There is a pretty good article on the recent research reported in the Royal=
Society's journal here: http://tinyurl.com/2f98ho
The data cited in the article matches my own measurements - a
60-millisecond "beep" at 3400 (or so) Hz. Also the compelling argument to
me is that birds which have had the outer retrices removed cannot make the=
sound, though they continue to dive.
My sonograms of 6 different examples of the dive sound all show two
distinct frequencies being produced, not at multiples of each other, so
they are not harmonics. This says to me that the sound is not vocal, too.
Also I have never heard or seen an Anna's make this sound while perched and=
I don't know anyone who has.
So far I'm staying with the theory that the sound is from tail feathers,
remarkable though it is.
Doug
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Doug Von Gausig
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