Big city living doesn’t stop peregrine falcons from mating for life

As humans encroach more and more on their habitat, some peregrine falcons have become city-dwellers: at least 25 pairs now nest on man-made structures like skyscrapers and bridges in Chicago, for instance.

In their natural habitat, where they build their nests on isolated cliff ledges, peregrine falcons (Falco peregrinus) mate for life. But like any other resident of the big city, these Chicago transplants live much closer to their neighbors than they normally would. That gives them more opportunity to be unfaithful. So are peregrine falcons stepping out on their mates in Chicago?

A team of scientists with Chicago’s Field Museum and the University of Illinois, Chicago examined the breeding patterns of the city’s falcons and have published the results of their study in the journal PLOS ONE confirming that even cityslicker pairs do indeed stick together.

Photo by Stephanie Ware, The Field Museum.

“Peregrine Falcons that now live in the Chicago region are living in very different conditions than you’d normally see for these birds, so we wondered if the falcons’ mating habits had changed too,” John Bates, Associate Curator of Birds at The Field Museum and a co-author of the study, said in a statement.

“They’re in much closer proximity to each other than they’d be in a more rural environment, and we thought they might be more promiscuous with more potential mates nearby.”

chicks. Photo by Mary Hennen, The Field Museum.

There were other reasons to suspect Chicago’s falcons might cheat on their mates, Bates explained, even though that turned out not to be the case. “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.”

The scientists used field observations and DNA testing to determine whether or not Chicago’s breeding pairs stayed faithful to each other. A group run by Field Museum scientist Mary Hennen, the Chicago Peregrine Program, monitors the city’s nesting falcons, measuring and banding the young each year while also collecting blood samples. The ankle bands allow researchers to identify the falcons and observe which individuals are nesting together, and the blood samples make it possible to determine parentage.

DNA testing showed that, among the 35 pairs tested, only one of the parents cheated. But even that might not have been a case of infidelity — the researchers believe that a male lost his mate and then paired with a new female who laid eggs that were not his that same season.

“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,” Bates added. “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.”


  • Caballero, I. C., Bates, J. M., Hennen, M., & Ashley, M. V. (2016). Sex in the City: Breeding Behavior of Urban Peregrine Falcons in the Midwestern US. PloS one, 11. doi:10.1371/journal.pone.0159054

This article was first published by on 22 Jul 2016.


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