Five Things Everyone Should Know about…Birds, Buildings, and Avian Mortality

1 l Glass is invisible to birds. When birds see clouds or vegetation reflected in glass, they perceive it as open sky or habitat. Also, if they see plants on the inside of a building through glass, or if they see completely through to the other side of a building, they don’t recognize that there is a solid surface in front of them.

2 l Timing and location increase collision risks for birds. When birds are in close proximity to expanses of glass that reflect habitat, they are in a dangerous situation. This can occur when birds are foraging during the day any time of the year. It can also happen during migration, when many bird species launch into long flights around sunset; they fly for several hours and then land to rest and refuel. When they land near houses in unfamiliar territory, they risk running into glass as they search for food or fly toward reflections of what they typically use for cover from predators and weather.

3 l New buildings present opportunities to decrease collision related bird deaths. With new construction, a good way to design a bird-friendly home or commercial building is to use glass that has patterns etched into it, which breaks up reflected habitat images. This is called fritted glass; an added benefit is that it also helps control heat and light. For existing buildings with large glass expanses, films can be applied that decrease reflectivity. A great source for more information is

4 l Bird feeder placement matters. One best practice is to put feeders relatively close to a house or building. This may seem counterintuitive, but this way, when birds leave the feeder suddenly because they are startled, they can’t build up enough momentum to hurt themselves if they mistake nearby glass windows for habitat. Placing netting between feeders and windows is another good solution. Also, because birds can perceive ultraviolet (UV) light, window decals have been developed that reflect the UV portion of the electromagnetic spectrum. Birds can see these extremely well, but people cannot, and sunlight still passes through. These decals need to be reapplied frequently because they degrade in sunlight. Other ways to break up habitat images are creating patterns with washable tempera paint and installing Acopian BirdSavers, which are evenly spaced nylon strings that hang in front of window glass.

5 l Birds face many risks beyond architecture. Buildings and glass are not the only major hazards for birds. Habitat loss is the primary factor causing avian mortality. Beyond that, best estimates point toward cats as the biggest direct mortality factor caused by humans. Building collisions are right up there at number two. Other mortality sources from humans include collisions with automobiles and communication towers.

Anna Pidgeon is an associate professor in the Department of Forest and Wildlife Ecology.