Montana Pronghorn Migration: Millennia-Old Journey Blocked by Swollen Missouri River

In Montana, the winter of 2011 was one of extremes: extreme cold, extreme snow and massive numbers of winter-killed big game. In many cases, deep snow forced herds of mule deer and pronghorn to gather on roads and railroad tracks, where many of them perished. Other herds were forced to migrate much farther south than normal in an effort to find open ground. One such herd of pronghorn, pushed far to the south of their traditional wintering grounds, was forced to cross the frozen Missouri River in an effort to find food and respite from the head-high snow. But as the snow and ice began to melt and the weakened herd started its return trip to summer fawning grounds, the pronghorn came across a barrier they had never encountered before: the raging spring flow of the Missouri River. Now, after making the longest land migration in U.S. history to survive - 260 miles - the herd now finds itself trapped on the wrong side of a mile-wide torrent of floodwater. Some of the antelope have tried to make the swim only to be turned back. Some have made it across. Some died trying to make the millennia old journey to the herd's historic breeding ground. It's now fawning time, and the pronghorn that remain trapped behind the still-raging waters of the Missouri are giving birth far from where they should be, while scientists and researchers can only watch and hope the herd somehow survives as the next generation is born far from where they should be. Scientists don't know if these pronghorn will retain their ancestral migratory instincts, or if they'll even survive. For the past few months researchers have monitored and documented the herd's attempts to cross the swollen Missouri. These photos, from wildlife photographer Michael Forsberg, chronicle the arduous task these pronghorn face.© 2011 Michael Forsberg /michaelforsberg.com
WWF Biologist Dennis Jorgenson and photographer Michael Forsberg have been following the trapped herd's plight for weeks. Jorgenson estimates there are over 600 pronghorn trapped on the south side of the river. "Every day they come down to the river looking for a place to cross," says Jorgenson. "We see groups of 15 to 20 animals that will jump into the river and try to swim across the mile-wide river. A very few of them make it. Most of them will turn back. The largest group we witnessed actually make it across was a group of six bucks."© 2011 Michael Forsberg /michaelforsberg.com
"They come down in these small groups and just stand there for a while, just looking," Jorgenson says. "Eventually one will leap into the water, and then the others will follow. It was a pretty remarkable sight."© 2011 Michael Forsberg /michaelforsberg.com
For the ones that don't make it and are forced to turn back, Jorgenson says, their instincts may actually be working against them. "Some animals don't eat on migration, so that drive to migrate is taking precedence over their need to eat. What we're seeing is some of these animals that can't make it across the river dying when they have access to forage."© 2011 Michael Forsberg /michaelforsberg.com
Many people don't realize it, says Jorgenson, but pronghorn are a highly migratory species. "These animals are adapted to moving large distances, in some cases a 400 mile roundtrip between summer and winter grounds," he says. But the extreme nature of this winter has pushed the animals far beyond their normal grounds. "These are Saskatchewan pronghorn that normally winter around Glasgow."© 2011 Michael Forsberg /michaelforsberg.com
Compounding the problem, says Jorgenson, are the numerous man-made barriers along migration routes that force the pronghorn to travel long distances out their way and further weakening them. "Something like a woven-wire fence is a pretty formidable barrier to a pronghorn," says Jorgenson. "I've seen pronghorn forced to walk seven miles out of their way to find a path around barriers, which expends even more resources, especially in a harsh winter. Part of what we're doing with our research is to identify and hopefully mitigate those potential migration bottlenecks."© 2011 Michael Forsberg /michaelforsberg.com
It's not known exactly how many pronghorn have perished along the banks of the Missouri, but Jorgenson says the numbers could be grim. "In July, Montana FW&P will be doing aerial surveys to get a handle on the number, but Michael was finding animals frozen to death," he says. "Animals that just got to a point where they couldn't go on. It's going to have repercussions."© 2011 Michael Forsberg /michaelforsberg.com
As for the pronghorn that remain trapped on the south side of the river, their fate, both short and long-term, remains cloudy. "It remains to be seen what some of the remaining pronghorn will do and how they'll fare," says Jorgenson. "I wouldn't be surprised to find that a lot of these animals will not go across, they'll just give up."© 2011 Michael Forsberg /michaelforsberg.com
So what does that mean for the future this herd's ancient migration and the future of the herd itself? "We don't know yet," says Jorgenson. "Some of them may try to stay and summer here, but I'm not sure what it's going to look like for the fawns."© 2011 Michael Forsberg /michaelforsberg.com
Estimates of mortality among the regions pronghorn range anywhere from 50 to 70 percent. Only the aerial surveys will give an accurate picture. But according to Jorgenson, a winter like this drives home the importance of making sure migration corridors remain open. "When you lose half of a population it makes identifying and protecting their natural travel corridors even more important."© 2011 Michael Forsberg /michaelforsberg.com
Sadly, there's not much that can be done for the pronghorn trapped by the raging waters of the Missouri. "We can't do anything about natural phenomena like a bad winter," says Jorgenson. "All we can do is learn from the experience and try to make changes to what we can control, like keeping travel corridors open, so the next time this happens maybe they're in better shape to deal with it."© 2011 Michael Forsberg /michaelforsberg.com

In Montana, the winter of 2011 was one of extremes: extreme cold, extreme snow and massive numbers of winter-killed big game. In many cases, deep snow forced herds of mule deer and pronghorn to gather on roads and railroad tracks, where many of them perished. Other herds were forced to migrate much farther south than normal in an effort to find open ground. One such herd of pronghorn, pushed far to the south of their traditional wintering grounds, was forced to cross the frozen Missouri River in an effort to find food and respite from the head-high snow. But as the snow and ice began to melt and the weakened herd started its return trip to summer fawning grounds, the pronghorn came across a barrier they had never encountered before: the raging spring flow of the Missouri River.