By Richard Podolsky, PhD

Millions of birds die during spring and fall migration or while on their breeding grounds because they cannot differentiate between what is the real world and what is reflected in glass.

Bird Migration: “Twice a year, every year, for the last 160 million years, most of the earth’s billions of birds embarked on a spring and fall migration – north in the spring to find food and mates and south in the fall for food and warmth.  For most of these 160 million annual pilgrimages the usual risks were being eaten by a predator, dying in severe weather, or death from not finding enough food to meet the high energy demands of long-distance migration”. 

To these “natural” risks—which help cull birds and keep their numbers roughly the same from year to year—we now must add the alarming number of birds that die by flying into glass installed on the facades of urban skyscrapers or on sliding doors and picture-windows on residential homes worldwide.  By all conservative estimates, the annual toll of death by glass is between 365 and 988 million bird deaths per year in the United States alone.

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It’s important to view death by glass in perspective.  While contributing hugely to anthropogenic mortality, death by glass isn’t even the number one killer of birds. Research indicates that that distinction belongs to free ranging felines. One hundred and ten million free ranging cats are estimated to kill four times the birds killed from collision with buildings, more than 2.6 billion birds annually in North America alone.  

This will be the focus of an upcoming article.  All told, cats, glass, habitat loss, climate change, unregulated harvest, and other forms of human-caused mortality have contributed to a thousand-fold increase in global bird extinctions during the Anthropocene (Figure 1).

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Hazards to birds and the mortality caused in the United States.

Three Billion and Counting

This cumulative mortality adds up to devastating numbers. A recent, comprehensive assessment of bird populations in North America reveals staggering declines. North American bird populations have declined by 3 billion breeding adults since 1970, that’s approximately one out of every three birds.  It is hard to hear this, but this number corresponds to nearly one-third of the wild birds in North America, a loss that puts at jeopardy the entire ecological fabric of North America. 

The losses are in all biomes; forests alone have lost 1 billion birds and grasslands 720 million birds.  More than 90% of this loss can be attributed to 12 bird families, including sparrows, warblers, blackbirds, and finches.  Dark-eyed Juncos are down by 168 million, Meadowlarks are down by 139 million, White-throated Sparrows down by 93 million and the Red-winged Blackbird population has declined by 92 million. To name just a few.  And these numbers are just for the United States.  Imagine what that number is worldwide because these mortality factors ignore international borders.  

Fortunately, it’s not all bad news. Populations of North American ducks and geese have grown by 56 percent since 1970.

Through a Glass – not lightly!

Glass is ubiquitous in the built environment, but it plays a diabolical trick on the way birds perceive their environment.  Most birds have much better visual acuity than mammals, including humans.  It has been estimated that eagles and falcons, if they could read a printed page, would be able to do so at a half-mile, and songbirds can fly through thick forested vegetation at full speed miraculously avoiding collision with trunks and branches.  

But when it comes to their ability to differentiate what is real from what is a reflection in glass, birds are woefully ill equipped.  A patch of blue sky, a cloud, a tree reflected in a pane of glass, are perceived by birds as being real.  The tragic outcome is that birds think nothing of flying towards a reflected cloud or a patch of blue sky or trying to perch on a reflected branch to court a potential mate.   Birds are fragile and carry too much speed to allow them to survive a dive into glass.

Don’t Go to the Light!

Mortality from glass is compounded by artificial night-lights.  Most birds migrate at night and historically relied upon stars and the moon to guide them on their annual flights north and south.  But the earth’s surface at night is now covered with artificial lights that look just like stars, causing a double whammy: night lighting obscures the stars and the moon and replaces them with lights below.  

The upshot is birds become disoriented – the moon and stars above are now replaced by lights below, leaving birds wondering which way to go.  Adding these insults to injury, on foggy and stormy nights birds are attracted to artificial night lights causing them to “fallout” into urban areas where they collide with buildings.  Cities like New York, Toronto, LA and San Francisco, Chicago, Boston and Philadelphia are death traps for millions of birds when the perfect storm of fog combines with a peak migration.  

The result can be thousands of dead birds falling to the ground or landing on rooftops of neighboring buildings.  You don’t see many of these fallen birds because scavengers get them almost immediately.  Several species on the Birds of Conservation Concern list due to their declining populations, are highly vulnerable to building collisions, including Golden-winged Warbler, Painted Bunting, Canada Warbler, Wood Thrush, Kentucky Warbler, and Worm-eating Warbler.  

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Bird Migration Through a Hall of Mirrors 8

Not all these birds die from collisions.  Many birds die from exhaustion from flying incessantly in circles around lighted buildings, using up their energy stores reserved for migration.  Some of the most common birds to die by collision or exhaustion here in the United States include, but are not limited to, Ruby-throated Hummingbirds, American Robins, Gray Catbirds, and Mourning Doves.

I have spent the bulk of my consulting career as an “industrial ornithologist,” advising developers on how to reduce or eliminate bird mortality in the built environment.  Just after the September 11, 2001, attack that brought down the two World Trade Center towers in lower Manhattan, I was hired by the property owners and their architects to advise them how they might rebuild the World Trade Center with skyscrapers that employ state-of-the-art techniques to minimize bird collision.  

It was well known that the original Twin Towers of the World Trade Center, along with the Empire State Building and many of the prominent skyscrapers in Manhattan, were responsible for thousands of bird deaths during each migration.

I would give the same advice to homeowners and renters to bird-proof their homes that I gave my clients to bird-proof their skyscrapers.  And it is even more important because more than half of the birds that die by diving into glass do so at private homes, not skyscrapers.

We CAN Make a Difference!

If we have lost 3 billion birds since 1970, and I have no reason to doubt this number, that means that 75 million birds have died each of the last 40 years that have not been replaced from breeding (recruitment).  And if it is also true that something like 500 million birds die per year at buildings, if we were to bird-proof just 20% of our buildings, especially private residences, we would save 75 million birds per year, or enough to at least stop the annual decline.  This, by the way, is not even taking a swipe at the losses due to cat predation. 

So, where do we start?  We start with our homes.  As human populations rise, so to do the numbers of buildings.  Hence, actions to reduce bird mortality from building collisions are vital at all types of buildings.  Thankfully, there is a lot we can all do as individuals to help save birds and the ecosystems upon which they rely.  Here is my list:

  1. Make Windows Safer

Consider installing screens, netting, shutters, exterior shades, placing films, decals of falcons, or paint, even string or tape, to warn birds and eliminate reflective glass.   Awnings and overhangs can be helpful.  Where possible, use bird-friendly glass that is fritted, opaque, etched, stained, or frosted and thereby more detectable and avoidable to flying birds.  

There is even ultraviolet (UV) patterned glass that can warn birds, passerines, hummingbirds, parrots, and other birds that can see into the ultraviolet spectrum of light, a range invisible to humans.  Walk around your property and look for shrubs or trees that are reflected in windows and try to reduce or eliminate those situations.  Work with your neighbors and local businesses and public buildings to share techniques to make windows safer for birds.  

Finally, support legislation for bird-friendly building designs, such as those offered by the Green Building Council, and advocate for a lights-out campaign in your community, especially during migration months of May and June and September and October.  Recent studies have shown that when such remediation is taken, collisions can be reduced by 90%!

  1. Keep Cats Indoors

Self-explanatory, but so critical.  Apparently, even well-fed cats will kill birds just for sport, so, keep them indoors or, at the very least, fix a bird warning bell onto their collars.

  1. Avoid Pesticides, Herbicides and Plastics

Pesticides are toxic to birds and to the insects they eat.  A recent study reported a decline of 82 percent in the biomass of flying insects over the past quarter century.  Herbicides kill the plant life that birds rely upon.  So, consider reducing or eliminating the use of pesticides and herbicides around your homes and gardens.  Buy organic foods instead of big agriculture products that use chemicals.  Also, urge your representatives to support the Saving America’s Pollinators Act. The bill, H.R. 1337, that will require the US Environmental Protection Agency (USEPA) to control the use of neonicotinoid, a class of neuro-active insecticides that is killing our pollinators.

Try to avoid using pesticides and herbicides on your lawns and in your gardens and reduce reliance upon single-use plastics including bags, bottles, wraps, and disposable utensils.  If you must use disposable plastic, be sure to recycle it.  Advocate for bans of plastic bags, Styrofoam, and straws and encourage local businesses and restaurants to offer reusable bags and to phase out single-use plastics.

  1. Drink Shade-grown Coffee

Most of the world’s coffee farms are sun-grown and thus destroy the forests that birds and other wildlife need for food and shelter.  Sun-grown coffee is also more likely to employ environmentally harmful pesticides and fertilizers.  So, try to purchase, organic, free-trade, shade-grown coffee that is grown below a forested canopy where migratory birds overwinter.

  1. Become a Bird Watcher and a Citizen Scientist

Ornithology, (like astronomy), is a scientific field that benefits enormously from contributions made by citizen scientists.  So, consider joining a local Christmas Bird Count in your area and getting involved at a local hawk watch.  There are also Feeder Watch programs that will allow you to combine feeding birds with monitoring birds.  Finally, log your bird watching sightings using eBird, a social media interface.

  1. Support Bird Organizations

From among the myriad of bird conservation organizations, I particularly like to suggest folks join and support American Bird Conservancy, The National Audubon Society, and the Cornell Laboratory of Ornithology.  However, there are many others worthy of your support.

  1. Lights Out!

Finally, there is one simple way to decrease mortality: turn lights off!  Across the United States and Canada, “Lights Out” programs are available that encourage building owners and occupants to turn off lights that are visible from outside during spring and fall migration.  The first of these, Lights Out Chicago, was started in 1995, followed by Toronto in 1997. If your night lights are necessary for safety reasons, be sure they are down-shielded rather than pointing upwards.

The Last Words

The worldwide, cumulative loss of bird abundance is a signal to us all to do what we can, first and foremost, in our own backyards.  Birds are our canaries in a coal mine, sentinels to environmental health and thus to our own health.  The massive decline of birds and insects is a warning that the earth’s biological systems are in big trouble.  

Unless we all do our part, we run the risk of experiencing a global avifaunal collapse, followed by a loss of ecosystem integrity.  On a more philosophical level, the collapse of bird populations marks the end of a certain kind of life.  A life filled with the dawn chorus of singing birds, a life filled with bugs soaring in the air, a life filled with biodiversity, fecundity, and pollution-free forests, lakes, streams and oceans.

Richard Harris Podolsky lives on the Maine coast.  He has a bachelor’s degree from the University of Wisconsin, a master’s degree from Rutgers University, and a doctorate from the University of Michigan.


  1. Loss, S.R., Will, T., Loss, S.S. and Marra, P.P., 2014. Bird–building collisions in the United States: Estimates of annual mortality and species vulnerability. The Condor, 116(1), pp.8-23.
  2. Loss, S.R., Will, T. and Marra, P.P., 2013. The impact of free-ranging domestic cats on wildlife of the United States. Nature communications, 4(1), pp.1-8.
  3. Rosenberg, K.V., Dokter, A.M., Blancher, P.J., Sauer, J.R., Smith, A.C., Smith, P.A., Stanton, J.C., Panjabi, A., Helft, L., Parr, M. and Marra, P.P., 2019. Decline of the North American avifauna. Science366(6461), pp.120-124.
  4. Van Klink, R., Bowler, D.E., Gongalsky, K.B., Swengel, A.B., Gentile, A. and Chase, J.M., 2020. Meta-analysis reveals declines in terrestrial but increases in freshwater insect abundances. Science, 368(6489), pp.417-420.


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