A crowd of 50 nestled around a small table on the third floor of Evanston’s Main Library to witness the banding of the wild birds. May 30 marked the fifth occurrence of this annual event, which provides a means to monitor the peregrine falcon chicks that hatch each year beneath an overhang on the south side of the building. For the past five years, Mary Hennen, the director of the Peregrine Project at the Field Museum, has made the trip to Evanston to head the banding procedure.

The falcon parents, Squawker (father) and Nona (mother), have been tracked from their places of birth using similar processes, migrating from Wisconsin and Iowa, respectively. This year’s group of four healthy chicks is the third clutch conceived by the couple. The fledglings, three females and one male, hatched in early May after a month of incubation. The banding usually takes place at least three weeks after the chicks have hatched. “If you do it too soon, they’re too fleshy,” says Ms. Hennen, “If you wait too long, they’re too big.”

The chicks are fitted with colored bands engraved with a series of numbers that group the birds by region. This system, utilizing specific colors to denote different parts of the country (purple for the Midwest), allows scientists to track the movements of the falcons. “We are able to look at the longevity and dispersal of the birds,” says Ms. Hennen.

The sex of the chicks is determined and blood samples are drawn. Though Ms. Hennen says the samples are used primarily to “look at the health of the bird,” she added that, only recently, they have begun to explore the genetics and lineage of the peregrines.

Keeping with tradition, the peregrine chicks were named to reflect references to falcons in culture and literature. The three females were named Brigid, after Brigid O’Shaughnessy of “The Maltese Falcon”; Margaret, after Mistress Margaret Hussey, who was once compared to a falcon by poet John Skelton; and Rebecca, after Rebecca West, author of “Black Lamb and Grey Falcon: A Journey Through Yugoslavia.” The only male, however, was dubbed Neal, in honor of Neal Ney, the avid bird enthusiast and former library director, who retired earlier this year.

Ms. Hennen has dedicated herself to the study of peregrine falcons since 1989, when the bird was still marked as an endangered species on both the state and national levels. The detrimental effects of the widespread use of the synthetic pesticide dichlorodiphenyltrichloroethane (DDT) caused the bird nearly to disappear from the Midwest, she said. Since the ban of DDT in 1972, however, the peregrine falcon population has been afforded a chance to rebound, a feat for which urban areas provide a prime setting.

Habitually cliff-dwelling birds, the falcons have resorted to nesting in the ledges, overhangs and nooks of tall buildings and bridges. Ms. Hennen says she hopes her efforts will aid the creation of a self-sustaining peregrine falcon population in the Chicagoland area, but also noted the collaborative effort necessary for such an endeavor to succeed. The Peregrine Project faces the difficult task of minimizing contact with the birds, while still keeping tabs on them.

In the past decade, the work has begun to pay off. In 1999 the peregrine falcon was removed from the National Endangered Species list, and the bird has been reclassified in Illinois as being only “threatened.” With each new generation, the falcons become more numerous. At some point in the next few weeks, the falcon fledglings will begin to fly, and, hopefully, within the year, will migrate elsewhere to find mates and build nests of their own. To monitor the progress of Evanston’s peregrine falcons, go to http://www.epl.org/falconcam/, where the “FalconCam” updates every few minutes.