Free-roaming horses (Equus ferus caballus) occur throughout arid and semiarid regions of the western United States, where they can decrease plant biomass and diversity, impair water quality, and reduce forage available to native wildlife and domestic livestock. Management of free-roaming horses on Bureau of Land Management (BLM) and US Forest Service lands is determined by protections and population targets established by law, but these do not apply to other federal or Tribal Lands, where relatively little is known about the abundance and distribution of free-roaming horses. To address this information gap, we conducted the first comprehensive survey of free-roaming horses within the Navajo Nation, which is the largest Tribal Land holding in the contiguous United States and covers portions of the states of Arizona, New Mexico, and Utah. We used stratified random sampling and double-observer distance methods to produce estimates of horse abundance corrected for detection bias. During the summer of 2016, we used fixed-wing aircraft to survey 4 975 km of transects across our 67 089-km² study area. We observed 4 290 horses distributed among 527 groups and estimated 38 223 horses lived within the study area during the survey period (standard of error [SE]: 6 052, 90% confidence interval: 29 365–47 080), with 29 394 horses in open areas (SE: 5 511, 90% confidence interval [CI]: 21 328–37 460) and 8 829 horses in forested areas (SE: 2 331, 90% CI: 5 417–12 240). Overall density of 0.570 horses/km² (SE: 0.090, 90% CI: 0.438–0.702) was 23% higher than density of horses and burros (Equus asinus) in all BLM herd management areas (HMAs) in 2016 and exceeded by 17% the density in Nevada, the only state with an HMA of comparable size to the Navajo Nation. Our results will inform management of a free-roaming horse population that this study has revealed to be the among the largest in the United States.

自由漫游马（Equus ferus caballus）它发生在美国西部的干旱和半干旱地区，它们会减少植物的生物量和多样性，损害水质，并减少可用于本地野生动植物和家畜的饲料。土地管理局（BLM）和美国森林服务局土地上的自由漫游马的管理取决于法律规定的保护措施和人口目标，但不适用于其他联邦或部落土地，因为那里对丰度的了解很少和散布自由漫游的马。为了解决这一信息鸿沟，我们对纳瓦霍族（Navajo Nation）内的自由漫游马进行了首次全面调查，那瓦霍族是美国毗连的最大部落土地，覆盖亚利桑那州，新墨西哥州和犹他州的部分地区。我们使用分层随机抽样和双观测器距离方法来生成针对检测偏差校正的马丰度估计。2016年夏季，我们使用固定翼飞机对67 089公里的4 975公里横断面进行了测量²学习区。我们在调查期内观察到527组中的4 290匹马，估计有38 223匹马生活在研究区域内（错误标准[SE]：6 052，90％置信区间：29 365-47080），其中29 394匹马在开阔地区（SE：5 511，90％置信区间[CI]：21 328–37 460）和8 829马匹在林区（SE：2 331，90％CI：5 417–12 240）。马的总密度为0.570马/ km ²（SE：0.090，90％CI：0.438–0.702），比马和子（Equus asinus）的密度高23％。在2016年，所有BLM牧群管理区（HMA）的密度都超过了内华达州的17％，这是唯一一个规模与纳瓦霍族相当的州。我们的研究结果将告知自由漫游马种群的管理，这项研究表明这是美国最大的马群之一。