Journey North Whooping Crane Spring 2006

Meet Tom Stehn

Feb. 24, 2006

Dear Journey North,

Every winter, 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.

The whooping 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. Research has 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 even 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.

During 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 not survive.

With 30 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 estimated 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 and occasional 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 about.

The Aransas-Wood 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 is going to end. Only with continuing efforts to protect the species and provide the habitat that it needs can we expect the species to survive for generations to come.

Tom Stehn, U.S. Fish and Wildlife Service