Faced with traffic noise and other humanmade din, city birds have taken matters into their own hands: they adjust their songs in real time to make themselves heard. Scientists have long known that bird songs vary in different settings  and that urban songbirds in particular sing at a higher pitch. But they didn't know whether the variety arose through evolution—higher frequencies carried better over low-frequency urban rumble, allowing higher-singing birds to entice mates and breed high-singing offspring—or if the birds were adapting their songs to the surrounding noise in real time. To find out, researchers exposed 21 house finches to a spectrum of ambient noise recorded on a busy Mexico City street. The finches shifted the minimum frequency of their songs higher as their surroundings grew louder and returned to lower frequencies when the noise subsided. The results, which will be published online tomorrow in Biology Letters, show the birds change their songs as needed to beat the noise.