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 in 2011, the agency changed from doing a weekly Whooping Crane census to conducting a survey that takes place for approximately one week each December.
Unlike a census, a survey incorporates 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. Unfortunately, the margin of error for this survey is quite large, equaling plus or minus 39 cranes for the estimated wild flock of 338 during the winter of 2016.
In January 2017, Dr. Bruce Pugesek, Montana State University and Tom Stehn, 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.
The abstract is provided below. To download the entire text of the paper, click this link: WHOOPING-CRANE-CENSUS-OR-SURVEY
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.