A Devastating Toll
Millions of migratory birds - colorful warblers, tanagers, grosbeaks, thrushes and their kin - are being killed each year by flying into plate glass windows.
Dr. Daniel Klem, Jr., a researcher in the Department of Biology at Muhlenberg College, Allentown, Penn., believes that somewhere between 98 and 980 million birds succumb annually in the U.S. to such collisions. He also suggests these estimates are conservative and that such carnage, in all probability, is one of the principal reasons many of our most desirable birds are declining in numbers.
Such a devastating toll represents from 0.5 to nearly 5.0 of the total estimated avian population of the United States. Klem's figures were derived by designating one to ten casualties annually to each structure in the country based on 1986 U.S. Bureau of Census data which indicated there were approximately 93.5 million housing units, 3.9 million commercial buildings, and 96.6 thousand schools.
The fact that birds sometimes kill themselves by striking glass windows has been noticed for well over a century, but it wasn't until the post-World War II building boom and the years since that the annual casualties mounted into the multi-millions.
The widespread use of residential picture windows was probably the single most contributing factor to the spiraling rates of mortality. Increased numbers of office buildings, shopping centers, schools, and institutional structures have all added to the toll.
The mortality rate of birds from natural causes has always been extremely high. Predators, disease, parasites, and adverse weather during migration effect a terrible toll. The additional burden of human-related causes of death have increased in proportion to human population and for some species might be more casualties than they can sustain.
Millions of migratory birds are killed by striking tall buildings, TV and radio towers, lighthouses, and edifices such as the Perry Monument on Lake Erie. Untold millions more are killed by cats (more people, more pets), automobiles, the effects of pollution and chemicals, oil spills, utility wires, poaching and, indirectly, through the fragmentation and destruction of habitat in the northern hemisphere and sub-tropical America.
Window-kills should not be confused with cases where birds - such as cardinals and robins - dash repeatedly at their reflections in windows. Such actions are examples of territoriality in which the individual bird, usually a male, mistakenly perceives a threat from another bird in the reflected image he sees in the window. Such behavior, if continued over a period of weeks or even an entire season, can be annoying to humans, but is seldom if ever fatal to the bird, although all parties concerned might be the worse for wear.
On the other hand, when a bird strikes a window in free-flight, it does so with considerable velocity and the results are significantly more serious. Sometimes the bird is merely stunned or sustains superficial injuries from which it may recover, but in over half of all recorded incidents the impact results in death.
Autopsies reveal the victims sustain injuries similar to humans who suffer blows to the head, i.e., possible skull fractures, intracranial hemorrhaging, swelling of the brain within the confined skull, neural damage, and rupture of the blood/brain barrier. If death isn't immediate, many victims die when vital functions fail, or they become prey to household pets or wild animals.
The studies by Klem and other ornithologists indicate that 225 North American species have been involved in such mishaps. This represents approximately 25% of all bird species endemic to the United States and Canada. A high percentage of birds killed in the spring and fall are migratory species. Species attracted to feeders and bird baths are killed year-round. Analysis of statistics reveals there is no correlation between window strikes and the sex and age of a bird or the weather, or the facing direction of the window. Although collisions happen at all times of the day, even at night, a significantly higher proportion occur during morning hours when birds are most active.
The fact of the matter is birds and most animals (including humans under many circumstances) cannot readily distinguish the presence of a pane of transparent glass from an unobstructed space or passageway. Further complicating the situation is the propensity of glass to cast reflections. An expanse of plate glass that reflects sky, tree limbs, or foliage, can be just as deadly, if not more so, than one that is totally transparent.
Jean Peters, a legal assistant who works in downtown Columbus nd a volunteer for the Ohio Wildlife Center, retrieves many dead and disabled migrant birds that have struck windows or reflective surfaces on high rise buildings. She is assisted in this effort by a network of office workers who report casualties to her.
A typically bad day was September 5, 1992, according to Peters. The day dawned cloudy and cool in the wake of a passing cold front. A total of 15 casualties including Ovenbirds, Black-and-white, and Black-throated Green warblers were picked up off the sidewalks and from the gutters of downtown Columbus. Many others probably fell onto building ledges, off-sets, and other inaccessible places and were never found.
"The buildings where the birds seem to hit most are the Huntington Center, and the Riffe and Rhodes towers," Peters says. "Buildings that have glass-enclosed walkways also claim numerous victims."
In foggy or misty weather with low ceilings, nocturnal migrants sometimes collide with tall structures in horrendous numbers.
One September night the 555-foot high Washington Monument took a toll of 576 vireos and warblers between 10:30 P.M. and midnight. A newly erected TV tower on the outskirts of Topeka, Kansas killed 1,090 birds of 61 species over a period of several days. A TV tower at Eau Claire, Wisconsin killed an estimated 20,000 vireos and warblers the night of September 19 -20, 1963.
Until they were outfitted with filters, airport ceilometers accounted for some of the greatest bird-kills reported. The night-flying migrants would circle like moths through the powerful beams of light, in the process colliding with each other.
But residential windows continue to kill the most birds for the simple reason there are so many more houses. That and the fact many residences are surrounded with shrubs and trees, Also there is a growing trend for people to move into rural areas, sometimes for weekends and vacations, in other instances as permanent residents.
Some windows attract many more birds than others and in cases where window kills are heavy, the inhabitants of the homes often feel remorse, if not outright guilt. "What can be done?" they ask. "We have tried everything, but nothing seems to work."
In one case of continuous and systematic monitoring of two houses, Dr. Klem writes, "I found annual kills of 33 and 26 birds respectively. For both homes, one out of every two strikes was lethal, and small (hummingbirds and sparrows) and large (cardinals to bobwhites) species were equally vulnerable."
According to Columbus veterinarian Donald Burton, who is director of the Ohio Wildlife Center, there is no perfect solution other than covering the entire surface of a window with netting, which probably obstructs the view from within less than a window screen and is virtually invisible from without. The Nature Center at Dawes Arboretum near Newark uses this technique.
In new or remodeled buildings and houses, windows can be installed at an angle so that the pane of glass reflects the ground instead of the sky or trees. Such a strategy has proved very helpful.
Many people are unaware that birds are being killed at their windows because the victims are small, frequently fall behind shrubbery, and more often than not are eaten by predators.
Peters emphasizes that it is a good idea to call the Ohio Wildlife Center whenever a bird strikes a window. If injured, the odds are fairly good it can be nursed back to health and released. If the injuries prove fatal, the dead bird can be prepared as a specimen for scientific study.
Here are some other steps for making your home windows safe for birds:
¥ With the exception of window feeding shelves, feeders and bird baths should be located a safe distance away from windows because many fatalities occur when birds are flying away from such places.
¥ Window screens are a great deterrent but are not practical for many picture windows.
¥ Decals, including cut-outs of raptors, and leaded glass decorations are only moderately successful.
¥ Vertical exterior tape stripes not more than 10 cm apart are an excellent deterrent.
¥ Interior vertical blinds with the slats half open will cut down on some casualties.