Using a computer, the biologists generated two cardinal songs -- identical except for normal and high frequencies -- to match what birds would hear in the wild. By hooking up an iPod to a speaker, the biologists then played their synthesized tunes to 26 cardinals.
The birds were less engaged in their responses to the higher-pitched warbles -- they sung back fewer times and did not fly past the speaker as frequently. Any benefit to being heard over rush hour traffic and other noise, the authors write, "might be lost" by these lower response rates. Not even the birds, it seems, like hearing one another shout over honking horns.