Discoveries in the Windy City

Tragic though it is that so many birds lose their lives to the lights and windows of our modern cities, much can be learned from their remains.
In Chicago, which borders Lake Michigan, ornithologist Dr. David Willard saves the disoriented birds he finds and gleans as much information as he can from the dead ones. For 20 years now, Dave and his colleagues at the Field Museum of Natural History have monitored one low-rise building near the waterfront during the migration periods. With the help of many other Chicagoans who have donated the bodies of birds found at virtually all of the prominent buildings around the city, museum staff have acquired over 25,000 specimens in that time period. Their discoveries are fascinating and largely encouraging.

This two-decade-long accumulation of specimens has enabled museum staff to study population trends in the more common species. Although there is a lot of year-to-year variation due to weather conditions during migration or the success of the breeding season, if any species is declining that decline should be evident in the statistics gathered. For most species the numbers remain constant. Two exceptions are the Ovenbird and the Nashville Warbler, both of which appear to be gradually diminishing in numbers.

Most birds die unnoticed, hidden by vegetation, quickly swallowed up by the earth or consumed by scavengers. This is especially true of highly secretive birds such as marsh dwellers. But when these enigmatic creatures fall victim to man-made obstacles, we may find that they are more abundant than originally thought. Such is the case with the Black Rail.

The first Chicago-area nest ever found for this species was discovered at Lake Calumet in 1875, but after that sightings were extremely rare. Three Black Rails have now been recovered from buildings around the waterfront, suggesting that these birds are more common in the area than birdwatching records indicate. The same holds true for the Connecticut Warbler of which a disproportionately higher number are picked up in Chicago than are seen in the wild.

Beyond mere statistics, the museum's study of these unfortunate birds has yielded differences in migration patterns between the sexes, and between the young and the old. The thrush family offers one such example. David has discovered that 95% of the Hermit Thrushes that succumb to the buildings during fall migration are immature birds, while among Swainson's Thrushes only 50% are immatures. Why is this happening? No one knows yet.

The most remarkable discovery to date involves the American Woodcock, a most remarkable bird. The species' bizarre courtship ritual begins with the male on the ground giving a comical "peent" sound as he throws his head back. This he does many times over. Then he lifts off and flies in wide circles hundreds of feet into the sky producing a whistling sound with his wings. Upon his descent he gives a series of chirps which ends abruptly as he nears the ground. The female chooses to mate with the male who gives the best performance.

Published literature indicates that female woodcocks mate, then lay their eggs within 100 yards of the male's display arena. Surprise! David and his colleagues found woodcocks travelling with very well-developed eggs in their ovaries, a condition unusual in migrating birds. One bird was even carrying a shelled egg which would have been laid very soon after that night's migration. Since there is no woodcock display site anywhere near downtown Chicago, all females obviously do not nest close to where they mate.

This indication of their breeding precociousness is reinforced by the ratio of females to males picked up by the Chicago crew. In the spring females outnumber males 4:1. This could simply mean that woodcocks have a biased sex ratio. More likely, many of the males linger in the south, hoping to mate at the first opportunity as the females begin moving north.

Although the species composition is almost identical for Chicago and Toronto, the numbers differ. For example, only a handful of Fox Sparrows or Tennessee Warblers (which rank sixth and tenth respectively in the Chicago data) usually appear on the FLAP lists. This may mean that certain species favour particular parts of the continent as their summering grounds or it may be indicative of the migratory paths they choose to take to their breeding destinations. There is much yet to be learned. The Chicago specimens provide an unprecedented opportunity to study North America's migratory birds, perhaps even to witness evolutionary changes over a short period of time.

(Adapted from an article written by Dr. David Willard for the January / February 1995 issue of "In the Field", the bulletin of the Field Museum of Natural History.)


Chicago's Top 10 List

Song Sparrow
Dark-Eyed Junco
Swamp Sparrow
White-Throated Sparrow
Hermit Thrush
Fox Sparrow
Ovenbird
Lincoln's Sparrow
American Tree Sparrow
Tennessee Warbler