Crows may have higher cholesterol because they eat our leftover fast food Charles Wollertz / Alamy

Crows living in urban areas have higher blood cholesterol levels than their rural counterparts. That may be due to humans feeding them, and the fast food we leave behind.

Crows are “experts at raiding human trash cans and dumpsters,” says Andrea Townsend at Hamilton College in New York. Some of the food they scavenge is fast food, which is often high in cholesterol, but it’s unclear how this may be affecting the birds’ health.

Townsend and her colleagues measured cholesterol in blood samples taken from 140 crow nestlings in rural, suburban and urban areas in California. They also measured the birds’ body mass and fat reserves, and tracked their survival rates. They found that the more urban the surroundings, the higher the blood cholesterol in the crow nestlings living there.


To see if access to the kinds of foods that raise cholesterol in humans were responsible, the team ran a “cheeseburger supplementation experiment” where they left McDonald’s cheeseburgers near nests in rural New York. Elevated cholesterol doesn’t appear to affect all species in the same way, and has actually been linked to better body conditions in some animals, so Townsend didn’t have reservations about leaving behind burgers for the nestlings.

The burger-fed crows had higher cholesterol than nearby rural crows that weren’t given fast food. Those that ate the burgers had cholesterol levels more similar to crows living in cities, being about 5 per cent higher than their burger-deficient neighbours.

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Townsend says these results are consistent with the handful of other studies on cholesterol in animals that live in near humans, including foxes, sparrows and even sea turtles living near more densely populated Canary Islands.

“All of these species tend to have higher cholesterol levels in places where they interact with people,” says Townsend. Previous research has shown that places with higher human population density also have a higher proportion of processed food waste.

She and her team found that while urban crows had lower survival rates overall, the cholesterol didn’t seem to hurt the crows, and even boosted the body condition of the nestlings, leaving them a little plumper.

Still, Townsend doesn’t recommend flinging burgers to birds at the park. “We know that excessive cholesterol causes disease in humans, but we don’t know what level would be ‘excessive’ in a wild bird,” says Townsend. “It is also possible that, if elevated cholesterol does have negative effects, they would show up later. In humans, it can take many years for excessive cholesterol to lead to disease.”

Jukka Jokimäki at the University of Lapland in Finland was surprised the fast food didn’t have a bigger negative influence on the birds’ health. “There are probably some other urban-related factors that are more important for the crows,” says Jokimäki, adding that other aspects of city life may tax crows’ health, such as car collisions and disease.

Journal reference: The Condor: Ornithological Applications, DOI: 10.1093/condor/duz040