Great tits are tweaking their tweets to be heard in noisy urban areas, but for their country cousins it is like they are speaking a different language.
Scientists at Aberystwyth University found male great tits in 20 UK towns and cities sang at a higher pitch to be heard above the man-made noise.
Rural birds were confused by urban bird song while city birds "didn't understand the lower rural pitch".
The male great tit sings to defend his territory and attract a mate.
Research student Emily Mockford visited 20 towns and cities in the UK to capture the bird song.
The singing was played back to rural male tits during the breeding season when they are at their most aggressive, but there was a "slower and weaker" response than normal from the countryside birds.
Scientists also recorded countryside great tits and found they sang at a lower pitch and, in turn, city birds found the rural droll difficult to understand, the study found.
Project leader Dr Rupert Marshall explained how the city bird song was captured.
He said: "We just stood there and pointed a microphone at them (the great tits).
"We went for medium-sized areas which were close to rural locations so that's why we avoided London.
"We played the urban bird song on a speaker to rural males during the breeding season. Usually this would provoke a strong reaction - the tits get quite worked up about it and would normally attack the speaker.
"But there was a slower and weaker response from the rural males.
"They were less aggressive and not quite sure what to make of it. It was like the city birds were speaking a different language.
"Likewise, we found city birds didn't understand the lower rural pitch."
Project scientists said the urban great tit reacted to man-made noise by raising the pitch of its songs, but in quieter rural locations a few miles away the pitch was found to be lower.
PhD student Ms Mockford said: "The next step is to find out what the females make of these different songs - will they want to mate with a guy who sings too high or too low?"