SA: One of the ways you were able to prove that song knowledge is innate¿rather than learned¿for certain species was by depriving young Phoebes of their ability to hear.
DK: We¿d done a bunch of experiments, but we knew that the final step before we could declare that they do not learn was to prevent them from hearing themselves practice. So we removed the cochlea from the ears of a few Eastern Phoebes [Sayornis phoebe], and they still perfectly produced these beautiful songs. They should not have been able to develop normal songs after being deafened if there was any learning component at all.
SA: You¿ve compared the Three-wattled Bellbird of Costa Rica to the Humpback whale [Megaptera novaeangliae] because their songs evolve rapidly within each generation. How have the bellbirds¿ songs changed since people started recording them?
DK: We have a series of recordings going back to the mid-1970s, giving us nice documentation of their songs in three dialects. In two of the dialects, the songs of the Seventies are drastically different from the songs today. In the third dialect, the one we¿re working with more carefully, we can plot many of the microchanges over time.
One change is a very loud whistle that has been declining in frequency [pitch] since the 1970s. The frequency has gone from about 5,500 Hz, or cycles per second, down to about 3,700 Hz. That¿s an enormous drop, and it¿s an average drop of 70 Hz per year from the 1970s to 2001.
SA: Is the Bellbird unique among birds in that its songs evolve this way?
DK: These birds are relearning their songs probably all the time as they monitor what other birds are singing. This kind of change has been found in only a couple of other birds, including the Yellow-rumped Cacique [Cacicus cela] in Panama. It¿s a blackbird and it lives in colonies. The songs within colonies change within a generation.
In birds that have fairly short lives, such as Indigo Buntings [Passerina cyanea], which live a couple of years, once the male develops his song he sticks with it throughout life. Each individual male is not constantly relearning his songs over time.
SA: Why do you think the Bellbirds¿ songs change?
DK: Probably like in most lekking systems, relatively few males are successful. The males display [before an audience of females], and many of the females agree as to which is the best male. The females are probably in charge of this system that enables males to show how long they¿ve been around, whether they¿re singing the songs of the local dialects and keeping up with the changes. So the successful males could be changing their songs, forcing the other males, especially the younger birds, to keep up with them. It may be a way for the females to be able to identify the dominant males or the ones who have been in the population the longest.
SA: And one of the ways you were able to prove that Bellbirds learn their songs is that you found they could mimic other birds.
DK: A friend told me about a city called Arapongas in Brazil. If you say "Arapongas" and emphasize the "pong," more or less they¿re describing the song of the Bare-throated bellbird that lives in southern Brazil. The town is named after the bird.
People keep Bellbirds in cages in this town. My friend heard a caged bellbird making sounds like a Chopi blackbird [Gnorimopsar chopi] there. He found out that it had been raised with Chopi blackbirds and had learned elements¿the whistles and the purr¿of their songs. This was a nice experiment done by bird fanciers, and we were able to obtain what I see as unequivocal proof that this one bellbird learned its sounds from the blackbirds.
SA: Why do you find the Bellbird project so compelling?
DK: It¿s difficult to think objectively once you see these birds because they are so charismatic. They hop around on their perches, they square off, they push each other off perches, they scream in each other¿s ears, they stick their heads into other birds' mouths. They¿re just extraordinary.