Apart from the whole bird versus mammal thing, crows and humans have a lot in common. Like us, these clever corvids have excellent memories and a penchant for puzzles. They can make and use tools, and move through the world in complex social groups. They’re even somewhat monogamous—just like us.
Now, the less delightful news: Crows, too, are gluttons for junk food—and their bodies are not immune to its effects. In a study published today in the journal The Condor: Ornithological Applications, scientists found that city-dwelling crows tend to have higher cholesterol than their rural cousins. The culprit, it seems, are the human-like diets they adopt, including the fatty, calorie-dense McDonald’s cheeseburgers that line many an urban alleyway.
This fatty spike isn’t necessarily caws for alarm, though, and there’s not yet evidence that elevated cholesterol has a long-term impact on crow health. But the case with crows has always been complex, as these birds are among the few species that actually thrive in urban environments.
In the United States, the American crow (Corvus brachyrhynchos) effectively commands a coast-to-coast range, due in part to its plucky combination of mental acuity and apparent stomach of steel. While countryside crows are often found munching on fruits, nuts, insects, and small mammals, given the opportunity, these opportunistic omnivores will gobble up pretty much anything they can get their beaks in, including straight-up garbage.
In a lot of cases, this indiscriminate dining comes in handy: Food is food, whether it’s served on a silver platter or fished out of the sludge at the bottom of a dumpster. But the modern human diet—especially in Western countries—contains a lot of highly processed, calorie-rich fare, and the contents of city refuse are often a far cry from what these birds (or any other animal) evolved to eat.
For years, researchers have known that people in urban areas tend to have higher cholesterol than those in rural regions. So study author Andrea Townsend, a behavioral biologist at Hamilton College, wasn’t terribly surprised when an analysis of 140 American crow nestlings from Davis, California showed the same pattern.
To tie the find more directly to diet, the researchers then devised a Super Size Me setup with a new cohort of crows in upstate New York. During the nesting seasons of 2017 and 2018, Townsend and her colleagues spent their days tossing cheeseburgers within sight of known crow nests—bait that inevitably piqued the interest of parent birds, which hauled the greasy treats back to their young.
The experiment required a steady supply of fast food, and the team became regulars at the local McDonalds in the village of Clinton, where they’d often place orders of around 100 burgers at a time. “We got a lot of looks of shock and horror from the employees,” Townsend says. (The crows, on the other hand, “seemed delighted with the whole study design,” she says.)
After several weeks on the cheeseburger diet, nestlings had more cholesterol in their blood than baby birds that hadn’t been provided with extra food. Crows only remain in the nest until they’re about a month old, after which point they get much tougher to catch, Townsend says. Because of this, it’s not clear whether these particular birds maintained higher cholesterol, or suffered any ill effects in the long term.
The fast food fix may have even helped the birds: Before the cheeseburgered nestlings flew the coop, they were larger and heavier. In the crow community, it’s actually a good thing to be “chubby” as a kid, Townsend says, and portly young birds tend to be healthier as adults.
This result seemed to be bolstered by another one from the nestlings the team had studied in California. During the two or three years after these crows fledged, the researchers kept tabs on them to see if having higher cholesterol at a young age impacted survival. Contrary to Townsend’s expectations, it didn’t: In broad strokes, crows from across the cholesterol spectrum seemed to fare about the same.
The reasons for this aren’t entirely clear. But while there’s a clear cutoff for “desirable” cholesterol levels in humans, no such statistic exists for our feathered friends—and having higher cholesterol in youth doesn’t necessarily translate to having high cholesterol in adulthood.
In other words, crows may simply be putting the molecule to good use. Despite its bad rap in nutrition circles, cholesterol is essential for animal life, and forms an important structural component of cells. It’s important enough that our livers churn it out on the regular—and unusually low cholesterol levels in humans can cause health problems of their own.
So when it comes to bird health and cholesterol, “there isn’t a simple answer at this point,” says Anne Clark, an ecologist and crow expert at Binghamton University who was not involved in the study. But, she adds, maybe that’s to be expected. “The relationships between humans, food, and urban animals is bound to be complicated, in the same way that getting a good diet in the modern age is complicated.”
Still, the lack of negative effects in these birds shouldn’t be taken as an invitation to throw caution (or cheeseburgers) to the wind, Townsend says. The study only followed its feathered subjects for a couple years—but crows can live more than 15 years in the wild, and upwards of 50 years in captivity. Had the cheeseburger intervention or its follow-up been more prolonged, it’s possible things might have looked different, she says.
These results also won’t necessarily hold true in other species. Sparrows, foxes, iguanas, and turtles also seem to get a cholesterol boost from urban environments, but the ultimate consequences of these changes remain mostly unclear. With their dietary flexibility, crows may be among the creatures best adapted to scarf down the occasional burger, says Corina Newsome, a wildlife biologist and conservation ecologist at Georgia Southern University who was not involved in the study. But that won’t be the case for all animals—or even all birds, she says.
Things get even more complex when considering the impact of urbanization as a whole on animal wellbeing, says J. Drew Lanham, a wildlife conservationist and ornithologist at Clemson University who was not involved in the study. While Townsend’s team didn’t find a relationship between cholesterol levels and crow mortality, one troubling trend did emerge: The more urban a bird’s habitat was, the lower its likelihood of survival.
“There’s something going on in the urban environment, either psychological or physiological, that’s having an impact on wellbeing,” Lanham says. “It makes me wonder about things like stress…and what an urban environment exacts on body and mind.”
These lingering uncertainties are perhaps all the more reason to keep wildlife wild, Newsome says, especially when it comes to nutrition. After all, “if it’s not healthy for us, it’s probably not healthy for them.”