City life is a singular experience. We cram ourselves into territories far too small to support us and stack our homes on top of one another. We encounter other people at a rate that would have shocked and terrified our ancestors. In order to survive the city’s stresses and compression, we build up, we speed up, and we toughen up. For better or for worse, we change. And the same holds true for other animals. But some things stay the same; experts say that, even in crowded conditions, Chicago’s peregrine falcons stick to their naturally monogamous mating habits. The research was published in the journal PLOS One. 

Scientists have good reason to keep an eye on these birds. Fifty years ago, the widespread use of DDT nearly wiped out the U.S.’s entire population of peregrine falcons (Falco peregrinus). A ban on the pesticide, combined with aggressive conservation programs, have helped the birds bounce back in a major way. But like so many modern humans, the birds have urbanized. In Illinois alone, almost 90 percent of the peregrine falcon parents build their nests on Chicago’s buildings and bridges. This is a big shift for the falcon pairs, who historically maintained large territories around their cliffside nests. In isolated conditions, monogamy might be a natural choice (where would a bird even find someone else?) but the falcons are isolated no longer. Could living in such close proximity to others encourage falcons to sleep around?

To find out, a team of Chicago-based scientists tracked the nesting behavior of 350 birds at 20 nesting sites in nine Midwestern cities; they also ran DNA tests on the chicks. Three-quarters of the samples came from Chicago, perhaps because members of the Chicago Peregrine Program were already banding and taking blood samples from every falcon they could find. The ankle bands allowed observers to identify the inhabitants of each nest. By testing the birds’ blood, the researchers could figure out if a couple’s babies were 100 percent their own, or if one of the parents had been stepping out. 

Analysis of the DNA results and nest-watching revealed that, almost to a bird, the falcons remained loyal to their partners. Out of 126 baby birds, only 2 were being raised by a bird other than their biological parent, and even that was a special case: The male bird had hooked up with the already-pregnant female after his own mate had died. He was a stepdad, not a cuckold.

Co-author John Bates is associate curator of birds at The Field Museum in Chicago. He says that he and his colleagues were a little surprised by the results. “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,” he said in a press statement, "but it turns out that almost all of the mated pairs in the city remain monogamous through the years.”

Even greater than their loyalty to each other was the falcons’ loyalty to their nesting sites. It makes sense; while a partner might die in a collision with a building or a power line, a safe nesting niche is forever.

The researchers plan to continue keeping an eye on the birds. 

“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.”

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