Pigeons crowd a streetlight in New York City. Image credit: Emmanuel Dunand/AFP/Getty Images

 
When Rebecca Calisi first moved to New York City, she ran into two immediate concerns. First, the New York City Department of Health and Mental Hygiene had encouraged her to have her children tested for lead sometime that year, since the state mandates that children under 6 be tested on an annual basis. The idea seemed sensible, given they had moved into one of the city's prewar apartments, which are notoriously full of lead paint. Next, she needed to find a new research subject. As a biologist with a specialty in avian biology, Calisi knew a readily available bird species would be best. And in a city like New York, there’s nothing quite so omnipresent as pigeons.

“You could walk outside and sit down on a bench and your subject would come right up to you,” she tells mental_floss.

It was only a short stretch for Calisi to connect the two items on her to-do list—lead levels and pigeons—and before long she was measuring the blood of pigeons from zip codes all over the city. What she found in her survey of 825 birds over five years correlated precisely with data from the city’s health department. Just like human New Yorkers, pigeons are very attached to their neighborhoods. They live in a small area for the majority of their lives, and the birds were exposed to many of the same sources of lead as human residents. Sure enough, her initial results showed that the birds’ blood showed similar levels of contamination. In other words, Calisi found, pigeons were like urban canaries in a coal mine. They were perfect biomarkers for lead contamination.

Now an assistant professor in the Department of Neurobiology, Physiology, and Behavior at the University of California-Davis, Calisi has a new study out in the journal Chemosphere detailing her findings [PDF]. The results clearly showed the link between the health of a city’s human inhabitants and its wildlife.

As cities and their populations expand, engulfing areas previously left for farmland or untouched wilderness, encounters between humans and wildlife are on the rise. Consider the coyotes roaming the streets of New York, or bears in cities all over Southern California. While these confrontations are often framed as antagonistic, viewed through the lens of biological research like Calisi’s, the encounters can actually be mutually beneficial.

For example, Calisi says, “If somebody had been monitoring the levels in lead in pigeons in Flint, Michigan, might there have been warning signs before children started getting sick?” Lead isn’t the only hazardous substance studying birds could detect. She hopes to expand her research to investigate other heavy metals, pesticides, pollutants, and fire retardants in other cities as well as more rural populations.

Calisi isn’t the only person intrigued by what the animals we live beside can teach us about our environment. Scientists at the University of Rochester Medical Center recently found a link between highway pollution and cardiovascular health. The Rochester-based team took lab rats on a road trip between Rochester and Buffalo. The rats, exposed to the same type of pollution as drivers or people living near highways, showed immediate health effects that lasted for up to 14 hours. Their heart rates quickly dropped after exposure to airborne pollutants, and their nervous systems were negatively impacted as well. The study offered new insights into why urban hospitals often seen an uptick in heart attacks on smoggy days.

Then there’s the paper that revealed how artificial lights impact the reproductive systems of European blackbirds (Turdus merula). A study by ecologists and evolutionary biologists at the Max Planck Institute for Ornithology in Radolfzell, Germany found that even low intensities of artificial light can alter the timing of reproductive development in songbirds. Birds exposed to light with an intensity even 20 times lower than the light emitted by a normal street lamp developed their reproductive system as much as a month earlier than birds kept in the dark at night. They also molted earlier. For humans living in the incandescent glow of urban areas, artificial light can be just as harmful: Disrupted circadian rhythms are linked from everything to metabolic disorders to an increased risk of cancer.

A blackbird in Paris. Image credit: Joel Saget/AFP/Getty Images>

 
If it seems like city life is killing its inhabitants—feathered, furred, and bipedal creatures alike—there’s reason for hope. Urban wildlife don’t just reflect the risks of urban living; their rapid evolution hints at how humans might also change to cope. The pressures are undoubtedly great, but for animals that find ways to adapt, there are numerous opportunities to thrive. In one recent study, ornithologists showed that European blackbirds from cities are far less sensitive to stress than their rural cousins. The city birds had a much lower hormonal stress response, which scientists hypothesized could be the result of selective pressures in an urban environment.

These pressures make up a large part of what Jason Munshi-South, an evolutionary biologist, investigates from his lab at Fordham University. With a collection of students and colleagues, Munshi-South has tracked down white-footed mice [PDF] in slivers of parks throughout the city, as well as salamanders and rats. Creatures this small can roam throughout the city and incorporate human infrastructure into their daily needs—though, like pigeons, they tend to stick to a small home area. White-footed mice and salamanders have found a way to exist in tiny pockets of wilderness, surrounded by the rush of 8 million people. And those challenges have led to some surprising rapid adaptations.

“They’ve evolved to eat different diets, to deal with pollution. Their overall life history may evolve if populations are crowded—they’ll have offspring at a younger age. Those are the things we found with white-footed mice,” Munshi-South explains to mental_floss. “With rats, similar things are going to apply. You’ll see adaptations in the skeleton for living in different structural environments.”

For example, he says, “You have a lot of rats in New York City living in underground infrastructure and coming up and down, not burrowing into the ground.”

A white-footed mouse. Image credit: Charles Homler via Wikimedia Commons // CC BY-SA 3.0

 
Genetic tests on the rodents have revealed that these changes extend even into the animals’ genomes. Munshi-South said they found evidence that the city critters are becoming distinct from their country relatives simply because of the extreme pressures to survive. And while the differences aren’t yet large enough to separate the two groups into distinct species, it’s a possibility in the future.

The ability of these animals to adapt and thrive in city environments—to live alongside humans and even act as early warning systems for certain pollutants—might seem to suggest that nature will triumph in the face of human progress. But neither Calisi nor Munshi-South are particularly optimistic when it comes to conservation.

“I guess we’re lucky that some species can live in cities and adapt, since most of them can’t,” said Munshi-South. “But I don’t think we’ve figured out a good way to use urbanization as a tool to prevent broader habitat loss yet.”

Calisi just hopes that the species who do manage to build niches in cities will stop being regarded as nuisances or invaders. Instead, she says, we should view them as partners—and acknowledge that we’re all in this rat race together.