A long, long, time ago, in a land not that far away from
here, I worked on birds. Bird song, to be exact; learned bird song, to be even
more exact. Why do songbirds sing the way they do? Why is it so consistent in
some species, and why do other species have dialects?
It may be more easy to understand how geographical variation
occurs in learned birdsong, than to understand the absence of it. Mistakes can
occur in the learning process (cultural mutations), the transmission of the
sound is different in some places putting natural selection on the song
frequencies, and perhaps other species interfere with the vocal communication.
These are all good reasons why the song of a species might differ between
places. However, most songbird species –
all of which learn their song, show a high consistency of species specific
song. How do they do that?
Although the particular notes in songs differ between songs,
one individual may even have several songs, and between males within a species
there will be variation; there is a certain overall structure, which makes a
great tit song recognizable from a blue tit’s song or a chaffinch’s song. This
is called the ‘syntax’ of bird song, where each note is a ‘syllable’ in the
analogy with human speech.
One explanation for how birds maintain their species’
typical song, even if the particular notes (syllables) in the song differ over
time and distance is that birds have a filter, or bias, for what kind of song
they learn. If you think about it, this makes some sense, since young birds
learning their song will hear lots of sounds and bird song from other species.
A pre-set bias will give them some guidelines of which songs to learn.
But clearly, there is variation between species in syntax,
so how did this evolve? One would think that, after so many years of biologists
studying bird song (and they’ve been at it for decades – ever since the Bell
Telephone Laboratories made it possible to make sounds visible on paper in the
40’s) there would have been some good handle on this issue. Not so,
unfortunately. One particular technical problem that has bothered progress is
that syntax is hard to quantify. There have been attempts, but this always
ended in semantic debates about definitions, and well, very little progress is
made when that happens.
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| A chaffinch male, picture taken from feedyourbires.co.uk |
Well, there is where this paper comes in. This is a study on
the song of chaffinches. A bird that has been the focus of bird song studies
ever since the beginning (1954).
This is a very common bird throughout Europe (and also in other parts of
the world, having been deliberately exported on a few occasions), and lives on
the mainland of Europe, but also on almost all of the islands in the periphery
of the continent in the Atlantic, such as Britain, the Azores, the Canary
islands). The nice thing is that the colonization route of the chaffinch to
those islands had been figured out already quite a while ago, so that we had a
repeated evolutionary experiment at our hands, not unlike the finches at the
Galapagos. They first colonized the Azores, then went on to the Canary Islands,
with Gran Canaria the last island to get colonized by the finches.
Many, many, many, many, recordings of chaffinches later, on
all those wonderful locations (which I was lucky to be part of on some of those
locations, such as the little gem of an island El Hierro), we created a
database of chaffinch song in Europe.
That was the practical part. Then the hard statistics came
in. I am not going to try to explain this here in detail, but in essence, per
population, the songs were analyzed to see which were the ‘atoms’, or the parts
that always occurred as a unit, such as a syllable (song note) or group of
syllables. Then, zoom out, and repeat this process: which units (classes of
syllables) always occur together. You can see how you can start to quantify
things this way. The more you can cluster units within units within units, the
higher the redundancy in the syntax of a song, i.e. the more stereotypical a
song is.
Mainland Europe chaffinch song is highly stereotypical.
Songs sung by birds in Holland and Spain follow very much the same syntax. This
starts to differ once you get on to the islands, first the Azores, then the
Canary Islands, where this structure starts to fall apart, until at the last
island, Gran Canaria, there is almost no syntactical structure to be found.
Clearly, with every colonization event, syntactical structure in the song
disappeared a little.
Why did this happen? There are a few possible explanations,
and read the paper for those, but one thing that we argue is that populations
that go through bottle necks, such as at each colonization event, there is
strong selection to recognize anything that might possibly be a conspecific.
The learning biases that I mentioned in the beginning were thus selected to
become less restrictive, i.e. less biased. A wider range of songs passed for
species’ specific song. If this happens a enough times, you end up with no
structure in your song. Which is what you find in Gran Canaria.
And now this work has resulted in a wonderful publication, in Current Biology, available online, but in press still:
