Bird Mimic Evolution
I have a family of catbirds in my yard and have been
listening for weeks to one in particular deliver its incredible soliloquy
from morning til night every day.
I know catbirds, like mockingbirds, are mimics. How did this trait evolve
in these thrushes? What possible advantage is there for a bird to mimic
other birds and even environmental sounds?
This question has been intriguing and puzzling birders and ornithologists
for generations. I don't think anyone has satisfactorily answered yet. A
recently published book on bird song, by Kroodsma, The Singing Life of Birds
: The Art and Science of Listening to Birdsong has probably more detail than
the average person wants to know. I have not read it all yet, but I don't
think the mimic question has been answered by this author, who has been
researching bird song for 30 years. He does show that bird song is far more
complex than anyone would expect.
Up-dated July 2008
As you know, bird song is primarily a way to (1) attract mates and (2) defend
territory against *same* species, of course. But there are reasons to mimic
other species. As described in "Bird Song - Biological themes and variations"
(Catchpole, 2003), it's proposed that mimicry of *other* species can discourage
those species from taking up precious nesting sites and food sources, either by
creating the illusion of a competing male of the other species, or the illusion
of a predator (Harcus 1977, and Rechten 1978). In some unusual cases, it's been
suspected to simply be a copying error (Hndmarsh 1986), but this appears to be
Another book (which I cannot locate at the moment) reminds us that from the female's
perspective, elaborate bird song is in general a sign of "fitness". Mimicry may have
had other origins, but ultimately been reinforced by sexual selection. This would be
similar to the way in which humans have high regard for those who speak several
foreign languages; it's a sign of intelligence. In the Handbook of Bird Biology
(Cornell, 2004), they describe a Marsh Warbler that (presumably) learns songs in
Africa during the winter to impress females in Europe during breeding season, since
there would be few African birds to discourage from European nest sites. This would
also probably explain why certain birds, such as the Lyre Bird, will readily mimic
the sound of chainsaws and cameras, which are *probably* not perceived as direct
threats to nesting sites and food.
We should also note that there are a good number of "casual" mimics that are perhaps
not as well known, such as certain Jays mimicking hawks (usually Red-Tailed), in
addition to the "classic" mimics such as Thrashers (e.g., Mockingbird), etc.
Mimicry seems surprising considering that studies show that many songbirds have
"filters" in their brains that filter out patterns (if not syllables, too) of other
nearby species (Marler & Peters 1977, 1988a), which would in normal cases prevent
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Update: June 2012