For six decades, the U.S. Fish and Wildlife Service has monitored the population of the last wild migratory flock of whooping cranes primarily via an annual census. Now, Aransas National Wildlife Refuge and the National Wildlife Refuge Systems Inventory and Monitoring (I&M) program are devising a way to improve that count.
The flock of endangered cranes migrates between nesting territory near Canadas Wood Buffalo National Park and wintering territory near Aransas Refuge on the Gulf Coast of Texas.
In 1950, the first whooping crane aerial census found 31 cranes, all wintering within 20,000 acres of salt marsh at Aransas Refuge. Since then, the population has grown to almost 300 birds and expanded onto 140,000 acres of salt marsh, some refuge land, some privately owned. Technological advancesincluding GPS (Global Positioning System) and highresolution satellite imageshave been huge. Yet the cranemonitoring method has changed little.
So, refuge and I&M staffs are developing a formal protocol for monitoring wintering whooping cranes. The protocol will describe the surveys objectives, sampling design, field methods and data analysis procedures. It will be submitted soon for professional peer review to ensure that the methods are scientifically defensible and professionally valuable.
Although still in draft stage, the protocol updates the method. Previously, one observer flew over the wintering grounds and marked the location of cranes on paper maps. The planes flight paths often were different, which resulted in some areas being searched more thoroughly than others. The new method will use two observers searching out opposite sides of the aircraft while navigating along evenly spaced transects. GPSequipped laptops will record the cranes locations digitally on highresolution satellite images. This will improve data collection consistency.
The protocol also updates how we analyze the data, so we can be confident in the resultswhich is vital to the Service mission and in line with the Conserving the Future visions call for sound science. The previous method was coined a censusbecause it was assumed that the population and search area were small enough to count each bird without error. But as the population grew, the chances of counting each bird decreased, and the censuss design did not allow the Service to gauge reliability.
The draft protocol includes a survey method called distance sampling. It is useful even when some whooping cranes are missed. For more than 30 years, distance sampling has been successful for counting wildlife from whales to deer. By measuring the distance each group of cranes is from the aircraft and using distance sampling, refuge staff members can credibly estimate the number of cranes present, even when they dont see them all.
The refuge and I&M staffs thoroughly tested a distancesampling approach before trying it with live cranes. They analyzed data from previous surveys and learned that nearly 95 percent of the whooping cranes recorded were less than 500 meters from the plane. They flew experimental surveys and learned the new method could accurately estimate the number of crane decoys present. They tested conventional distance sampling against a cuttingedge modification and learned that better results were obtained by incorporating crane locations and habitat information into the analysis.
More than 60 years of Service census data have taught biologists a lot about where the whooping crane population has been. The new method, which retains much of the old, now provides statistically defensible estimates of abundance, while providing a better understanding of how whooping cranes use habitats. These innovations help the Service continue recovery of North Americas largest endangered bird by informing conservation planning, land protection efforts and policy decisions.
Brad Strobel is a wildlife biologistuntil recently at Aransas National Wildlife Refuge in Texas and now at Necedah Refuge in Wisconsin.