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.
Researchers studied urban blackbirds in the city of Vienna and country blackbirds in the nearby Vienna Woods. They investigated the correlations between the frequency and amplitude of their song under controlled conditions.
It emerged that the animals were able to produce higher tones at higher amplitudes. In the city, blackbirds sing preferably at these high frequencies that they can produce particularly loudly.
The researchers examined which effect is better suited to avoiding the acoustic masking by traffic noise: the higher frequency or the higher amplitude that results from it.
"The higher volume of the higher-pitched song is more effective than the higher frequency," said researcher Erwin Nemeth in a statement.
"So we assume that the increased volume is the main cause of the higher frequency singing by city birds," he said.
"By actively selecting high-frequency sounds, the city birds can increase their capacity to sing loudly and in this way counteract the acoustic masking of their song by the ambient noise," lead research Henrik Brumm added.
Researchers said despite the numerous unfavourable environmental conditions they encounter there, many wild animals have colonised cities as a new habitat.
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.
However, the urban habitat also offers certain advantages, for example a more abundant supply of food and new breeding options. Many animals have thus adapted surprisingly well to city life.
