Spring 1998

[TEXT ARCHIVE WEB-PUBLISHED FEBRUARY 2002.
ORIGINAL PRINT PUBLICATION DATE: SPRING 1998.]

Birds and Buildings: Lethal Combo

By Sheryl De Vore

In the last 20 years, more than 26,000 migrating birds died crashing into a single building along the Chicago lakefront. David Willard, manager of the bird collection at the Field Museum, has retrieved these birds from McCormick Place Convention Center during spring and fall migration and preserved them as specimens. The numbers seem staggering, yet they represent, in fact, a small fraction of the bird fatalities that occur annually when vast numbers of traveling birds collide with skyscrapers, TV towers and picture windows.

"There is hardly a prominent building in downtown Chicago that has not caused bird fatalities during migration," he says. Willard's collection began in 1978 when a birder told him that birds were hitting the windows at the McCormick Place Convention Center near the museum. The next morning, he wandered over and discovered four dead birds, including a yellow-billed cuckoo. Willard resolved to collect the birds and see what he could learn about migratory patterns and population trends.

"For several years after that, we casually checked the building sometimes finding birds, sometimes not," says Willard. "By 1982, we realized that, as unfortunate as these casualties were, here was an extraordinary data source. We now have 15 years of excellent data."

Birds migrate at night using the stars as a navigational tool and often following a corridor along a Body of water such as Lake Michigan. Lit buildings can disorient them, attracting them to their deaths. Willard speculates that, from a vantage point over the lake, the low, dark mass of McCormick Place may appear to be a cluster of trees. It might look, to birds, like a haven of food and shelter.

"Our sample now consists of 26,000 birds salvaged," says Willard. on one particular day, he found more than 200 birds. "That sounds appalling. But you need to put that into some sort of context. In the spring of 1996, for instance, we picked up 2,000 birds along the Chicago beaches that died over the lake in a single severe storm, which is a natural phenomenon."

The McCormick Place data show that numbers of the most common species collected there have remained fairly constant. "If these birds were in massive decline, there would be fewer at McCormick Place," he says.

Exceptions include the ovenbird, Nashville warbler, and rose-breasted grosbeak. "Rose-breasted grosbeak numbers have declined steeply at the McCormick Place study site," he says.

"Those data reflect the breeding bird survey research that shows the rose-breasted grosbeak is declining in the Upper Midwest," says Doug Stotz, a Field Museum conservation biologist. "Pinpointing trends in this way is of major conservation value."

"Unfortunately, the only real increase we are seeing is in the European starling," says Willard. This species was introduced from Europe to North America in the early part of the century and has become a pest here.

Willard has also uncovered some interesting rare finds, including three black rails, a species rarely seen in Chicago, plus the state's first specimen of a painted bunting. While these are exciting to discover, Willard says the single rare specimen offers few clues to population trends. That type of information can only be gained from reviewing data of the more common species, those that are regularly found at the data site.

"The 10 most frequently collected species account for more than two-thirds of the individuals found there," says Stotz. These include the song sparrow, dark-eyed junco, swamp sparrow, white-throated sparrow, hermit thrush, fox sparrow, ovenbird, Lincoln's sparrow, American tree sparrow, and Tennessee warbler. Because Willard and other Field Museum personnel and volunteers carefully preserve each specimen they collect, they are gathering other information such as sex ratios, measurements, and breeding status. Mary Hennen, bird collections manager for the Chicago Academy of Sciences, is using data on hermit thrush weights in a study of that species. She too collects birds regularly, in her case at the John Hancock building.

One of Willard's most intriguing finds involves the American woodcock, which performs a fascinating aerial mating display in March and April in the Chicago Wilderness region.

"We found a highly skewed ratio of female woodcocks to males at the data site," says Willard. "What's really interesting," he says, "is that these females' ovaries are highly developed, and that means they are almost ready to lay eggs.

"The doctrine on woodcocks says they mate and nest all within 500 yards," says Willard. But his McCormick data spawned an intriguing theory that females may mate one place, then migrate elsewhere to lay their eggs.

While these data provide insights into bird population and biology, Willard is concerned about the migration fatalities at buildings, TV towers, and even home picture windows. Indeed, estimates range from 96 million to 960 million birds are killed annually from hitting buildings in North America.

How can we undo the lethal combination of lights, glass, and migrating birds? "If an urban building can have the lights turned off at night, the number of birds killed can be reduced," Willard says.