Mom asks Daniel if he thinks city birds sound different from birds in the countryside.

He says a bird is a bird. They all make the same sounds.

He is wrong -- and the science behind why is one of the most quietly remarkable things happening in cities right now.

Researchers studying great tits -- a small songbird found across Europe -- compared their songs in ten major cities including London, Paris, Prague, and Amsterdam with songs from birds of the same species in nearby forests. In each of the ten cities, the urban birds sang at measurably higher minimum frequencies than their forest counterparts. The louder the urban noise, the higher the birds tended to sing.

The reason is the city itself. Much of the background noise in urban environments -- especially traffic -- is concentrated at lower frequencies. Birds that sing a ...