Date: Wed, 11 Sep 2002 23:24:15 -0500
Reply-To: BB for Hummingbirds and Gardening for them in the Southeast <HUMNET-L@LISTSERV.LSU.EDU>
From: BB for Hummingbirds and Gardening for them in the Southeast <HUMNET-L@LISTSERV.LSU.EDU>
Subject: Re: [HUMNET-L] There are hummers on Pelee Island.

Bob/Humnet:

As to the evolutionary history of birds crossing the Gulf of Mexico, I
agree with much of your analysis, but I might remind you that you are only
telling part of the story. It is true that at the height of the last Ice
Age the gulf was much smaller (if one wants to see its dimensions, you
need only look at a bathymetry map and the outline of the 300 foot depth
contour), but it is also true that within a thousand years after the end
of the last Ice Age, the Gulf was much larger in area than today. Indeed,
for birds heading due north from the Yucatan 8-9,000 years ago, the first
land encountered might not have been until they reached Missouri or
Illinois, if their northward path were narrow enough. After the Pleistocene
ice sheets melted, sea level rose rapidly to about its present stand. But
where today the Mississippi River delta stretches deep into the gulf
toward the Yucatan, there was then no land, and the Mississippi discharged
into a deep embayment near present-day Cairo, Illinois. This is because
during the Pleistocene, the river cut a deep valley well below present-day
sea level. It took thousands of years for the Mississippi to fill that
valley and build the present deltas. The valley and embayment were quite
narrow at the northern apex, but in southeastern Louisiana the coast
was just south of where Olga now lives, ninety miles north of Grand Isle,
and at that latitude the embayment covered all of the land between Baton
Rouge and Lafayette, where now the Atchafalaya Basin's vast forests occur.

Natural selection might then have favoured birds that avoided a direct
northward crossing in spring, leading to populations that tended east or
west. Who knows?

In any case, Ruby-throats have presumably been around for a good deal
longer than 10,000 years, and have survived the waxing and waning of the
Gulf of Mexico (not to mention the waxing and waning of the eastern
forests where they breed).

This leads me to one other observation. We should probably all be careful
about making sweeping observations about the behavior of trans-gulf
migrants. If your observations of night-time arrival at Ft. Morgan don't
jive with my observations in Louisiana, it may be because different
distances and trajectories are involved. Ft. Morgan is a good bit farther
from any point in Mexico than is Grand Isle or Venice on the Louisiana
coast. It may just take birds longer to get to you. Or conversely, your
night-time arrivals may be from birds jumping off from some point in
Penninsular Florida, the Keys, or even Cuba cutting off the curve, rather
than making a longer overwater flight from Mexico. Again, who knows?

David Muth
New Orleans

[David Muth]

 -----Original Message-----
From: BB for Hummingbirds and Gardening for them in the Southeast
[mailto:HUMNET-L@LISTSERV.LSU.EDU]On Behalf Of RubyThroat@AOL.COM
Sent: Wednesday, September 11, 2002 5:24 AM
To: HUMNET-L@LISTSERV.LSU.EDU
Subject: Re: [HUMNET-L] There are hummers on Pelee Island.

Ron and all:
It might be noted that neotropical migrants, including hummingbirds, make
most of their overwater journey at night and PROBABLY at some higher
altitude than ground level flights.  Regardless of what has been published
over the years, by own experience operating a bird banding station on the
northern Gulf Coast shows ALMOST ALL INCOMING BIRDS FROM THE TROPICS
ARRIVE IN THE DARK OF NIGHT, and not in the afternoon as is so often
suggested.  Example, we close our nets at dusk and have no birds in the
woods.  We open our nets before dawn and the woods are fully of newly
arrived, skinny migrants.  Not as scientific as radar predictions and
speculations, but it is a fact.

Exception to the above almost always occur when northbound migrants will
have encountered opposing winds on their night flights.  Head winds can
change the dynamics of arrival times.  In those cases, many thousands of
birds, including masses of hummingbirds, will "drop in" on the coast
almost anytime of DAY.  The term in the birding community is FALLOUT.

In my opinion, it is a leap of faith for migrants to cross open water.
There is great wisdom in night flights, and those that attempt to do
otherwise are probably subjected to much more danger from predation.  It
seems to me that these birds that pack up all their genetic materials, the
very essence of their species, and then launch themselves over open water
must have some innate sense of what they face.  The fatting process alone
appears to indicate this high degree of preparation for such a journey.

As for the Gulf of Mexico, remember that only 10,000 years ago it was a
much smaller body of water and not nearly so wide as now.  Apparently
humans in the Americas lived and foraged in places that are now nearly 300
feet under water.  I suspect that as the Gulf widened and deepened,
successful migrants were those that had the most fat.   This "set of
instructions" would surely have been passed on to their offspring.  Those
birds flying with the old "narrow Gulf instructions" would have run out of
gas and been eliminated from the gene pool of breeding birds.  Perhaps
this is oversimplified, but it probably is pretty close.

I see little in the way of a deterrent for hummers to cross open water, if
they are programmed (and prepared) to do so.  A Ruby-throated hummingbird
can easily put on a single gram of fat in one day!  In addition to that, a
bird about to cross an open body of water could also add a gut full of
high energy food prior to leaving.  These little guys have been doing this
for some great long time and are pretty clever in their preparations.
Mother Nature deals harshly with those that do it incorrectly.

Bob Sargent
Trussvillle, Alabama

==================================

Date: Thu, 19 Sep 2002 06:30:50 -0400
Reply-To: BB for Hummingbirds and Gardening for them in the Southeast <HUMNET-L@LISTSERV.LSU.EDU>
From: BB for Hummingbirds and Gardening for them in the Southeast <HUMNET-L@LISTSERV.LSU.EDU>
Subject: Re: [HUMNET-L] Migration question: trans. vs. circum-Gulf

In a message dated 9/18/02 12:39:09 PM Pacific Daylight Time,
ddemchec@USGS.GOV writes:

Isn't trans-Gulf migration for RTHU a given?  I know the folks in Rockport
TX get a lot of hummers, especially in the fall.  I have seen many, many
hummers along the La. coast in the spring.  Is it mostly trans-Gulf in the
spring, and mostly circum-Gulf in the fall?

Dennis Demcheck
Baton Rouge

Cat-Man and Humnetters
This one is another of those scenarios that needs a LOT MORE STUDY.  My
personal view is that migration is both transgulf and circumgulf for
Ruby-throated.  I think that the vast majority of spring arrivals are
transgulf, but again that is strictly my opinion.  In the fall, who knows
for sure?

Along the Alabama Gulf Coast I see little evidence that would support some
sort of arc flights or overland travels in the springtime.  The folks along
the Florida coast, all the way down to Tampa get big influxes of
Ruby-throated in a similiar fashion as they do with passerines.  This is
the same as with our masses of birds that arrive along coastal Alabama.

In the fall, I SUSPECT that most of our Alabama migrants depart the United
States in mixed flocks with other neotropical migrants.  Viewing doppler
radar scans in early evening in the fall indicates that the flocks do
apparently leave from well inland and do in fact fly out over the Gulf of
Mexico.  Unfortunately, to my knowledge, we don't have any means of
observing those flocks to determine if they indeed head into the Yucatan
Peninsula area.

Prodded on by ignorance.
Bob Sargent

Footnote:
As much as I love Rockport and its many hummers, the numbers that are seen
there during the Festival are a only a tiny, tiny, tiny fraction of the
totals that are migrating out of the United States daily.  I find it
difficult to equate the Rockport phenomena with the true picture of
Ruby-throated migration.