"With our semi-feral herds, it is only after all the natural terrain spaces are taken that lower ranking individuals or groups get the shed," McDonnell says.
When horses huddle in a group or look for shelter during a thunderstorm, it might be the wind and rain that they are trying to avoid, not a killer bolt from the sky, writes Marcia King in the April edition of The Horse. Dr. Sue McDonnell of the University of Pennsylvania has studied horses that are pastured most or all of the time, horses at liberty or in large pastures, and semi-feral herds. During thunderstorms, she says, most horses show no noticeable response to either thunder or lightning independent of the severity of the rain and the wind. "We consistently observe that with thunder and lightning preceding a storm, horses just continue on as they would, doing whatever else they were doing before the weather change. But when the wind picks up and the rain picks up, horses may seek natural or artificial shelter. So, we conclude that it is not the electrical storm from which they are seeking shelter." McDonnell also found that stabled horses didn't exhibit any changes during thunderstorms. They continued to munch their feed and continued their activities, oblivious to the thunderclaps and lightning flashes. "It could be a hellacious storm and still no change," she says. When wind and rain accompany thunderstorms, horses will seek the shelter of trees and natural changes in the terrain. "For example," McDonnell says, "they'll often go down to the creek beds, which are typically lined with trees. They usually stand on the side of the creek. They often huddle in their social groups and become more tightly compacted, just as they would in a driving snowstorm or rain that is independent of thunder and lightning." Outside horses also seek refuge in manmade shelters -- if nothing else is available.