Bird Tricks for Surviving Life in the Big City: Have Big Brains or More Babies

(CN) – If common knowledge says animals with bigger brains are better suited to the complexities of urban environments, then why are there so many pigeons in cities? Research published in Frontiers in Ecology and Evolution on Tuesday provides evidence for two city survival strategies for birds: bigger brains and breeding often.

With roughly 4 billion people already populating cities and urbanization undercutting biodiversity, the question of which animals exist best in this extreme environment becomes ever more pressing.

“Although we suspected that they must be more than one way to thrive in cities, it was surprising to find that actually having a large brain only works in certain conditions, but not in others,” said lead author Ferran Sayol, of the University of Gothenburg and the Gothenburg Global Biodiversity Centre in Sweden, in an email.

Birds are often good indicators of biodiversity not only because they are easy to observe, but also because they are sensitive to changes in habitat, including the harshness of urbanization.

“The species that can tolerate cities are important because they are the ones that most humans will have contact within their daily lives, and they can have important effects on the urban environment within our cities,” Sayol added.

Every city has its avoiders and exploiters, those that struggle and those that thrive. It’s widely accepted that big brains bode well for survival in complex environments, which requires more mental capacity to respond to sudden threats, find new resources, and communicate over all the hustle and bustle.

But brain size isn’t the only factor determining survival. A bird can make up for having a small brain simply by reproducing more often, lowering the necessity for each brood to survive – as long as some of them do.

The team of European researchers analyzed existing data on 629 bird species across 27 cities around to world, studying relative brain to body size, life history, and ecology, in the context of a quantitative estimate of urban tolerance.

While researchers were only able to get data from 34 fresh brains, the rest were measured by filling empty skulls with lead shot or plastic microbeads and calculating potential skull volume from there. Sayol personally made a trip to the Natural History Museum at Tring in the United Kingdom to measure the brain sizes for 105 species with microballons.

Turns out birds with smaller brains that produce more offspring, like swifts and pigeons, tolerate urban environments just as well as birds with big brains and fewer babies, like crows and gulls.

“In particular, species that invest in a high number of breeding attempts over their life (i.e. have a low brood value) are more tolerant of urbanization, even when having a small brain size. In contrast, for species that invest in few reproductive events, a brain that is larger than expected by their body size is key to provide urban tolerance,” researchers explained in the study.

Broader habitat also increases avian urban tolerance, but the researchers acknowledged that these are broad generalizations and there are endless life history events that may also impact urban survivability.

Still the research “has implications for guiding conservation in urban landscapes. For instance, if we want to increase biodiversity in cities, a solution could be to try to make it more green so then more low-breeding-average brained species could be able to live in it,” Sayol explained.

Whether or not the social distancing in response to Covid-19 is a plus for birds depends on the species, Sayol said, since “less people doing activities in green areas might be good for species that avoid contact with people. However, for some urban birds that rely on human food sources (i.e. sparrows that eat food leftovers in terraces) this new situation might be challenging for them if they don’t find enough food.”

The research was funded in part by the British Ornithology Union.

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