Ted Floyd wrote:
Ah. Colby has brought up the strange case of the missing Swainson's
Thrushes:
Hello everyone,
I guess this partially answers my unasked question about whether
Swainson's and other species migrate at some altitude not picked up by
the equipment typically used. But I'm still curious about the effects
of the altitude at which a species migrates on detection rates. How
high can the typical software "hear?" Related to that, what influence
does topography have on detection. I live in a "hollow" between hills
that are several hundred feet higher to the north and to the south. As
birds migrate through my area, they need to be high enough to be over
those hills, so how unlikely am I to detect the vast majority of birds
migrating over my site because I am so low?
Just curious.
Jody Enck
Ithaca, NY
> Interestingly, ZERO Swainson's Thrushes were heard. Interestingly,
in all my nights
> listening (a couple dozen) in Utah, I never heard a Hermit or
Swainson's Thrush in either
> the spring or fall. I don't know what these birds do between the
Cascades/Sierras and
> the Continental Divide given my experiences thus far as I'm certainly
quite baffled at this
> point...
It's interesting, isn't it? In their recent monograph on the birds
of western Colorado, Bob Righter et al. state the following for
Swainson's Thrush: "Even during spring and fall migration, rarely
found outside of breeding habitat, suggesting that most birds migrate
through the mountains and mountain valleys." Coen Dexter, one of the
coauthors of that monograph, put it even more dramatically to me
(personal communication in front of 100+ folks when I was doing Q&A at
a public talk; insert frowney-face here): There do not appear to be
any credible records in western Colorado away from mountains and
mountain valleys. Conversely, if you go east of the Divide, you can
see 100+ per day on spring migration in Colorado.
Anecdotally, that was my impression back in my Nevada days. Swainson's
Thrush is common as dirt in the broadleaf forests of, say, the Ruby
Mountains (northeastern Nevada). But it was notable enough in the
lowlands for us to put it on the hotline. And those lowland records
tended to be from the far west (Reno area) and south (Las Vegas area).
I wonder if they were mainly Russet-backed (Pacific slope) birds, not
Olive-backed (everywhere else) birds.
Best, --Ted Floyd
Lafayette, Boulder County, Colorado
--
Jody W. Enck, PhD
Human Dimensions Research Unit
Department of Natural Resources
119 Fernow Hall, Cornell University
Ithaca, NY 14853 607-255-8192
www.dnr.cornell.edu/hdru/ <http://www.dnr.cornell.edu/hdru/>
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