Smithsonian National Zoological Park l Friends of the National Zoo



September/October 2009 cover
$62.53 million raised; 78.2% of $80 million goal

grapes with the apes

No-Strike Zone

The Smithsonian’s National Zoo goes beyond protecting the animals inside the exhibits. Special glass and other methods prevent migratory birds from getting hurt or killed.

by Jodi Legge

Imagine sitting at your dining room table looking out of the big picture window into your backyard. Without warning, a bird smacks into the window—a scary noise for those inside the house, but almost certainly deadly for the bird outside.

An estimated 100 million to one billion birds are killed each year after colliding with glass, most often a window. This can include windows in homes, small businesses, or even large skyscrapers.

“It is possible that five percent of the total peak bird population is affected,” says National Zoo Bird House biologist Dan Boritt. “If you take the upper estimate of one billion birds killed, glass strike likely becomes the largest non-natural killer of birds in this country.”

Unfortunately for the birds, the situation will only get worse. As human populations grow, more and more houses and buildings will be constructed—and almost all of those will have windows for birds to fly into. Adding to that, glass is a preferred material used in green construction, leading to even greater chances for bird strikes.

Which birds are affected?
While any species of bird can strike a window at any time, migratory birds are most often affected. Because these birds are attracted to lights and are often exhausted during migration, they are more vulnerable.

Some species commonly killed include: Below left: Swainson’s thrush (Catharus ustulatus), below right: ruby-throated hummingbird (Archilochus colubris), below center: downy woodpecker (Picoides pubescens), as well as American robin (Turdus migratorius), and Cooper’s hawk (Accipiter cooperii). (Dan Boritt/NZP)

Swainson’s thrush ruby-throated
downy woodpecker

A Serious Impact

Windows allow humans to see what’s going on outside, but the glass is an invisible barrier to birds. Neither tinted nor clear glass is visible to birds; instead, they often see plants, shrubs, trees, or even open sky reflected in the glass. Thinking it part of their natural habitat—or a passageway from one location to another—the birds fly toward windows. Depending upon the speed of the bird, which can be 20 to 60 miles per hour, the impact can be fatal. Even if the force of the impact doesn’t kill the bird immediately, it is most often left dazed—easy prey for predators.

“It is difficult to get a true measure of the number of birds killed each year because many of them die after leaving the site of the impact,” says Boritt. “The numbers could actually be much higher than we think.”

Bird window strikes can happen any time of the day or night, any day of the year, to any species. A significant number of migratory bird deaths are caused by lights at night attracting and confusing birds—some die later from the collisions, others lose the strength and energy needed to complete their migration.

Migratory birds that fly long distances from one place to another are particularly vulnerable. During spring and fall migrations the number of birds killed due to window strikes may significantly increase—especially at locations near migratory stopover sites. In major cities where large numbers of migratory birds travel, it is not uncommon for birds to strike skyscrapers.

“You add the number of window strikes to the increasing number of birds that have lost their habitat for environmental reasons and you’ve got a serious problem,” says Smithsonian Migratory Bird Center director Russ Greenberg. “We are talking about huge losses to many different bird populations.”

While the speed and force of the impact can cause instant death, birds that are stunned and do not die immediately may actually be fatally injured, suffering from traumatic brain injury or internal bleeding. Often they leave the site and die somewhere else. However, even if the bird is not fatally injured from the strike, it may end up lying stunned on the ground for a prolonged period. This leaves it highly vulnerable to predators such as raccoons, sea gulls, foxes, crows, or cats.

scuba diving in Caribbean
The use of patterned glass at select spots around the Zoo has reduced bird strikes significantly. (Dan Boritt/NZP)

Searching for Solutions

In the fall of 2006, with the opening of Asia Trail, the bird strike issue became very relevant to the National Zoo. In the space of a week, several hummingbirds were found dead near the glass enclosure by the sloth bear exhibit. Former Zoo director John Berry was extremely concerned about this potential problem and asked that a team be formed to collect data and determine some viable solutions.

The team, chaired by Russ Greenberg, included staff from the Zoo’s departments of pathology (Tabitha Viner), animal care (Dan Boritt and Paul Tomassoni), and exhibits (Jeff Baxter), as well as the Zoo’s Conservation and Research Center, in Front Royal, Virginia (Scott Derrickson). The group soon determined that the bird strike issue was not just exclusive to the new exhibit, but was unfortunately happening all over the Zoo.

“As a conservation organization, with an emphasis on green practices and understanding the natural world, the National Zoo has an obligation to learn about this problem and help to solve it,” says Greenberg, who established and chaired the bird strike team.

The first action the team took was to analyze the Zoo’s animal mortality database and identify the three hot spots that accounted for a majority of the bird strikes: the sloth bear exhibit on Asia Trail; the orangutan and gorilla yards; and Lemur Island.

The team then consulted with Daniel Klem, a world-renowned expert on avian glass strikes; surveyed what other zoos were doing about bird strikes; worked with Friends of the National Zoo program supervisor Dan Rauch to establish a volunteer team to monitor the park for bird strikes; and searched for attractive, economical, and effective materials with which to treat the problematic glass surfaces.

The team decided the best option would be to treat the glass with a custom vinyl material, designed especially to prevent bird strikes on windows. The vinyl creates a pattern on the glass that makes it visible to birds. Researchers had found that in order for birds to be deterred from flying toward the glass, the vinyl pattern could not have gaps larger than two inches. “This indicates that those commonly used solitary hawk silhouettes don’t really work,” says Greenberg.

Patterns were applied to the glass at the various trouble spots around the Zoo, from a bold opaque pattern at Lemur Island to a more subtle bamboo design on Asia Trail and the ape yards. Then came a year-long study, comparing the number of bird strike fatalities before and after the vinyl treatments. The results showed the applications reduced the number of bird strikes at the treated spots to almost zero.

Despite this successful reduction of the total number of birds dying at the Zoo, the solution caused some visitors to complain that the vinyl patterns obstructed their view of the animals. In response, the Zoo’s bird strike team worked with curators to increase portals for viewing and photography, while maintaining most of the lifesaving, protective patterning.

The results of an additional six months of monitoring showed that adding more portals did not contribute to a secondary increase in bird strikes—a win-win situation for birds and visitors. The bird-strike team continues to grapple with its commitment to reduce the number of bird strikes at the Zoo, but also find ways to make sure visitors have the best animal-viewing experience possible.

Going forward, the team will continue to monitor the bird strike issue at the Zoo, and has also raised the issue among other zoos, educating them about what works and what doesn’t. After all, other zoos are similarly concerned about animal welfare, yet they also tend to have many glass surfaces that can cause bird strikes.

“Because of our measured approach to solve this problem—through data collection and study—we now have the statistics to show the difference between treated and untreated glass,” says Greenberg. “This means the National Zoo can take a leadership role on this issue and help find real solutions, not only for the other concerned zoos, but for other organizations and businesses as well.”

—Jodi Legge is deputy associate director of communications for Friends of the National Zoo.

More: "Preventing Bird Strikes at Home"


If you have a comment about Smithsonian Zoogoer magazine, please email it to us.

Smithsonian Zoogoer 38(5) 2009. Copyright 2009 Friends of the National Zoo. All rights reserved.