Lately I’ve been thinking a lot about stereotype. Not the racial or ethnic kind, but the degree to which a bird’s song is the same each time it’s sung. In a couple of recent blog posts, I showed that Violet-green Swallows and Evening Grosbeaks produce stereotyped vocalizations in complex strings, and therefore I argued that they were exhibiting true singing behavior. Shortly afterwards, Andrew Spencer asked me an intriguing question: why does a vocalization have to be stereotyped in order to qualify as a song? Are there any birds that sing non-stereotyped songs?
I can’t find an answer in the scientific literature. Among researchers, the definition of bird “song” (as opposed to “call”) has been pretty controversial over the years, but the question of stereotypy isn’t the controversial part. Most authors simply take for granted that bird songs should be stereotyped — or “crystallized,” in the terminology often used for learned songs.
And perhaps for good reason: I haven’t been able to find any clear-cut examples of unstereotyped courtship song. Sure, there are the “subsongs” of young, inexperienced males, and birds may sing variable songs at different times of year — but it seems that when an adult male really needs to impress a female, in almost any species, stereotype becomes important.
This may be because females, at least in some species, prefer stereotyped songs. In the Zebra Finch, for example, the degree of stereotypy varies depending on what the male is doing. When he’s by himself, each song is different from the last, albeit in very subtle and minor ways. But when a female shows up nearby and the male’s got courtship on his mind, he begins to sing directly to her, and those subtle differences go away: each song now becomes a precise copy of the last. Woolley and Doupe (2008) showed that females prefer this stereotyped courtship song over the more variable undirected kind — meaning that females are listening not just to how males produce their songs, but how they reproduce them.
Is stereotypy easier or harder?
Which demands more of the singer—stereotyped or non-stereotyped vocalizations? My guess would be the former; it seems intuitive that having to produce a sound multiple times in precisely the same way, without any tolerance for variation, should be the more difficult task. But when it comes to demands on the listener, things might be the other way around. Recognizing a sound is likely to be easier when the sound only takes one form. If the singer is trying to send the same message in a slightly different way each time, it actually requires the listener to recognize a class of similar sounds rather than a single sound – presumably a more difficult cognitive challenge.
Let’s consider an example from human speech. A common word like “birthday” is not going to sound exactly the same every time you say it. Sometimes you might say it quickly, sometimes slowly; sometimes while laughing or when your nose is stuffed up; sometimes with your voice rising toward the end of a question. Sometimes you might even sing it. In all these cases, the spectrogram of your voice saying “birthday” is going to look different—and yet, every single time, our brains reliably classify these sounds as renditions of the the same word. That takes some mental firepower—it’s difficult for computers to do, which is why speech transcription software remains imperfect.
Perhaps it’s no coincidence that the best examples I could find of birds that rarely give stereotyped vocalizations are also the most intelligent birds: parrots and ravens. Nothing that these birds say in the wild meets the classic definitions of bird “song,” but they are apparently lifelong vocal learners — ravens, like parrots, can be taught to speak in captivity — and they apparently recognize other individuals of their species by voice, even though no two utterances are likely to be precisely the same. Compare these calls from a Lilac-crowned Parrot in Mexico:
Here’s an unedited sample of consecutive calls from the same bird, showing how different types of calls grade into one another:
Even these parrots have the ability, and apparently the need, to reproduce almost exactly the same call over and over, even if they don’t do it with quite the same precision as most other birds. Is this lack of perfectionism a sign of intelligence? Or merely a sign that they’re not in an amorous mood? Are there any species in which the females are attracted to vocal innovators instead of virtuoso self-imitators?
At the moment, nobody really knows.