Audet and his colleagues tested 53 bullfinches from different parts of Barbados — something he was inspired to do after being terrorized by bold city birds at a restaurant.

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"Barbados bullfinch are always watching and trying to steal your sandwich," Audet told CBC News.

So what's the difference between a bold city bruiser and a demure country bird? According to cognitive tests, an awful lot: The city birds really were more bold by some measures (they were quicker to eat food presented to them in a familiar dish by a human who then hid, meaning they cared less about how likely a human was to interrupt their meal), and better at solving problems — getting to food that had been placed in jars or drawers, for example.

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They weren't any better or worse at learning to distinguish between different colored dishes that gave them access to food, and they were actually more cautious about obtaining food with unfamiliar objects nearby than their rural cousins were (a trait referred to as neophobia).

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But surprisingly, there was one other area where they came out ahead: They had stronger immune systems. Audet and his colleagues thought it might make sense for better cognitive abilities to be associated with weaker immune systems, because the birds don't have infinite resources to fuel their brains and bodies. But it seems that city birds really can have it all.

Taken all together, these traits seem to serve a pretty obvious purpose. When humans are around, it means that birds have more reason to learn to access food in new and challenging places. A bird in the country is pretty much always going to find seeds in the same sorts of places, but a city bird can, for example, learn to snatch sandwiches off restaurant plates. That's where the boldness comes in, too. And perhaps a fear of novel objects is just part of the same routine: A city bird encounters new and strange dangers more often than one in a rural environment, so it needs to be smart about sussing out situations before blundering in.

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Fifty-three birds from one country is a pretty small sample size, so we can't assume that these findings are true for birds all over the world — or even for birds all over Barbados. But the results suggest that humankind's influence might have a profound effect on the behavior (and health) of urban birds. It's especially interesting in Barbados, where birds likely aren't isolated enough from one another to become different species, or even to change much from generation to generation.

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"Barbados is a very small island, and it would be surprising if urban and rural bullfinches were geographically isolated, even if the island has a high population density and the original vegetation of the island has been destroyed and replaced by sugar cane and other anthropogenic plants for over 350 years," the authors write in the study. "Enhanced boldness, problem solving, and immunocompetence in urbanized bullfinches might all be experience-driven responses to environmental variation in food, human disturbance, and pathogens."

In other words, these changes probably aren't due to long-term natural selection: They're made on the fly.