my most important goal is to figure out how many whooping cranes
are in the Aransas-Wood Buffalo flock that winters
on the Texas coast.
Armed with a Cessna 172 single-engine high-wing aircraft and a 74-year-old
pilot who has been flying for over 40 years, we go forth once
a week and try to find
every whooping crane.
cranes in winter as in summer are spread out over a range of about
50 miles. We methodically fly transects
about 5 miles long
by 1⁄4-mile wide until all the crane winter range of about 54,600 acres
is covered. This takes about 8 hours. I plot all whooping cranes sighted
on 9 aerial photos of the crane marshes attached to a clipboard on my lap.
shown that on average, we find about 95% of the flock on any given flight.
Whereas in Wood Buffalo National Park where the only white objects
are the whooping cranes, along with the occasional set of moose
antlers or white tree trunk that can fool you at a distance, we have
thousands of white objects to sort through at Aransas. I sometimes
think that every white pelican, great and snowy egret for miles around
flies to Aransas every week just to get counted. Throw in the occasional
piece of white styrofoam trash washed up into the marsh along with
white refuge boundary signs, our eyes have much to sort through to
find all the whooping cranes. But one thing at Aransas makes it easier
to find whooping cranes than in Wood Buffalo. There are no trees
at Aransas. In fact, Aransas salt marsh is very unique in that there
is nothing out in it taller than a 5-foot whooping crane, the tallest
bird in North America. So while in Wood Buffalo cranes can be hidden
from view on the back side of a clump of spruce, they have no place
to hide from us at Aransas.
The cranes have figured out how to overcome
this handicap and resort to
movements to throw off my count. We can spot a pair of cranes, only
to have them take off and fly up to 3 miles to a prescribed burn
to feed on acorns or to a freshwater pond to get a drink. Sometimes they
move further. For example, on 21 December 2005, a family group seen
by us just after 8 AM was sighted again 3 hours later in a place about
10 miles away. The crane had flown across San Antonio Bay to a completely
different part of the winter range. A color band on the adult male allowed
me to know we’d seen the same family group earlier. But sometimes
we are fooled by such movements. That is why we fly once a week and wait
until we get consistent counts before the final tally for the size of
the flock is announced.
My best estimate is that the peak size of the Aransas-Wood Buffalo population
this winter reached a record 220, surpassing last winter's record count
by 3 cranes. One whooping crane is not at Aransas but is wintering about
30 miles north of Mexico in the Rio Grande Valley of extreme south Texas.
This is presumably the juvenile that in 2004 separated from his parents
in migration and wintered with sandhill cranes about 70 miles north of
Aransas. This whooping crane doesn’t know where Aransas is since
its parents never showed it the fabulous wintering grounds where blue
crabs are abundant and there are lots of shallow water ponds and tidal
flats to roost in to remain safe from bobcats during the night.
the fall and early winter, four whooping cranes that had arrived at Aransas
turned up missing and are presumed dead. Since crane families don't split
up during winter, a loss of an adult or a juvenile in a family is easy
to detect. The losses included three juveniles along with a 28-year-old
male. However, we were unable to find the carcass of any of the birds.
Thus, my current estimate of the size of the flock is 216, one less than
last winter's peak size of 217. In the fall migration, a 28-year old
female died in Saskatchewan with her carcass recovered although the cause
of death was not determined. Whooping cranes can live to be
about 30 years old, so she was an old bird.
Of the 31 chicks that fledged in Wood Buffalo in August, 30 of them
have arrived at Aransas. The high point of the fall migration was
the arrival at Aransas of one family group with two juveniles that had
been seen in Saskatchewan on December 10 and was found at Aransas on
December 16, proof of a very rapid migration trip. The following week,
the very last
juvenile to arrive at Aransas was a real surprise. I did not find
it until December 21, and only one adult was with the juvenile. Perhaps
the other adult had gotten sick or injured during the migration which
delayed their trip south, and unfortunately, one parent apparently did
new juveniles added to the population, the flock should have shown
a substantial increase from last winter. There were
an estimated 215 present in the flock last spring, so mortality
between spring and fall was apparently much higher than average. An
25 whooping cranes, or 12% of the flock, died between spring and
fall. Research has shown that the majority of mortality occurs in migration.
Mortality factors include collisions with power lines, disease
shootings. This higher than average mortality is very disappointing.
Although the nesting pairs did their job this summer with good
production, the flock apparently
has run into other difficulties that we can only speculate
Buffalo whooping crane has made a remarkable comeback
from only 15 birds in 1941, but one of the aspects that makes
my job so fascinating is that no one knows how this comeback story
to end. Only with continuing efforts to protect the species and
provide the habitat that it needs can we expect the species to survive
Stehn, U.S. Fish and Wildlife Service
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