This makes sense. Sabri Ben-Achour for NPR reports that birds sing different tunes in cities than they do in the country. The buildings, noise and other factors in city life make it hard for birds to hear each other unless they sing at different pitches than the surrounding noise. This doesn't seem like such a big deal until you read the part where this may very well affect the species as a whole.
So if birds from the city can't flirt with birds from the country anymore, "those birds are actually going to be less likely to mate with each other," he says. "I mean, literally they're going to stop being able to speak the same language."
I hope that doesn't happen. Keep tweeting birds!
/via Jason Fried on Twitter.