[texbirds] Some insights into extreme winds affects on trans-Gulf migration
- From: "John Arvin" <jarvin@xxxxxxxx>
- To: "New Texbirds" <texbirds@xxxxxxxxxxxxx>, "LABIRD-L" <LABIRD-L@xxxxxxxxxxxxxxxx>
- Date: Wed, 10 Apr 2013 18:55:05 -0500
This is a message I got from a buddy, Ken Able, one of the foremost
migration researchers. He is responding to conversations we were having
about the possible effects of extremely high tail winds on trans-Gulf
migrants. I hope the formatting holds together.
Hi, John:
I am not registered to post on either TX or LA bird lists, where I see some
discussion taking place about strong winds and trans-Gulf migration. Most
small passerines fly at air speeds of 20-30 mph (~28 kts). There is
evidence from tracking radar studies going way back showing that birds
actively adjust air speed depending upon the wind speed in which they are
flying. Exactly how they judge this remains an open question, but if birds
are flying in situations in which they have visual contact with fixed or
nearly fixed objects on the ground, one can at least imagine them being
able to do it. Regardless of how, the evidence is quite clear that they do
so. So if flying into a head wind, air speed is increased, in a tail wind
it is decreased, with obvious energy-saving advantage. The slopes of the
air speed-headwind component relationship are generally in the range of
-0.3 to -0.5, so we are not talking about doubling or halving air speeds in
response to realistic wind speeds.
Given this morning's conditions, you could have birds achieving ground
speeds close to 90 mph, even if they adjusted their air speed downward. So
long as the wind flow is laminar, I doubt that there would be a problem, as
Clay Taylor says. The birds are moving with the air mass at a normal air
speed, maybe even a somewhat slower than normal air speed. It is only the
ground speed that is ridiculous and as long as there is no small-scale
turbulence, it should not be a problem I would think. Turbulence is
another matter entirely and could seriously screw things up.
So these guys are getting a nearly free ride across the Gulf, but there is
danger ahead in a couple of forms. If they actually run into the front
with all the associated nastiness, they could be in danger. If over land,
most birds no doubt put down before they get into trouble. There is the
real risk of being carried too far northward on the strong air flow ahead
of the front, however. Folks at more northerly latitudes have seen these
dramatic overshoot events many times over the years and they can result is
significant mortality when a bunch of tired and hungry insectivores wind up
in a blizzard with sub-freezing temperatures for several days. Numerous
examples have been chronicled in the pages of NAB (and its many
predecessors) over the years and in some of the worst events, measurable
effects on regional breeding populations were documented. Ken
John C. Arvin
Research Associate
Gulf Coast Bird Observatory
103 West Hwy 332
Lake Jackson, TX 77566
jarvin@xxxxxxxx
www.gcbo.org
Austin, Texas
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