The presence of free-roaming horses in the United States is an iconic feature of the American landscape. These wild equids, which include horses and burros, inhabit vast territories, primarily in the western half of the country. Their existence results from animals escaping or being released from domesticity over centuries, establishing herds adapted to challenging environments. The lands they occupy span millions of acres, covering deserts, mountains, and coastal regions, which presents unique management challenges.
Legal Status and Management Agencies
The definition of a “wild horse” in the US is legally specific, referring to “unbranded and unclaimed” horses and burros found on public lands. These animals gained federal protection under the Wild Free-Roaming Horses and Burros Act of 1971 (WFRHBA). This legislation declared them “living symbols of the historic and pioneer spirit of the West” and mandated their protection and management.
The responsibility for overseeing these herds falls primarily to two federal departments. The majority of the population is managed by the Bureau of Land Management (BLM), part of the Department of the Interior. The U.S. Forest Service (USFS), part of the Department of Agriculture, manages a smaller number of herds in designated Wild Horse Territories. Both agencies must manage these animals to maintain an ecological balance on the public lands they inhabit.
Primary Geographic Distribution in the Western US
The vast majority of federally protected wild horses reside on public lands across ten western states. These populations are concentrated in designated Herd Management Areas (HMAs), encompassing approximately 27 million acres. The primary habitat consists of arid and semi-arid rangelands, high deserts, and plateaus.
Nevada contains the largest population, hosting over half of all free-roaming horses and burros on BLM-managed land. Other states with significant populations include Wyoming, Utah, Oregon, California, and Arizona. HMAs are often characterized by scarce forage and water, necessitating careful management to prevent overgrazing and land degradation. The BLM ensures these herds remain at an Appropriate Management Level (AML) to sustain the animals and the health of the ecosystems.
Distinct Non-Federally Managed Populations
Other distinct populations exist outside federal jurisdiction and are managed differently. The horses of Assateague Island, split between Maryland and Virginia, represent two separate herds. The Maryland herd is managed by the National Park Service, while the Virginia herd is owned by the Chincoteague Volunteer Fire Department.
The feral equids of the Outer Banks in North Carolina, known as Banker horses, are another example. These Colonial Spanish Mustangs, such as the Corolla herd, are managed largely by non-profit organizations like the Corolla Wild Horse Fund. Management often occurs in cooperation with local and state entities, focusing on conservation and population control using methods like immunocontraception. These coastal herds live in unique environments, such as barrier islands and salt marshes.
How Herd Sizes Are Maintained
Wild horses have a high reproductive rate, with herds capable of increasing by up to 20 percent annually. This rapid growth, coupled with the lack of natural predators, necessitates active population control to prevent resource depletion on the range. The primary management tool is the removal of excess animals through “gathers,” or roundups, often utilizing helicopters.
Animals removed from the range are prepared for the Wild Horse and Burro Adoption and Sales Program, which places them into private care. Another strategy is the application of fertility control vaccines, primarily Porcine Zona Pellucida (PZP). This vaccine is administered via darting to mares, temporarily preventing pregnancy and slowing population growth. These actions aim to keep herd numbers within the established Appropriate Management Level (AML) for each Herd Management Area.