[texbirds] Re: Some insights into extreme winds affects on trans-Gulf migration
- From: Clay Taylor <Clay.Taylor@xxxxxxxxxxxxxxxxx>
- To: "jarvin@xxxxxxxx" <jarvin@xxxxxxxx>, New Texbirds <texbirds@xxxxxxxxxxxxx>, LABIRD-L <LABIRD-L@xxxxxxxxxxxxxxxx>
- Date: Thu, 11 Apr 2013 00:04:13 -0400
Hi John -
VERY interesting. It sounds like the migrants have a built-in "governor" that
monitors actual ground speed and in the case of a tailwind kicks into
fuel-saving mode to arrive with more energy, or perhaps to keep pace with the
emerging leaves, insects., etc. - go too far and too fast on consecutive days
and they might encounter wintry conditions with no food sources at the same
time as being drained of energy. Amazing stuff....
Clay Taylor
TOS Life Member
Calallen (Corpus Christi), TX
Clay.taylor@xxxxxxxxxxxxxxxxx
-----Original Message-----
From: texbirds-bounce@xxxxxxxxxxxxx [mailto:texbirds-bounce@xxxxxxxxxxxxx] On
Behalf Of John Arvin
Sent: Wednesday, April 10, 2013 7:55 PM
To: New Texbirds; LABIRD-L
Subject: [texbirds] Some insights into extreme winds affects on trans-Gulf
migration
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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