January 24, 2013

Whooping Crane Update

 Wade Harrell, U.S. Whooping Crane Recovery Coordinator

Surveys & Monitoring:
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.

Reports from Outside the Survey Area:
The following information is provided by observers who posted on Texas Parks and Wildlife Department’s Texas Whooper Watch website, as well information we have received.
• There are still eight whooping cranes using the Granger Lake area, two family groups with a chick each and one pair without a chick. Whooper Watch observers have been able to photograph some of the birds using small stock ponds on private lands near the lake. One observer reported “I was out yesterday (Jan. 16) on my own at Granger and got to watch five whoopers together at a farmer's pond. A nearby family group "took over" the pond that a pair was dancing in. While the pair stayed close by, the group with a juvenile (no transmitter that I could see) began dancing, all of them, especially the juvenile, then bathed in the pond! They would squat and dip their bodies, then shake their feathers off. Then they would dip they heads under the water, then bring them up and shake their heads.”
• Though it is infrequent, five whooping cranes (a family group and a pair) northwest of El Campo continue to be seen by local biologists and wardens.
• An experienced birder reported a pair of whooping cranes using farm fields in conjunction with a larger group of sandhill cranes near the Loyola Beach area of Kleberg County. We are still gathering evidence to confirm the sighting but if you bird this area regularly, please keep an eye out and report any sightings to the Texas Whooper Watch.

Count or Estimate?:
Questions have come in regarding the difference between a “count” (or census) and an “estimate” (or survey). As it relates to whooping cranes, a census is an exact count where a survey is an estimate of the true number.

From about 1950 to 2010, the Aransas National Wildlife Refuge staff attempted to conduct a census of the whooping crane population. The census would account for every individual bird in the population regardless of where they were found. This was possible because the population was small and stayed in a relatively small area.

In wildlife biology, there are few cases where one can successfully complete a count/census of truly wild animals. This is mostly because, by nature, wildlife species move around in their environment. Also, they live in places where it might be easy to overlook them. Animals raised in zoos and released in the wild can be marked with transmitters or bands, which help biologists get an exact count. This is what has been done with the eastern migratory population of whooping cranes that are part of a reintroduction project.

When the Aransas- Wood Buffalo whooping crane population was small and inhabited a limited area, it was reasonable to assume that most of the individual birds could be accounted for. Fortunately the whooping crane population has grown both in numbers and winter range over the past 20 years.
What was originally a primary wintering range of the Blackjack Peninsula is today extended to encompass more than 30 miles of the Texas coast. Click here to see the survey map.

In addition to the expanded range, we know the birds are moving in and out of the survey area. Data from the satellite tracking study shows the radio-marked birds move frequently, sometimes on a daily basis. We can no longer assume that we can properly account for every individual in the winter population because of the expanded range and increased population that moves around on the landscape.

To make the transition from counting a few endangered species to surveying a growing population, the field of biology has developed methods for estimating the number of individuals in a population through the use of surveys within a given area. It is especially important the method be consistent and can be repeated. The basic survey method we use to estimate the wintering whooping crane population is Distance Sampling, a widely used technique that has been tested on many wildlife species in a variety of habitats.

Knowing the whooping crane winter range is extensive and the birds move around, we are certain we will miss some of the individuals when conducting an aerial survey. To account for this, we use statistical models that extrapolate the “estimated” number of individuals in a survey area. We get an estimate of all of the whooping cranes in the survey based on the individual birds that are actually seen on the various flights. We conduct several flights using the exact same counting method every time -- consistent and repeatable.

This type of statistical model typically produces an estimate of the true number of species with an associated confidence interval, i.e. we are 95% confident that the true population is between 220 and 270 individuals. In this case, the population estimate would be 245 individuals +/- 25.

Several comments expressed concern that the first Distance Sampling estimate conducted last year had too large of a confidence interval to be able to detect change in the birds’ population. This year, our second time to apply the new survey methodology, we are working to narrow that confidence interval. We are doing this by flying more survey days, this is more data to include in the modeling to increase the precision. We are also continuing to refine the statistical model. Adding data and refining the model is a typical scientific process that will lead to increased statistical precision of our whooping crane population estimate.

That said, the Aransas Wood-Buffalo population is made up of wild birds so there is always going to be some uncertainty when estimating the population. We are confident that the finalized survey protocol will be able to detect change in the birds’ population over time and will provide us the information needed to make critical management decisions.

In human terms, a real life example can be found in the classroom where a teacher takes attendance to count students. She/he gets a count, an exact number. It is possible because each student is known and can be accounted for. That afternoon, the class attends a ceremony in the auditorium. Looking over the auditorium, the principal wants to gauge how many students are in attendance. How does she/he get an accurate accounting if students are coming and going? How can it possibly be known who didn’t attend school that day? The same principle applies to whooping cranes. It was possible to count the birds when there were less but the growing population and expanded range makes it hard to continue counting individual birds. Thankfully there are more whooping cranes and they are finding new habitats to winter, feed and raise their young. 

Satellite Tracking Study:
The U.S. Geological Survey, International Crane Foundation, Gulf Coast Bird Observatory and other partners wrapped up this winter’s trapping season with a total of 12 whooping cranes marked with satellite transmitters. This important study has one more year of marking birds. Next winter, the team will come back to Texas to mark 10 more birds, which will wrap up the trapping portion of the study.

Since the last update, we have received 1.9 inches of rain. Most of this rain came on January 9, right after our last update. As is normal after a big rain event, we saw significant movement of cranes into the coastal marsh areas. They were undoubtedly taking advantage of the newly available freshwater and food. Salinity in San Antonio Bay dipped to less than 20 ppt for a couple days after the rain day but it has stayed between 22 and 27 ppt since then. We are hoping for another significant rain event to freshen up the coastal marsh and maintain optimal habitat conditions.

Food Abundance:
Refuge staff were able to complete a 1,095 acre prescribed burn last Thursday on the southern tip of the Blackjack peninsula, an important foraging area for whooping cranes. This brings the winter burn total to more than 8,000 acres. We have another 2,000 acres or so still planned for the winter. Our prescribed burning program is an important part of how we manage whooping crane habitat on the Aransas Refuge. It opens up new habitat for the birds to forage in and provides food resources such as live oak acorns that would not typically be available. The satellite tracking study is providing insight into how the whooping cranes use the recently burned areas.

For general questions regarding the methodology, I would recommend taking another look at the December 18 Whooping Crane Update. The website also has extensive information about the survey methodology that can be found here.