Debate continues in the Whooping crane world about the methods US Fish and Wildlife Service is now using to count Whooping cranes at the Aransas National Wildlife Refuge.
For almost 30 years, retired Whooping Crane Coordinator, Tom Stehn made a direct count of each bird while flying transacts over the refuge. The accuracy of that method was questioned because birds may move between the aircraft passes and been counted more than once.
The new system employed since Stehn’s retirement is called ‘distance sampling’. It consists of making multiple parallel survey flights over a large area and applying a formula to the number of cranes spotted to come up with a population estimate.
The concern is that distance sampling is generally used to estimate the size of large populations and there are fewer than 300 Whooping cranes at Aransas. Many feel that a direct count could provide a more accurate number.
According to a recent article on caller.com, distance sampling “…is not designed to produce an exact count…” A letter sent by Whooping Crane Conservation Association President, Lorne Scott’s letter to Secretary of the Interior, Ken Salazar, asking for a return to the previous census method has caused an influx of comments on the WCCA website.
Bloggers have also been very active on the victoriaadvocate.com site. The Operation Migration Field Journal has also received many questions and comments.
Stehn, who conducted aerial census surveys for almost three decades, believes the distance sampling method is only accurate to within about 15 percent, which he maintains is an unacceptable error range. Using distance sampling surveys in 2012, the US Fish and Wildlife Service estimated the population at 254 cranes, It includes a plus or minus range of 62 birds – or, approximately a quarter of the population.
In his December 27th report Whooping Crane Recovery Team coordinator Wade Harrell noted, “To date, we have completed seven survey flights (November 28, 29 and December 5, 12, 13, 14 and 17). Over the next several weeks, we will be working to compile and analyze the data collected and expect to provide an estimate on the number of cranes by late January.”
In Harrell’s most recent report (January 24th) he said, “We are still crunching the numbers on the winter whooping crane population estimate and hope to publish it within the next few weeks. We would rather report the best estimate the first time rather than have to revise it later. We understand how important the information is and will make it a priority to circulate the information as soon as it is completed.” Read the full report here.
Click the link to read USFWS’s 2011-2012 Whooping Crane Abundance Survey.