Many urban birds sing at a high pitch to differentiate their songs from the low-frequency sound of road traffic in noisy cities, a new study has found.
Scientists from the Max Planck Institute for Ornithology found that the real reason for this behaviour is that songs at a higher pitch are also automatically louder.
The birds can make themselves heard far better in city noise by increasing the volume of their song than by raising its frequency.
To attract mating partners and defend their territories, urban robins sing in the latter night when the traffic noise decreases after the evening rush.
Many other bird species, including blackbirds, sing in urban environments at a higher pitch. So their song is easier to detect in the lower-frequency traffic noise.
In cities they must deal with greater numbers of humans and with more light and noise pollution than they encounter in rural settings, they said.