As a contractor with the United States Postal Service, Louise Lowder has been delivering mail to remote ranches in southwest Wyoming for years.

She’s often observed pronghorn migrating across the sagelands. But on a recent snowy post-storm day while on her route between Kemmerer and La Barge, she encountered an assembly of antelope so large it made her jaw drop.

“I’d say there were at least a thousand head,” Lowder said. “They were everywhere.”

Lowder said she’s come across herds of a few hundred antelope. But, “I’ve never seen anything this big.”

Lowder captured this image of the animals as they traveled a remote road near Highway 189, and later photographed the herd as it stretched in a single file line far into the distance, heading south.

The animals were likely prompted to migrate suddenly by the heavy snow that fell in late November, said Greg Nickerson, a writer and filmmaker with the Wyoming Migration Initiative (and former WyoFile reporter).

“When that first heavy snow falls up at the higher elevations where a lot of the big game animals are spending the summer, they start thinking about migrating,” Nickerson said. Because pronghorn are not well-equipped to travel through heavy snow, they “are usually the first … That means going down into the basins, particularly the Green River basins.”

Animals will travel in single-file formation to conserve energy, Nickerson said. Another way to do that, he said, is to take advantage of plowed roads.

“When the whole desert is covered in snow they are looking for whatever advantage they can get,” he said. “That could be a windswept ridge. These plowed gravel roads and highways are also attractive to them.”

That, however, sets up the potential for vehicle-animal collisions. Which is one more reason why winter is a time to be extra vigilant while driving.

Wyoming Game and Fish recently urged motorists in southwest Wyoming to reduce speeds and be extra watchful of migrating animals this time of year.

Already this winter, the agency reported, a number of vehicles have collided with antelope on highways and interstates in the southwestern part of the state. Collisions and animal deaths appear higher this winter than normal, according to a Game and Fish release.

“Poor visibility, coupled with slick roads this year, have made it challenging for both motorists and wildlife,” Green River Wildlife Management Coordinator Mark Zornes said in the release.

As the original North American ungulate, Nickerson said, pronghorn have evolved to match these landscapes, and have been following the same migration corridors for millenia.

“That worked great for millions of years, but now it’s harder for them with all the barriers we are throwing up with highways, fences, things like that,” Nickerson said. “Anything we can do to make it easier on them to continue to do what they’ve always done is good.”

(WyoFile is an independent nonprofit news organization focused on Wyoming people, places and policy.)