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Birds living in cities have larger brains because they have to be more resourceful about finding food, scientists have discovered.
Urban birds have to adapt to an environment that is not their natural home and find new ways of feeding themselves.
This means that over generations their brains have grown bigger as they think in new ways about how to survive.
Big-headed: Urban birds, including the crow, have larger brains because they have to be more resourceful about finding food, scientists claim
The research is the first time that scientists have shown that the size of the brain is a key part of city life for avians.
The scientists identified 'urban adapters' as crows, wrens and tits which all come from the same family and all have larger brains compared to their bodies.
Other small-brained species such as barn swallows do survive in cities but are not true 'adapters', said evolutionary biology expert Dr Alexei Maklakov, the lead researcher.
'A centre of a modern city is a novel and rather harsh environment for most species and the ability to sustain a varied diet or develop novel foraging techniques and perhaps utilise non-standard nesting places, can be beneficial,' he said.
A wren: Researchers identified 'urban adapters' as crows, wrens and tits which all come from the same family and all have larger brains compared to their bodies
His team from the Evolutionary Biology Centre in Uppsala, Sweden and the Donana Biological Station, in Seville, Spain studied 82 species of birds from 22 families.
They focused their attentions on 12 cities in Switzerland and France to find out about the relationship between brain size and survival.
Family traits emerged as the key to understanding brain growth and birds from the same family were more likely to see the change.
Species which tend to avoid cities include pied flycathers, reed bunting and whitethroat.
Rural dweller: The reed bunting is one of the species which tend to avoid cities
The team also identified some birds with small brains which have survived in the city by finding niches which resemble their natural environment.
'We were interested whether behavioural flexibility can increase the chance of a given species to successfully colonise cities,' said Dr Maklakov.
Previous studies have highlighted the link between larger brains and behavioural innovation in birds and mammals.
The new research was published in the journal Biology Letters.