[TN-Bird] Re: Whigg Meadow - Swainson's Thrushes and N. Saw-whet Owl update
- From: kde@xxxxxxxxxxxxxxxxxx
- To: Scott Somershoe <Scott.Somershoe@xxxxxx>
- Date: Wed, 14 Sep 2011 09:40:57 -0400 (EDT)
Hey, Scott. That sounds similar to my experiences along our ridge
in west Knox Co the last few nights. Lots of Swainson's Thrushes
have been passing through. I've had a few in the woods during the
day but most seem to keep going. Also the NW movement of birds
you saw during the morning goes along with what I experienced on
Monday. Below is a recent post by Harry LeGrand (Ed's brother)
on the CarolinaBirds listserv discussing this phenomena.
Dean Edwards
Knoxville, TN
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The movement of birds you saw pouring over the gap during the morning is
what Dr. Sidney Gauthreaux at Clemson U. studied and published a paper on
(I helped him gather some of the data):
Gauthreaux, S.A., Jr. 1978. Importance of the daytime flights of nocturnal
migrants: redetermined migration following displacement, pp. 219-227; in
K. Schmidt-Koenig and W.T. Keeton, eds. Animal Migration, Navigation, and
Homing ...
He calls it "redetermined migration" -- birds flying in the morning in a
roughly perpendicular direction to their main flight axis (during
nighttime migration), in which they are presumably blown eastward of their
main migration route. In general, birds in the fall (on calm nights) fly
at Clemson to an average direction of SW (about 232 degrees). So, most
redetermined flights the next morning are back to the NW -- on average. If
you are on the Blue Ridge Parkway, in MOST places (where the BRP is
oriented NE-SW) birds do come upslope from the SE, and fly past you to the
NW. I've seen that many times. Ridge Junction Overlook is oriented in a
different direction -- on the NW-SE part of the BRP, so that complicates
directions. What direction were they crossing the BRP -- I'm guessing from
the map -- E to W (or NE to SW) (from Yancey into Buncombe).
Again, this is a correction for displacement -- and not a true migration.
These birds migrate at night.
Harry LeGrand
Raleigh
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