SSS 2009-10-22（在线收听）

Did you know birds sing in dialect? They do. The song of a great tit from the countryside is a far cry from that of his city cousin. And some song dialects can change nearly as fast as human slang—the Indigo Bunting changes tune from year to year.

To investigate the cultural evolution of such songs researchers have recently completed a study of adjacent White-crowned sparrow dialects from 1969 to 1998 in San Francisco. Biologist David Luther of the University of Maryland and ornithologist Luis Baptista of the California Academy of Sciences hypothesized that the pressures of urban noise would tend, over time, to eliminate the lower ranges of the bird's song and cause the sparrows to prefer to learn songs at the higher range. Simply put, birds that sang too low would be drowned out by rumbling buses, honking cars, or other typical city noises.

And that's exactly what they found.

The lowest frequencies of bird song in the Summer of Love were lower than those to be found during the Dotcom craze. And since sparrows are relatively short-lived—average lifespan of just two years—this effect spanned generations. Much like humans, the sparrows seem to be raising their voices to be heard over the sounds of the city.

At least one birdsong dialect died out entirely, though there were some bilingual birds, and the so-called San Francisco dialect came to dominate all songs, likely because it was tuned higher. 

It seems that the need for effective communication in the local environment is the fundamental driver of cultural traits that are passed on from one generation to the next. Ya hear that?!