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City life isn’t for everyone, and it can be especially challenging for birds. On top of finding food and places to nest, urban birds have to contend with noise, pollution, and glass surfaces that can produce deadly collisions. Yet, worldwide can be found in cities.

Ferran Sayol, a biologist at the University of Gothenburg in Sweden, wanted to know what makes some bird species so well-suited for urban environments—a question that is becoming more important for conservation as cities around the world continue to grow. Based on past studies showing the benefits of large brains for birds, he and colleagues hypothesized that bigger-brained species would be better able to solve the challenges of city life. 

In a in Frontiers in Ecology and Evolution, Sayol’s team confirms that hunch, but also reports another trait of successful city-dwelling birds: frequent breeding. 

The team started out by determining which species could be classified as urban. By comparing bird abundances outside and within 27 cities across the world, they identified species that were more abundant inside urban areas. Next, they used global databases and museum measurements to obtain brain sizes for 629 of the 1,219 avian species they recorded in those cities. For 436 species they also gathered information on how many times these birds raise a brood each year and over their lifetime.

Their analysis revealed that species with large brains relative to their body size, such as American Crows, Black-capped Chickadees, and American Kestrels, were present in higher numbers in cities versus adjacent rural lands. This brain-size advantage can help individuals find new food sources and avoid manmade hazards better than birds with smaller brains, the researchers say.

But that didn’t explain why some of our most familiar city birds have relatively small brains, like the Rock Pigeon, Mourning Dove, and House Finch. Turns out, while these species may not be as clever as their bigger-brained neighbors, they keep their populations afloat or growing by breeding several times each year to add new members and replace lost individuals. Mourning Doves and Rock Pigeons, for example, raise up to six broods each year, while American Kestrels and Black-capped Chickadees often raise just one.

Interestingly, species that combine both traits aren’t doing as well in urban areas. Some birds have large brains and breed up to three times per year, yet they aren't as abundant in cities. “You would expect them to be most successful,” Sayol says. But having a large brain means using a lot of energy, and the same goes for reproduction and chick-rearing. “You need to put energy in one or the other strategy to be successful,” he says. “Either you have a small brain and breed many times, or you breed a few times, but you have a large brain.”

While cities favor birds with these two contrasting strategies, species with average brain sizes tend to dominate in rural landscapes. “There aren't only small-brained or large-brained birds outside cities,” Sayol says. “We find that there is more variety.”

But what if one were to compare brain sizes of individuals of the same species living in urban versus rural environments? They are unlikely to vary, saysAlejandroGonzalezVoyer, an evolutionary biologist at the Universidad Nacional Autónoma de México, who wasn’t involved in the study. “Urbanization, from an evolutionary point of view, is still rather novel,” he says. 

The growth of cities isn’t only relatively new, it’s also happening quickly. In the U.S., urban areas made up 68 million acres, or 3 percent of land area in 2010, but by 2060 163 million acres, or nearly 9 percent of the country. As urbanization ramps up, Voyer doesn’t expect that birds with small brains will evolve larger brains to survive. Instead, he says, some rural birds may lose out as the countryside undergoes development, so conserving rural habitat will become increasingly important.

Against that backdrop, Sayol and his team now hope to more fully understand how extra brain capacity helps birds overcome urban challenges. Do bigger brains enable them to widen their diet, or navigate through the physical dangers of city life, or do they play some other role? Answering those questions could help planners and others create more bird-friendly places for species better suited to life in the big city.