Pro: New whooping crane counting method can be replicated
By By Dianna Wray - DWRAY@VICAD.COM
March 3, 2013 at 10:05 p.m.
Updated March 3, 2013 at 9:04 p.m.
The last known naturally migrating flock of whooping cranes has been carefully tracked since the flock was discovered on the verge of extinction in 1938.
Declared an endangered species in 1967, the whoopers have been counted for decades.
Tom Stehn, the longtime whooping crane coordinator for the U.S. Fish and Wildlife Service, conducted an annual census of the whoopers as the birds wintered in the marshes of the Aransas National Wildlife Refuge.
After Stehn retired, the method of tracking the flock size has been changed from a direct count to an estimate.
Is one method better than the other?
The new method of surveying the flock is better, a state expert contends.
"A big part of the reason for the change was to make it repeatable over time," said Wade Harrell, whooping crane coordinator for the U.S. Fish and Wildlife Service.
When Tom Stehn, the former whooping crane coordinator, started conducting surveys in the early 1980s, the flock was smaller, and it was more likely most of the whoopers were wintering on the Aransas National Wildlife Refuge.
However, over the years, the whooping crane flock numbers have increased, but Stehn's survey methods never changed.
"Tom would fly based on his knowledge, and he would make assumptions based on what he knew, but there was still a possibility he could have missed some of them," Harrell said.
After Stehn retired in 2011, it became clear that the survey method would have to be changed, Harrell said. Staff biologists tried to conduct test surveys using dummy birds, but they were unable to conduct an accurate direct count using Stehn's methods, Harrell said.
"The real difference here is that we don't assume we can count every bird. We assume with the numbers of birds and how spread out they are that counting every bird is impossible," he said.
Wildlife service has changed significantly since Stehn started conducting his surveys in 1982. Back then, it was possible for someone in the service to spend an entire career working with one species, like the whooping cranes, but Harrell said that doesn't happen anymore.
When wildlife officials decided to start conducting hierarchical distance sampling instead of a direct survey, Harrell said, they were designing a system that could be reproduced no matter who was conducting the count.
"People have asked why we don't test the old method, but I don't know how to do that. The only person who knows how to do that is Tom. We're now in a situation where we have to go to a scientific method that is more repeatable so that with different observers over different times you'll have accurate numbers," he said.
Bill West, general manager for the Guadalupe-Blanco River Authority, agreed with Harrell about the new method.
The GBRA has been embroiled in a lawsuit concerning the death of 23 whooping cranes during the 2008-09 season. The attorneys for the state maintained that the number of birds who died during the season has been exaggerated and that Stehn's count was inaccurate.
Both parties are still waiting for a decision on the lawsuit from U.S. District Judge Janis Graham Jack, but West said he was pleased to see the U.S. Fish and Wildlife Service change the survey method.
Robert Powell, a wildlife biologist who worked as a consultant on the case for the state and reviewed Stehn's surveys and methodology, said he thinks the new method is better and will yield more accurate numbers.
"I think it was a necessary change," Powell, a doctoral candidate at Texas A&M University, said.
The first survey analysis estimated the flock to be made up of about 257 birds with a 95 percent confidence interval, meaning the actual number could be between 178 and 362 whoopers, according to a release.
Harrell acknowledged that the range between 178 and 362 birds is a wide one, but said accuracy will improve over time as the organization continues using the new survey techniques.
The last survey conducted by Stehn was released in 2011 and stated there were 283 birds in the flock.
The new estimate comes in slightly lower, with an estimate of 279 birds in total, according to a release issued Feb. 15.
"This is an accepted methodology in the scientific community. The protocol that they follow will provide consistency, and that's the main thing to keep in mind. Consistency is key to the scientific process, irrespective of who is conducting the survey," West said.
West also pointed out that the method needed to change to reflect the growing size of the flock.
The report using the new survey method, issued in February, estimated the flock size at 257, plus about 22 birds thought to be living outside the survey area.
The goal in the coming decades is for the flock to swell to more than 1,000 birds. If this happens, it will be impossible to cover the ground to count every bird the way Stehn once did, West said.
"The objective is to grow the flock. The magic number is 1,000 birds, but as the flock does grow, the difficulty in counting will increase," he said.