Based on recommendations from the National Academies of Science, the BLM has partnered 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.
The BLM is currently working toward a plan that would survey one-third of the nation's HMAs using the new survey techniques annually.