An estimated one billion birds die each year in the U.S. from smashing into windows.

1. If so many birds are killed, why don’t I ever see any dead bodies?

Scavengers. Very vigilant scavengers that remove the evidence before you see it.

Migrant song birds fly at night and often strike buildings in the dark and very early morning hours. But scavengers like gulls, crows, magpies, vultures, rats and feral cats and dogs are waiting and often clean up the evidence before most people even see the carnage. Gulls have been observed cruising by buildings with bad collision rates, looking for dead and injured birds.

Both scientific research and anecdotal evidence proves that scavengers remember where dead and injured birds can be found. In a typical study, the scientists place a piece of attractive food as bait in a non-window area. The bait goes untouched while scavengers always find the nearby bird-kill sites.

Additionally, building maintenance staffs now acknowledge that they have been sweeping up bags of dead birds at dawn before the work crowd arrives. At skyscrapers with bright lights on at night, the morning ‘catch’ of dead birds can fill a barrel. Horribly enough, janitorial staff have been sometimes been observed sucking up live, but injured birds, with their sidewalk vacuums.

2. Why am I just hearing about this now? Is it a new problem?

Birds have been dying through collisions with windows for as long as we’ve had windows that reflected trees and sky. The problem grew dramatically throughout the 20th Century as building designs included larger and larger areas of glass.

There is no one answer for why it has taken so long to get attention. In general, the vast majority of ornithologists, the group most likely to be sensitive to the issue, focus their work in natural areas away from buildings.

Since the evidence of the problem –dead birds— is usually removed quickly by scavengers or maintenance staffs, few people have had to actually deal directly with the issue.

3. Why don’t birds see glass?

No one knows. What we do know is that approximately 225 different species have been documented striking windows. The good news is that birds do see ultra-violet light that humans do not. Research is underway to better understand the options this may allow for windows.

4. How many birds die from hitting windows?

Approximately one billion birds are killed every year in the United States, per experts like Muhlenberg College Professor Dan Klem who has done extensive analysis for three decades. Birds killed include healthy and sick, large and small, common and endangered.

How many of any given species are killed on a continent-wide basis is not known. However, we do have anecdotal reports that indicate how serious the problem can be. On one university campus, for example, the entire population of hummingbirds was killed. They did not die in vain, however: The event motivated the school to ensure that their buildings are now bird-friendly.

5. How many birds are there?  Is the one billion killed that significant?

The current expert estimate is ……..  Wait.  Does it really matter?  Even if you know for a scientific fact that the species of birds that will hit your next building are in great abundance, do you really want to do that?  Will your clients understand?  (There are great stories in the ornithology grapevine about architects showing up at meetings asking for help because their clients are complaining about their new “green” building killing birds.)

But to your question:  We don’t know the total number of birds for the US.  Published numbers range from 5 to 20 billion but that is for all 800+ species.  Detailed census work has only been done for about forty years and the population declines in many types of birds has been dramatic.  Songbird numbers are down fifty percent, for example.

So the simple is Yes, the one billion killed through window collisions is incredibly significant.

6. Don’t other things kill more birds? What about the West Nile Virus?

There are certainly other significant causes of the decline in bird populations: habitat loss, cell-tower collisions, cat predation and pesticide exposure are the most significant. You can get more information about these issues at the American Bird Conservancy (

We now know that window-collision is at the top of that list.

The spread of diseases like West Nile Virus is very significant killer for many bird species. Research is underway to get a better understanding of ramifications of WNV. The National Audubon Society has an updated webpage with the latest information.

7.  Won’t this problem be solved through evolutions? Won’t the smart birds avoid the glass?

There is nothing in avian biology to support a solution through evolution. And even if there were, evolution is a very slow process. And the construction of unfriendly buildings is expanding rapidly.

8. I’ve seen buildings in nature areas with lots of glass. If birds die at windows, why would they put them in nature centers?

Unfortunately the facts about birds being killed by window strikes have not been well-known within the conservation community (no doubt thanks to the scavengers who remove evidence). That is changing, and in the meantime, nature centers are taking preventive actions. The next nature center to be built in Chicago, the Ford Calumet Environmental Center, is designed based on the best bird-friendly techniques.

9. This will sound terrible but, ultimately, I don’t think my clients will change their design plans. What happens if they don’t?

Maybe nothing. Perhaps the window situation at your client’s building won’t impact birds very much. Perhaps the windows won’t reflect sky or trees may not be close enough to be reflected in the windows.

But if there are reflections of trees or sky or something that tempts birds, your clients can look forward to two significant consequences:

The dead birds will attract scavengers, which will hang around the building waiting for their next meal. Attracting scavengers of any kind – especially rats – is undesirable and a health hazard for human residents of problem buildings and passersby.

If the bird-kill situation is significant, the people who work or visit the building will be disturbed by the sight of dead birds. Worse yet, tenants working by a window will be distressed that birds are hitting the windows. Tenants in one downtown Chicago building have described, unhappily, their experience listening to the distress cries of birds as they strike windows or lie dying below.

10. What about lights? I read that lights are part of the problem.

Artificial lights are a very big problem. Birds are drawn to lights (like moths to a flame). Scientists can only speculate why but the fact is well-documented.

Birds fly into or around the lights incessantly until they fall, exhausted or near death, to the rooftop or ground. Janitors, as well as independent sightings, have verified that on some nights, especially in foggy, overcast conditions, thousands of birds may be mesmerized by the lights of a single building. Across an entire migration season, the death toll may be in the tens of thousands at some buildings.

Long-term scientific research at Chicago’s McCormick Place convention center has proven that there is an 80% reduction in bird fatalities when the lights are turned out.

Lights in lower-level lobbies are also a big hazard: The light attracts the bird, which can’t see the glass barrier in the way. Building owners are encouraged to adopt a policy of low-level lobby and atrium lighting throughout the night.

11. Do window decals work? I thought an owl or falcon decal would scare the birds.

Birds know that the decal is not a bird of prey.

Decals work only if used to fully obstruct the reflections of trees and sky. Birds will attempt to fly through anything larger than 4” wide by 2” high. Think about their skills at flying into and through tree branches and leaves.

What do you think?

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