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Name: Curtiss
Status: other
Age: other
Location: CT
Country: N/A
Date: 7/19/2005


Question:
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?


Replies:
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.

J. Elliott

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 rare.

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 such mimicry.

Paul Bridges


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