Anyone who has travelled on the London Underground at rush hour knows that city-dwellers aren’t always the most relaxed of folk.

Well, the same can be said for birds, according to a study. City sparrows are more aggressive than their rural counterparts, the researchers found. But the behaviour isn’t triggered by a lack of resources, such as a reduced availability of food, the biologists from Virginia Polytechnic Institute and State University (Virginia Tech) said. Instead, it is the opposite: an abundance of food in the city - often from people feeding them - is thought to be the cause of their aggression.

The birds may be more keen to defend an area with lots of food as it is more valuable or because it attracts lots of thieves, or simply because the birds have more time to put up a fight, as they are not searching for food, the researchers said. For the study, which was published in the journal Behavioural Ecology, the researchers set up music speakers in a range of urban and rural areas, through which they played the birdsong of a male song sparrow.

They measured the responses of other male song sparrows to the recordings, noting how often the birds approached or attacked the speakers. The males in the urban areas, where there was more food, were more aggressive than the rural ones. And when the researchers introduced extra food to the rural areas, they found that levels of aggression rose.

Researcher Sarah Foltz said: ‘Giving birds abundant and conveniently located food may free them to spend more time defending their territories, which they may want to do now that the territory contains more valuable resources. ‘Territories where we placed extra food were visited by more birds, so the song sparrows may have become more aggressive in response to this up-tick in unwanted visitors.

‘Other features of the local area didn’t seem to be related to aggression; birds with lots of neighbours or fewer nesting sites on their territory weren’t any more or less aggressive than those in lower-density areas with lots of spots for nests.’