Frequently Asked Questions
If so many birds are killed, why
donít I ever see any dead bodies?
Why am I just hearing about this
now? Is it a new problem?
Why donít birds see glass?
How many birds die from hitting
How many birds are there?
Is the one billion killed that significant?
Donít other things kill more
birds? What about the West Nile Virus?
Won't this problem be solved
through evolutions? Won't the smart birds avoid the glass?
Iíve seen buildings in nature
areas with lots of glass. If birds die at windows, why would they
put them in nature centers?
This sounds terrible but,
ultimately, I donít think my clients will change their building
plans. What happens if they donít?
What about lights? I read that
lights are the problem.
Do window decals work? I
thought a owl decal would scare the birds.
1. If so many birds are killed, why donít 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 Ďcatchí 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 weíve 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 Ėdead birdsó is usually removed
quickly by scavengers or maintenance staffs, few people have had to
actually deal directly with the issue.
3. Why donít 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
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 decline 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. Donít 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 (www.abcbirds.org).
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
8. Iíve seen buildings in nature areas with
lots of glass. If birds die at windows, why would they put them in
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. (www.studiogang.net/projects.htm)
9. This will sound terrible but, ultimately, I
donít think my clients will change their design plans. What happens
if they donít?
Maybe nothing. Perhaps the window situation at your clientís
building wonít impact birds very much. Perhaps the windows won't
reflect sky or trees may not be close enough to be reflected in the
But if there are reflections of trees or sky or something that
tempts birds, your clients can look forward to two significant
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
Long-term scientific research at Chicagoís McCormick Place
convention center has proven that there is an 80% reduction in bird
fatalities when the lights are turned out.
Both Chicago (http://cityofchicago.org/Environment/BirdMigration/)
and Toronto (www.flap.org) have
successful ďlights outĒ programs for the rooftops of downtown
buildings. Project Safe Flight is now underway in New York City:
Lights in lower-level lobbies are also a big hazard: The light
attracts the bird, which canít 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 4Ē wide by 2Ē high. Think about their skills at flying into and
through tree branches and leaves.