into buildings. With careful architecture and some changes in behavior, humans might spare some of them.

Most of us have had the disheartening experience of having a bird fly straight into a house window and drop to the ground, dead. The force with which the bird hits the pane is demonstrated by the sickening thump that it makes. It’s natural for us to wonder how the bird could have failed to see the house, and how millions of birds each year fail to see the glass buildings and brightly lit skyscrapers that they collide with.

The answer is that the birds see the houses and the tall buildings – they just don’t interpret what they see in the way that we expect them to. When confronted with a pane of reflective glass, a bird sees the reflection, not the glass. If the window is reflecting sky and clouds, the bird sees sky and clouds; if the window is reflecting trees or shrubbery, the bird sees a place to find rest and shelter. At night, when windows are brightly lit and there may even be decorative lighting illuminating the landscape all around, birds simply become confused and don’t know where to fly. The result is all too often a fatal bird collision with a building.

Small ways to avoid window strikes by birds have been around for years. Homeowners buy decorative decals to stick on windows to make the obstacle more visible to passing birds, and a number of municipalities have turned off lights on tall buildings and other structures at night to give night-flying birds a dark sky to travel through. The CN Tower in Toronto and the Sears Tower in Chicago are examples of tall structures that are now unlit at night. But these measures save only a fraction of the birds at risk.

Scientists and architects are starting to work together to identify the scenarios that are particularly dangerous for birds – they’re learning what makes a building friendly or unfriendly to birds.

Unfriendly:

• Lots of glass in close proximity to greenery creates lots of reflections that birds mistake for more greenery.
• See-through buildings with a glass exterior and a lack of interior walls look like open sky to birds.
• Open pedways, lobbies, and courtyards make birds think they can fly through.
• Buildings that are all reflective glass can reflect the skyline in the background and be virtually invisible to birds.
• Buildings with large plants inside, either real or artificial, that can be seen from outside invite birds.

Friendly:

• Non-reflective or patterned glass, or decorative film applied to glass in windows doesn’t create sharp reflections that look like trees and sky.
• Outside walls that have lots of varied surfaces rather than a virtually unbroken face of glass are much more visible to birds.
• Windows and walls set at an angle don’t confuse birds.
• Window screens make glass surfaces much less reflective.
• Multiple panes, preferably with dividers on the outside of the glass, break up the surface effectively.
• Glass that reflects wavelengths of light invisible to people but visible to birds may be on the way: this should help the birds see the glass and understand that they can’t fly there.
• Unnecessary exterior lighting should be turned off at night and curtains or blinds should be drawn on lighted windows.

It looks as if a new style of “bird friendly” architecture is on the way. That’s good news for the millions of birds that migrate, at their peril, past and through human communities each year.

Sources consulted: Birds and Buildings

Related articles: When Birds Attack Windows

The copyright of the article Migrating Birds and Buildings in Birds is owned by Rosemary Drisdelle. Permission to republish Migrating Birds and Buildings in print or online must be granted by the author in writing.