Counting the Birds


Getting a true census of wildlife populations is very difficult to do with natural free ranging animals.

First, most study areas are too large to ensure that no individual animal enters or exits the area during a survey. Second, various circumstances, including the unpredictable behavior of wildlife, the density of vegetation and observer fatigue, typically results in some birds within the survey area being missed.

In 2011, the refuge began utilizing the Distance Sampling method to estimate the population of the whooping cranes within a survey area. Distance Sampling has been used with many other rare and endangered wildlife populations, including fin whales, Karner blue butterflies and raptors. This method includes flying along transect lines set at specific distances within the survey area. From these transect lines, biologists count the individuals within each group of cranes and mark their GPS location. The data collected is used to determine the population within the surveyed area. The wintering grounds survey area included 140,000 acres on and around the Aransas National Wildlife Refuge.

In previous years, the refuge’s aerial surveys attempted to count every individual bird. Since 1941, the population has grown an average of 4.6% every year making it more difficult to count each bird. This is especially true when the birds are widely distributed.

To standardize the search effort and increase the efficiency of how we collect the data, each survey is conducted by a pilot and two observers looking out different sides of the plane. Upon detecting a crane the observers record the bird’s location on a high-resolution satellite image using a touch screen laptop equipped with a wireless GPS.

To test the precision and bias of distance sampling, the refuge did some experimental surveys using a known number of whooping crane decoys. During September 2011, 104 whooping crane decoys were placed on the Blackjack peninsula on Aransas Refuge at randomly selected locations. The two observers on the surveys did not know the location nor the placement of the crane decoys. They conducted four surveys of the whooping crane decoys and each time the decoys were moved between surveys. The team of biologists did a statistical analysis of the four separate surveys in three different ways and came up with estimates of 111, 104 and 96 decoys, a range of +6.7% to -7.6%.

The analysis methods used during the 2011-2012 winter capitalized on the relationship between whooping cranes and the habitat they use. Protecting their habitat is one of the most important things that can be done for whooping cranes. Understanding the habitat whooping cranes prefer and where that habitat exists on the landscape can help guide management decisions and conservation efforts.

Learn more about the annual survey and scientific report here.