then the horses. the chopper herds them toward a track. these wild mustangs never knew anything but freedom. now, the gates of the makeshift corral close behind them. animal rights groups say these round ups must stop. >> the issue is that the wild horses should not be rounded up at all. hence the word, wild. >> reporter: the bom responsible for controlling herd sizes insists the number must be reduced dramatically. at this nevada round up, more than 100 horses will be taken. less than a dozen left behind. >> we realize ten is a low number here, but if that's what the range in this herd area can support, it is what it is. >> reporter: for activists, too many horses taken and what they believe is the excessive use of helicopters has turned a year's long simmering controversy into boiling anger.