Male great tits are territorial birds and they sing to defend their patch – usually around 100m sq – against other males. They also use the songs to attract mates.
In their research, published today in the journal Proceedings of the Royal Society B, Marshall’s team recorded the songs they heard in city centres and also in the nearby rural areas. They found that, on average, the city songs were higher pitched than the countryside songs.
Marshall’s team also went a step further. “We played back the songs we’d recorded in the cities to the birds in the rural areas and vice versa. And we also played the city birds other city bird songs and the same in the countryside. We found that they responded much more strongly to the songs of the birds form the same area, birds with a similar noise background.”
This inability to recognise songs from members of its own species could lead to problems, said the researchers. If a city bird moves to a new area to attract mates or find food, its song may not be attractive to mates, nor would it be a warning to local males to stay away from its territory. “We know that great tit songs are largely determined by what they hear in the first year of their life and only small changes are known to occur after this stage,” said Marshall. Great tits do move their territories over the years, so those at the edges of towns might be most at risk.
Previous research on birds in continental Europe has suggested a difference in song pitch between rural and city birds. In 2006, scientists at Leiden University in the Netherlands recorded the songs of great tits in 10 European cities including London, Prague, Paris and Amsterdam and compared the songs with birds of the same species in nearby forests. They found that not only was the pitch of city songs higher but also the urban birds sang faster, shorter, songs.
Marshall’s study is the first to show that birds of a single species respond differently to the different songs. He said the next stage of the research would be aimed at working out whether female great tits also responded differently to the songs of males from different areas.
The changes in song could mark the first step in an evolutionary process called speciation – when an organism splits into two or more different species because of the differing environmental pressures facing different populations. But, given that such an evolutionary change would take many generations to occur, it is impossible to be certain whether great tits were already heading down this path.