A group of wild horses are rounded up in Eureka, Nevada. (Photo: Justin Sullivan/Getty Images file photo)

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A 12-year-old mare has died at the site of the ongoing Owyhee Complex helicopter roundup in Northern Nevada.

The cause of death was an internal infection, according to the Bureau of Land Management.

Since the roundup commenced on Nov. 2, four wild horses have died and 405 have been captured. Of those, 111 have been returned to the range — including 49 mares that were treated with fertility control and released on Monday.

No horses were captured on Monday while traps were being moved, but 24 stallions, 39 mares and 16 foals were transported to the Palomino Valley Wild Horse and Burro Adoption Center near Reno, Nevada, where they will be prepared for the BLM adoption program. Those not adopted will later be taken to off-range pastures.

The BLM plans to capture 680 wild horses in and around the Elko District’s Rock Creek and Owyhee Horse Management Areas. Of those, about 450 will be removed from the range.

A second phase of the roundup is scheduled to be held after Thanksgiving on the Winnemucca District’s Little Owyhee Horse Management Area. There, 920 wild horses are the be captured and 650 removed from the range.

Mares that are not transported will be treated with PZP-22 fertility control vaccine before release. So far, 111 horses have been returned to the range, according to the BLM.

The agency justifies the roundup as an effort to “remove excess wild horses in order to prevent further deterioration of Greater Sage grouse habitat within the Sagebrush Focal Area (in northern Elko and Humboldt Counties. Overpopulation of wild horses leads to the degradation of rangeland resources, which adversely impacts habitat for other species as well as the horses themselves.”