At the request of the U.S. Bureau of Land Management (BLM), scientists at the U.S. Geological Survey Fort Collins Science Center (FORT) are testing combinations of four techniques for estimating wild horse populations: mark-resight, simultaneous double-count, sightability bias correction modeling, and distance sampling. Field methodology to implement an integration of techniques has been developed and tested in Antarctic marine fauna populations; however, the equipment and aircraft used are not suitable for wild horse survey needs. Developing a similar approach using more readily available aircraft and less expensive technology could produce a very user-friendly and more accurate survey method than is currently available to wild horse managers.
The combination of multiple sources of information overcomes most of the deficiencies of the separate techniques alone and provides greater power and efficiency. For example, the major difficulty in the double-count technique—ensuring similar sighting probabilities for all animals—can be resolved by modeling sightability using covariates in a manner similar to the sightability bias correction model. However, unlike the traditional model, multiple observers provide sufficient information to estimate sighting models for each covariate from a single survey. Therefore, no pre-calibration of the model is required, and the assumption that the initial calibration applies uniformly over space, time, and observers is eliminated. Similarly, the requirement in distance sampling that all animals near the transect line are spotted can be eliminated by applying data from a double-count to estimate the detection probability on the line. Finally, both the mark-resight and distance sampling methods can be made more precise by incorporating sightability covariates.
Simultaneously using two or more methods increases the complexity of data collection for aerial crews. For example, combining sightability and double-count would mean that observers record all of the same data that are recorded for sightability models, with the addition of a GPS location used to later match the sightings of the two observers. Still, by including one additional measurement—perpendicular distance from the transect to observed animal groups (distance sampling)—all three techniques can be combined, tested, and compared during the same survey.
With input from various Federal and State wildlife survey experts, FORT’s aerial research team completed a work plan in the summer of 2003 and was in the air conducting surveys by October of that year. By testing the techniques at the fertility control field-trial sites, the research team had the advantage of knowing the true population numbers, against which the aerial survey results could be compared. Wild horses at these sites are known individually and observed from the ground 6–7 months per year.
Mark-resight sampling combined with sightability bias correction modeling has now been tested against known populations at the McCullough Peaks Herd Management Area (HMA), Wyoming; the Little Book Cliffs Wild Horse Range, Colorado; and the Pryor Mountain Wild Horse Range, Montana and Wyoming. Aerial photography was used to create "marked" individuals, followed by additional flights to collect "resight" data. Because horses exhibit unique coloration and markings, a photographic image served to "mark" an individual; then, photos from each flight were matched up to determine which animals were resighted. Results to date show this to be a promising technique for smaller populations and for habitats consisting of considerable tree cover and rugged terrain, where only a limited percentage of the animals may be visible. See Lubow and Ransom (2009) for details on the results of these tests.
In Nevada and Wyoming, several flights have been conducted pairing simultaneous double-count with sightability bias covariates. The HMAs surveyed were very large and more representative of large western herd areas than the fertility control field-trial HMAs. Since the true populations of these areas were not known, surveys were conducted before and after a removal of horses, and the statistical estimates compared to the known removal number. The results of these flights guided the development of a more statistically sound population estimate for each HMA, but also provided considerable data on individual observers and the effect position in the aircraft has on sightability of horses. The distance to and size of the horse groups also played a key role in the modeling. See Ransom (2012) for additional information on factors affecting sightability of horses. A manuscript on the final combined technique—using simultaneous double-count, sightability bias correction modeling, and distance sampling—is in preparation.
One other technique, forward-looking infrared (FLIR), was integrated into the study in 2004. FLIR has been used in surveying many other wildlife species and can potentially improve the sightability of wildlife in more vegetated or rugged habitats. Our test, conducted in Nevada, resulted in a less satisfactory population estimate than the simultaneous double-count survey performed at the same herd area. This technique might be more suitable for surveying wild burros, which are challenging to count because of their more solitary nature and preference for rocky, rugged habitat.