by Friends of the Wild Whoopers
Not all Whooping Cranes counted
Whooping Cranes over Aransas NWR. Photo by Kevin Sims ©2105
Aransas Wildlife Refuge biologist Tom Stehn conducted Whooping Crane census flights for 29 years at Aransas during which he tried to find every crane. When Tom Stehn retired from U.S. Fish and Wildlife Service (USFWS) in 2011, the agency changed from doing a weekly Whooping Crane census to conducting a survey that takes place for roughly one week every December. The change to a survey incorporated a technique called distance sampling where not every crane is counted but estimates of the cranes not seen are based on how far observed cranes were from the aircraft when sighted. This statistically-derived method provides 95% confidence limits for an estimated Whooping Crane population. Unfortunately, those confidence limits are quite large, equaling plus or minus 39 cranes out of an estimated flock of 338 during the winter of 2016.
Monitoring Whooping Cranes, Comparison article published
Dr. Bruce Pugesek and Tom Stehn, in January, 2017 published an article in the Proceedings of the 13th North American Crane Workshop entitled “THE UTILITY OF CENSUS OR SURVEY FOR MONITORING WHOOPING CRANES IN WINTER”. The article compares the survey and census methods of counting Whooping Cranes. An abstract for this paper is provided below, along with a link to download the entire 10-page article.
Tom Stehn commented to Friends of the Wild Whoopers about the article as follows:
“The article is not an easy read, but that’s the way science sometimes works. It is a rebuttal of some of the things that USFWS wrote regarding their crane survey method. In the article, Dr. Pugesek and I point out problems with the survey, including statistics, as well as what we consider as some of the “falsehoods” contained in what was written trying to justify the survey instituted after I retired. I feel the USFWS was overly critical with unfair and overstated criticism of the census method that USFWS had done for 60 years. And I firmly believe that with very thorough coverage of the crane area combined with the knowledge I had of individual cranes and their territories accumulated over 29 years of work and over 400 census flights, I could estimate the size of the whooping crane population in a manner much more accurately, and with a justifiable estimate of winter mortality, than what is being done on the current survey method. The bottom line is that a census is usually stronger scientifically than a survey if the species’ biological parameters allow a census to be conducted.”
Tom continues: “If current USFWS policy requires that a survey with confidence limits be continued, I recommend that after the annual survey is completed, that additional funds be spent doing a census in a manner similar to what I used to do. Doing some additional flying will allow comparison of the former census method with the current survey results which will provide more information about the crane population as well as better assess the survey methodology currently being employed. A census would involve thorough coverage of the crane range with transects no more than 500 meters apart. If there are now more cranes and a bigger area to cover, do the census over 1.5 days if need be. Also, territories of all family groups should be determined, with a follow-up flight or two in early March to show which juveniles have died during the winter. This estimate of annual winter mortality is very important, given what is known about drought and reduced inflows related to increased whooper mortality.”
THE UTILITY OF CENSUS OR SURVEY FOR MONITORING WHOOPING CRANES IN WINTER
BRUCE H. PUGESEK, 1 Department of Ecology, Montana State University, Bozeman, MT 59717, USA
THOMAS V. STEHN, 1613 South Saunders Street, Aransas Pass, TX 78336, USA
Abstract: We discuss recent changes in the monitoring program for endangered whooping cranes (Grus americana) on their winter habitat in Texas. A 61-year annual census was replaced in the winter of 2011-2012 with a distance sampling procedure. Justification for the change was, in part, based on criticism of the previous methods of counting cranes and the assessment of crane mortality on the wintering grounds. We argue here that the arguments, methods, and analyses employed to discount the census procedure and mortality estimates were applied incorrectly or with flawed logic and assertions. We provide analysis and logical arguments to show that the census and mortality counts were scientifically valid estimates. The distance sampling protocol currently employed does not provide the accuracy needed to show small annual changes in population size, nor does it provide any estimate of winter mortality. Implications of the relative merit of census and mortality counts versus distance sampling surveys are discussed in the context of management of the whooping crane.
Link to article
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