The BLM needs population estimates to determine whether and where excess wild horses and burros exist, and, if there is an excess, how many animals need to be removed from public rangelands. Population estimates also guide the BLM in applying fertility control to mares and adjusting herd sex ratios in favor of stallions or geldings to reduce on-the-range births. The BLM works to ensure that horse populations are in balance with other rangeland resources and authorized uses of the public lands.
Most BLM field offices base their population estimates on direct counts from either a helicopter or fixed-wing aircraft. All wild horses and burros one year of age or older are considered adults and are counted in these estimates. Foals are counted during these surveys to provide information on foal production; however, a foal is not counted as part of the population until January 1 of the year following its birth.
The BLM is able to count from the ground a few of the smallest herds that are easy to reach and observe. In these cases, animals are uniquely identified (to avoid double counting) based on natural markings, such as body color and face and leg patterns.
Many of the BLM’s Herd Management Areas (HMAs) share a common boundary with HMAs in a neighboring district or state. Managers coordinate with each other to reduce the potential to double-count or miss animals because of movement between HMAs.
In addition to collecting information about the location and condition of herds within HMAs, the BLM compiles basic data about the land, such as the amount and quality of forage and the availability of water. Information about wildlife or livestock use is also collected.
The direct count method currently used by most BLM field offices involves the counting of each wild horse and burro actually seen either on the ground or during aerial surveys. This method, the Government Accountability Office concluded in an October 2008 report, results in an undercount of populations.
The BLM has been working with the U.S. Geological Survey’s Fort Collins Science Center to develop methods that will achieve greater accuracy in population estimates. An Aerial Survey Team, assembled to identify and research available population estimation techniques, concluded that two techniques produced the best results, depending on factors such as vegetation and topography. The two survey methods, which the BLM is implementing, are known as “simultaneous double-count” and “photographic mark-resight.” Both of these methods use statistical corrections to account for animals not seen during surveys.
The simultaneous double-count method, which works best in open terrain with short vegetation, uses two observers who independently observe and record data on groups of horses. Their independently derived results are then used to estimate the number of horses that were not seen during the survey.
The photographic mark-resight method, which is most useful in steep terrain or tall vegetation, uses aerial photographs to identify individual bands of horses based on unique markings of band members. Bands “marked” (observed) during an initial survey are then compared to those seen during “resight” surveys to estimate the population.