12 June 2014 Last updated at 01:27
By Michelle WarwickerReporter, BBC Nature
Urban great tits have smaller black breast stripes than their rural neighbours, a study has found.
Scientists discovered significant differences between birds living in Barcelona, Spain and a nearby forest.
Smaller "black ties" appeared to improve male great tits' survival rates in the city, while in the countryside birds with larger ties fared best.
"The urban habitat favours a different kind of bird than the natural habitat," said researcher Dr Juan Carlos Senar.
His team captured and recaptured birds over several years and used a predictive computer model to compare survival rates of great tits with different sized ties.
"Recapturing a bird means that it is alive, and so we can relate survival probability to tie size," explained Dr Senar, who is head of research at the Natural History Museum of Barcelona.
The size of the distinctive black stripe on great tits' yellow chests is hereditary. It varies more in males, and is linked to their personality.
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