THE quarry is in sight. With water sloshing around his boots and sharp rushes
scratching his wrists, Don Kroodsma parts the vegetation and holds his
microphone aloft. He has been out on the marsh since dawn and needs just one
more recording to fill up the tape.
A little brown bird hops onto the tip of a bulrush. Cocking his tail over his
head he launches into a serenade, desperately trying to impress the female he
has spied. Kroodsma hits the record button as the air fills with a sound
reminiscent of an old-fashioned sewing machine鈥攖he harsh, tinny
chatterings of the North American marsh wren.
The bird stops and then, from across the water, another male replies in
song鈥攐r rather, repeats the original tune. Syllable for syllable, the
second bird copies the entire song sequence. Unperturbed, the first bird
launches into a different song, and once again it is repeated from across the
marsh as the second singer competes with the first. This is blatant
plagiarism鈥攁nd Kroodsma loves it. 鈥淭here鈥檚 great gamesmanship going on
here,鈥� he whispers. 鈥淲hen two males `countersing鈥� like this it鈥檚 quite a
jaw-dropping experience.鈥�
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Kroodsma has been trying to understand such behaviour for three decades and
his muddy boots and miles of tape recordings testify to his enthusiasm. 鈥淚鈥檓
fascinated by why some birds copy songs and some birds do other things like
improvise or make songs up,鈥� he says. 鈥淢y goal is to find ecological correlates
of these different bird behaviours.鈥� In other words, Kroodsma believes that the
lifestyle and habitat of birds explains why some plagiarise while others
extemporise.
Back in his lab at the University of Massachusetts, Kroodsma鈥檚 recordings
come under close scrutiny. This latest soundtrack carries clear evidence that
male marsh wrens are exemplary imitators. Kroodsma remarks that his field
results confirm previous lab research. 鈥淚f, in a laboratory, I take a baby marsh
wren and expose him to just five songs he鈥檒l learn those five and nothing else,鈥�
he explains. 鈥淏ut if I give him 50 songs he鈥檒l do a good job at learning all 50
songs. It鈥檚 like his brain is a sponge. He soaks up whatever you present to him
until you reach his limit.鈥�
This couldn鈥檛 be farther from the behaviour of the marsh wren鈥檚 close cousin,
the sedge wren. Tape recordings from the wild show that each male sedge wren has
a unique song pattern. There is no countersinging and it looks as though this
bird stubbornly refuses to breach copyright and steal the songs of his
neighbours.
But is he really so scrupulous? It鈥檚 possible that young sedge wrens imitate
the songs they hear around them while still in the neighbourhood of the nest,
but they then move away from their nesting sites. If this were the case then the
copied songs would simply become dispersed. It is as if a Scotsman and an
Englishman moved onto the same street. Their accents would be very different
from each other鈥檚, but similar to the accents heard in the town where they grew
up.
To test this possibility, Kroodsma collected 20 male sedge wren nestlings
from North Dakota and brought them back to his lab. He hand-reared them and
during their first few months of life exposed them to a series of taped sedge
wren songs, just as he had done for marsh wrens. 鈥淲e also kept the birds all
together in a room because they might possibly learn better from each other than
from a loudspeaker,鈥� he says. 鈥淲e gave them as much opportunity as possible to
濒别补谤苍.鈥�
The following spring, the young birds began to sing and Kroodsma recorded
them. When he analysed the songs the evidence was conclusive: the birds鈥� mature
songs revealed no sign that they had imitated the training tapes nor each other.
They were improvisers extraordinaire, with each individual composing his own
repertoire.
Why should two very closely related birds, with similar-sounding songs and
mating behaviour, differ so radically when it comes to their musical
inventiveness? What evolutionary forces have brought about this divergence in
development? Kroodsma鈥檚 hunch was that the answer lay in their lifestyles and
habitats.
Marsh wrens live in stable habitats such as bogs and swamps. Although some
populations migrate south in the autumn, they tend to return each year to the
same haunts. On the Atlantic coast, in the Gulf of Mexico and along the more
temperate reaches of the Pacific coast, the populations are sedentary, remaining
at the same location all year round.
Sedge wrens, meanwhile, lead a more nomadic existence. They migrate south for
the winter, returning in the spring to their nesting sites in the wet grasslands
of central North America. This environment is highly dependent on rainfall. If
rainfall is low, grasslands may dry up during the spring and summer, giving the
wrens no choice but to up-nest and move on. So sedge wrens are opportunistic,
perhaps grabbing the first suitable-looking spot they find during their spring
migration back up north, and rarely returning to the same site.
Kroodsma suggests that it is this difference in habitat that has led to the
two musical styles. The marsh wrens know their neighbours, and by copying the
songs around them they learn to speak a local dialect. 鈥淭hey live in the same
community till death do them part. I think that makes a big difference in what
kind of communication system they use,鈥� he says.
Indeed, the idea that some birds have local dialects is well established.
Much of the pioneering work was done by Peter Marler when he was at the
University of California, Berkeley, and later by Myron Baker when he and Marler
were together at Rockefeller University in New York. Over two decades they have
investigated the songs of resident populations of white crowned sparrows at
Point Reyes National Seashore, a national park just north of San Francisco. They
found a marked pattern of dialect differences, with a change in dialect every
mile or two.
鈥淭here is no doubt about the existence of dialects in many species of bird,
though they arise by different mechanisms,鈥� says Marler. 鈥淲hat we don鈥檛 know is
why birds use a dialect system.鈥� He adds that some scientists argue that
dialects help to split populations into local groups which have the potential to
respond somewhat independently to changing circumstances such as food supply or
climate changes. This might be an adaptation to an unpredictable environment,
increasing the chances that some birds will always survive and continue the
species.
Another possible effect of dialects was demonstrated by Baker. He found that
a female white crowned sparrow from one community, with its own dialect, always
responded better to a male from her own locality than to one from a distant
group. In other words, the girls go for the local lad who knows the lingo rather
than the foreigner who sings an unfamiliar song.
Kroodsma points out that local dialects might not have the same advantages in
itinerant species. The poor male might be in trouble if he only sang the dialect
of the place where he was born. If the following year he ended up in another
part of the country where the females did not understand him, he might have to
resign himself to bachelorhood. So, he argues, birds such as the sedge wren have
evolved to be wandering minstrels, relying on innovation rather than imitation.
And because all males improvise according to the same general rules, their
language is universal and recognised by females everywhere.
Now Kroodsma has further proof that his theories are not far off the mark.
鈥淭o test my hypothesis, ideally what you鈥檇 want to do is take some sedge wrens
and provide for them in one location with everything they could possibly ever
need,鈥� he says. 鈥淲hat you鈥檙e doing is taking a sedge wren and providing
ecological conditions for it that are more like those where the marsh wren
lives.鈥� You then run the experiment for thousands of generations, with the sedge
wrens living and dying in the same location. The question is, would they start
to behave like marsh wrens?
Of course, this sort of experiment is impossible. But there is a population
of sedge wrens in Central America that lives in a stable habitat. To Kroodsma鈥檚
delight, these turn out to have musical tastes that are quite different from
those of their close cousins the North American sedge wrens. 鈥淐entral American
sedge wrens do have song dialects and they do countersing with each other. They
imitate songs, more like the North American marsh wrens,鈥� he says. 鈥淪o here is
much stronger evidence of the idea that ecology has influenced whether birds
imitate or improvise.鈥�
If Kroodsma is right, his ideas should extend to other species. 鈥淚t鈥檚 pretty
well documented that where birds remain resident鈥攚here they know each
other鈥攖hen there tends to be a lot of song sharing,鈥� he says. 鈥淚 think the
stability of the community has had a tremendous effect on a large number of
species. There鈥檚 been strong selection for imitating precisely.鈥�
From his own work, Baker has noticed a correlation between population
stability鈥攚hether the same community of individuals stays
together鈥攁nd dialect formation. And at Point Reyes, he notes, migratory
populations of white crowned sparrows are less likely to have distinctive song
dialects than resident birds. 鈥淚f a species has evolved a migratory
habit鈥攆or whatever reason鈥攎aybe they have to give up the development
of dialects because over long migratory distances there鈥檚 no guarantee where
you鈥檙e going to end up in the summer,鈥� says Baker.
Kroodsma believes that there must be ecological explanations for the
development of song types in all songbirds. But this is a field of research
where general patterns are hard to find. Take the mockingbird, for example. Why
has it developed such an amazing gift for imitation that it even copies the
songs of other species? Marler believes that here imitation allows the male to
increase his repertoire so that he can attract and stimulate females, who are
impressed by variety and originality. 鈥淢aybe the mockingbird is also having
fun,鈥� says Kroodsma. 鈥淚f humans no longer have a monopoly on the idea of
language or dialect, why should they have a monopoly on something we label as
蹿耻苍?鈥�
Marler agrees that some birds may sing just for pleasure. 鈥淚f we make music
for fun then surely birds do the same,鈥� he argues. But the idea is contentious,
and probably depends on your definition of 鈥渇un鈥�. After all, some people think
it鈥檚 fun to wade through muddy marshes at dawn just to record the tinny rasps of
a little brown bird.