Cautiously Optimistic

Guest Author: Tom Stehn, Ret. Aransas NWR Biologist and Whooping Crane Recovery Team Co-Chair.

I hope you will celebrate the results of the USFWS 2017-18 winter whooper count posted below (505 in the primary survey area + 21 outside the survey area) = 526 total.

If you want to read details about the survey, check out: This link on the refuge web site.

However, just a cautionary note. Several things as follows indicate to me that this estimate of 526 may be too high.

1.  I calculated the % average population increase every decade from 1940 to 2010.  Flock growth averaged 42.6% per decade. So from my aerial census done in the 2010-11 winter of 283 cranes, one could anticipate, using the average flock growth per decade, a flock size of around 404 in 2020.  The best one decade growth rate equaled 87.2%. If the growth rate between 2010 and 2020 matches the best ever decade, anticipated flock size in 2020 would be 530. This is the only way I can come close to the current estimated flock size. 

2.  I have more faith in the accuracy of the nest count done in Canada every June than the winter survey done at Aransas after I retired.  I calculated the ratio between # of nests and the flock size the following winter for 10 years between 1967 and  2010.  This ratio equaled 3.87.  So given the peak of 98 nests recorded in 2017,  projected flock size would be 379 in the 2017-18 winter. If the highest ratio between flock size and # of nests is used (4.78), then the flock size could be 468.  Note that not all adult cranes nest every year, so I based my calculations on the peak # of nests ever recorded (98).

3.  The document by Butler and Harrell on the refuge web site entitled “Whooping crane survey results: Winter 2017-18” gives the annual long term flock growth rate at 4.55%. If you start with the last time a complete census of the flock was done in the 2010-2011 winter that found 283 whoopers at Aransas, a growth rate of 4.55% would derive an estimated flock size of 422 in the 2019-2020 winter. 

4.  The 95% confidence limits as stated by Butler and Harrell for the 2017-18 winter survey provide a flock size at Aransas of between 439 and 577 whoopers.  

So maybe flock growth in the past decade has been the best ever in the history of the flock.  Growth could be exponential. I sure hope so. Based on historic growth rates, that’s about the only way I can reach the current flock estimate of 526. 

So I will celebrate cautiously.  

Tom Stehn


U.S. FISH AND WILDLIFE SERVICE  Date:  August 21, 2018

Contact:  Wade Harrell, 361/676-9953 wade_harrell@fws.gov / Beth Ullenberg, 505/248-6638 beth_ullenberg@fws.gov

Estimated Texas Wintering Whooping Crane Population Breaks 500  –  Survey accuracy improved with shift from December to February time frame

The first winter after Hurricane Harvey ravaged the Texas Gulf Coast, an estimated 505 whooping cranes arrived on their Texas wintering grounds after migrating 2,500 miles from their breeding grounds in Wood Buffalo National Park in Canada.  Each fall the birds make their way back to Aransas National Wildlife Refuge and surrounding habitats, where they spend the winter.  Once they have arrived, wildlife biologists with the U.S. Fish and Wildlife Service survey the birds by air and analyze population trends. 

Biologists have completed analysis of aerial surveys of the Aransas-Wood Buffalo whooping crane population done last winter.  A switch in aircraft the previous winter and a shift to surveying later in the winter when a larger proportion of the population had arrived helped improve accuracy of the counts.  Preliminary data analysis indicated 505 whooping cranes, including 49 juveniles, in the primary survey area (approximately 153,950 acres) centered on Aransas National Wildlife Refuge near Austwell, Texas. An additional 21 birds were noted outside the primary survey area during the survey. This marks the 6th year in a row that the population has increased in size and the first time the population has topped the 500 mark.

“Breaking the 500 mark for this wild population is a huge milestone”, stated Amy Lueders, the Service’s Southwest Regional Director. “Seeing this iconic bird continue to expand demonstrates how the Endangered Species Act can help a species recover from the brink of extinction.  I have to credit our biologists and our partners and local communities who continue to invest so much time and effort to improve our ability to make sure future generations have the chance to marvel at the beauty of these amazing wild birds.” 

The U.S. Fish and Wildlife Service has implemented several small changes that have greatly improved the agency’s capacity to survey the birds. “After two years of testing a shift of our December survey timeframe to later in the winter, we believe our previous survey estimates were likely low given that not all the whooping cranes had completed migration by mid-December. We had indications of a later than expected fall migration over the last several years via migration reports and telemetry data. This is the first year that we have based our winter abundance estimate from a February survey timeframe rather than a December timeframe. It may seem like population numbers jumped more than usual, but in reality we are just capturing a more complete proportion of the population, with most birds having completed migration by early February” stated U.S. Fish and Wildlife Service Whooping Crane Coordinator Wade Harrell.

Harrell said biologists will continue to conduct flights in late January and early February for future surveys.  He also stated that staff at Aransas National Wildlife Refuge continue to make progress in recovering from the impacts of Hurricane Harvey.  “The good news is that the coastal marsh that supports our wintering whooping cranes was not significantly damaged by the hurricane and recovered quickly from any impacts, demonstrating how resilient intact wetland habitats can be.”

Whooping cranes are one of the rarest birds in North America and are highly endangered. Cranes have been documented to live more than 30 years in the wild. Adults generally reach reproductive age at four or five years, and then lay two eggs, usually rearing only one chick.

More information about the survey and whooping cranes can be found on the Aransas National Wildlife Refuge website at http://www.fws.gov/refuge/ Aransas/ or by calling the Aransas National Wildlife Refuge Visitor Contact Station at: (361) 349-1181.    

The mission of the U.S. Fish and Wildlife Service is working with others to conserve, protect, and enhance fish, wildlife, plants, and their habitats for the continuing benefit of the American people. We are both a leader and trusted partner in fish and wildlife conservation, known for our scientific excellence, stewardship of lands and natural resources, dedicated professionals, and commitment to public service.

For more information on our work and the people who make it happen, visit www.fws.gov. Connect with our Facebook page at www.facebook.com/usfws, follow our tweets at www.twitter.com/usfwshq, watch our YouTube Channel at http://www.youtube.com/usfws and download photos from our Flickr page at http://www.flickr.com/photos/ usfwshq.

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2 Comments

  1. Susanne Shrader August 23, 2018 5:25 pm

    I wanted to thank Tom Stehn for his very thorough and thoughtful analysis of the most recent Whooper winter account. I will be cautiously optimistic also. I would send him an email but I don’t know if it follows the same format as the non-retired people leaving comments below his.

    • Heather Ray August 23, 2018 9:05 pm

      he has a private email address, however, if you email your message to me I would be more than happy to forward it along with your return address Susanne. I can be reached at heather@operationmigration.org