Pre-Gather and Post-Gather Counts. The number of animals captured or removed from the land is used to inform population estimates. This technique can be used when a count has been conducted and is followed soon thereafter by a gather, in which a relatively large proportion of the horses are removed and the quantity is known. Another count is conducted soon after the gather. The difference between the two counts can be used to estimate the detection probability (Eberhardt, 1982).
All the methods except removals or captures can be conducted from the ground or from the air. In ground-based surveys, observers might traverse transects on foot, in vehicles, on horseback, or a combination of the three. Ground-based observers may be in prepositioned, stationary blinds to count animals with the mark-resight or double-observation methods. Cameras can be used to photograph animals at places of common congregation, such as watering holes (Cao et al., 2012; Petersen et al., 2012), and animals can be identified in a series of photographs over time by their markings; this procedure is typically used in a mark-resight analytical framework. Given the sizes of HMAs and their varied topography, it is usually practical and cost-effective to conduct surveys of horses and burros from the air. Helicopters and fixed-wing aircraft are the two aerial survey platforms typically used. In some cases, fixed-wing aerial surveys, which are less expensive than helicopter surveys, are adequate to locate and count animals, especially in areas dominated by sagebrush or other low-growing vegetation. In areas that have higher canopy and cover, however, helicopters may be needed for slower and more careful searching patterns. In aerial surveys, survey methods may be combined. For example, more than one observer may count animals as an aircraft follows a transect pattern by using distance sampling. Transect patterns can also be flown more than once during a survey to increase accuracy of population estimation, assuming that animals do not move substantially relative to flight paths between surveys.
Similarly, the Wild Horse Identification Management System (Osborn, 2004) was established in the Pryor Mountains to enumerate free-ranging horses by using unique coat-color markings and morphological characteristics in photographs. Lubow and Ransom (2009) used this approach in three HMAs (whose horse populations were of known size and were each smaller than 400) that were monitored weekly. Before correcting for detection probability, population size was biased (undercounted) by as much as 32 percent, but estimates accounting for heterogeneity of sighting probability (detection probability) were within 3-29 percent of the true number of animals known to be occupying the areas at the time of the surveys (Lubow and Ransom, 2009). The authors considered the cost of the more accurate models that quantified uncertainty in population-size estimates to be comparable with the costs of raw counts typically used by BLM (Lubow and Ransom, 2009), although the post-processing staff time required can be greater for this technique (Ransom, 2012b).
Remote-sensing technology can be used effectively to locate and count free-ranging horses and burros with a wide variety of sensors on satellites or manned and unmanned aircraft. The sensors can obtain high-resolution images at user-defined times and locations and can capture surface-reflectance characteristics at various spatial resolutions. Manned and unmanned aircraft can also take high-resolution videography that can be used to count horses and assess condition. New technology, including videography that detects movement patterns and measures speed of travel, can sense features with tremendous detail and accuracy. These methods will continue to be developed and improved and will allow even