Heard about these curious creatures on RadioLab’s Galapagos feature.
According to Wikipedia:
A Judas goat is a trained goat used in general animal herding. The Judas goat is trained to associate with sheep or cattle, leading them to a specific destination. In stockyards, a Judas goat will lead sheep to slaughter, while its own life is spared. Judas goats are also used to lead other animals to specific pens and onto trucks.
One of the most effective uses of Judas Goats was in the Galapagos islands, where they were trying to eradicate them as an invasive species. They did so by shooting the goats from helicopters…
After endless planning and meetings, we commenced project Isabella…In under a year, through an aerial attack [by helicopter], we ended up wiping out 90 percent of the goats on Isabela. But to give an example of the nature of this business, its relatively easy to remove 90 percent of a goat population from an island. As they become rarer and rarer, they become harder to detect. The become educated. So the goats start hiding. You end up flying around in an expensive helicopter not finding any goats.
So the way we deal with that is an interesting technique called Judas goats. Goats are gregarious and like being in groups. They’re herd animals. The technique we would use was you fire up the helicopter, capture goats live, take them back to base camp, unload them, put a radio collar on them, and then throw them back on the island. Instinctively, that goat will go find other goats. A week, two weeks go by. You fire up the helicopter and…start tracking the Judas goats until you spot it with other goats. And then everyone gets shot except the Judas goat. And then they do it again. Every two weeks for a year.
From: http://onward.nationalgeographic.com/2014/09/02/on-the-galapagos-the-betrayal-of-judas-goats/