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Peregrine Falcons in Chicago don’t cheat

A pair of Peregrine Falcons in Chicago's Uptown. Photo by Stephanie Ware, the Field Museum.
A pair of Peregrine Falcons in Chicago’s Uptown. Photo by Stephanie Ware, the Field Museum.

In their usual habitat on isolated cliffs, Peregrine Falcons mate for life. But what about the Peregrines that inhabit so many of our cities? Over the last few decades, reintroduction programs have brought the birds to urban areas throughout North America.

To learn whether the falcons’ typical breeding patterns hold true in the close quarters of urban life, researchers from Chicago’s Field Museum and the University of Illinois-Chicago studied the Windy City’s Peregrines using a combination of field observations and DNA testing.

They report in a paper published in PLoS ONE on July 15 that most mated pairs in the city remain monogamous.

“Peregrine Falcons that now live in the Chicago region are in much closer proximity to each other than they’d be in a more rural environment,” says co-author John Bates, associate curator of birds at the Field Museum. “We thought they might be more promiscuous with more potential mates nearby. Each spring this population also has migratory Peregrines passing through on their way to all parts of Canada, so we didn’t know what we were going to find, but it turns out that almost all of the mated pairs in the city remain monogamous through the years.”

In the decades since DDT nearly wiped out the species, reintroduction efforts have returned falcons to the Midwest. Populations are thriving in urban areas in 12 Midwestern states, as well as southern Canada, the northeastern U.S., and elsewhere.


The Chicago Peregrine Program, a group run by Field Museum scientist Mary Hennen, monitors the nesting falcons in the city and collects blood samples from the young each year. Many of the falcons they observe have leg bands to identify them, enabling researchers to know which falcons are nesting together.

The scientists also compared blood samples from falcon chicks and adults to determine parentage. DNA testing revealed that of the 35 broods tested, only one showed that the parents “cheated.” Even in this case, the researchers believe that a male lost his mate and then that same season, paired with a new female that laid eggs that were not his.

The researchers also learned that Peregrines re-use the same nests each year. At nest sites with at least six years of data, only six of 122 nesting attempts (5 percent) involved changes to a new nest site.


“Two of these involved males moving from other states to two new territories in the Chicago area,” the authors wrote. “One was a female that bred in Chicago, then was injured but after recovering went to breed in another state. Three changes were to nearby nest sites that occurred after previous breeding attempts had been unsuccessful or a mate disappeared.”

In the only other study to date of an urban raptor’s breeding behaviors, researchers in Milwaukee reported in 2015 that 34 percent of Cooper’s Hawk nests had at least one youngster sired by a male that was not the mate of the attending female. No less than 19 percent of all nestlings were the result of so-called extra-pair paternity.

The Peregrine study was the PhD. dissertation of Isabel Caballero, who is currently a post-doctoral fellow at Texas A&M University. She and her co-authors suggest that for mated pairs of Peregrines, it’s in their best interest to remain together, even in cities.


“In long-lived species like Peregrines and other Falconids in which bi-parental investment is substantial, females may avoid extra-pair copulations because males reduce their breeding effort when their paternity is in doubt,” they write.

“Whenever you have animals living in habitats that have been influenced by human development, you have to wonder how the animals’ life histories will be altered,” says Bates. “It’s important to do studies like this one to see how birds are adapting to living in human environments, so that we can monitor changes through time.”

Read the paper

Sex in the City: Breeding Behavior of Urban Peregrine Falcons in the Midwestern U.S. Isabel C. Caballero, John M. Bates, Mary Hennen, Mary V. Ashley, PLoS ONE 11(7): e0159054. doi:10.1371/journal.pone.0159054. Full text.

Read more stories about Peregrine Falcon.


Watch for a feature story about Chicago’s Peregrine Falcons in our forthcoming October 2016 issue. It will mail to subscribers in late August and will be available at Barnes and Noble and other newsstands on September 6.


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