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Mule deer on the move: Study finds that they'll travel more than 50 miles to summer and winter ranges

by CHRIS PETERSON
Editor | September 1, 2021 9:25 AM

The mule deer of Northwest Montana are largely travelers, sometimes migrating as much as 50 miles from their winter range to the summer range, a three-year study by Montana Fish, Wildlife and Parks found.

Since 2017, FWP biologists have collared and captured 134 total mule deer does in three distinct habitats. Forty-nine mule deer does were caught on the Rocky Mountain Front, 41 in the Cabinet-Salish Range and 44 in the Whitefish Range.

Not all the deer kept their collars — or survived the study. But on the Rocky Mountain Front, for example, of 44 deer that were studied, 34 were considered migratory, nine were residents and one was residential and migratory.

While some deer stayed in the same home range, which is to say they spent their lives both in the same winter and summer range, many were found to travel during the summer months to the proverbial greener pastures, looking for prime habitat not only to feed, but to raise their young.

For example, one doe captured on the west side of the Whitefish Range traveled to Glacier National Park’s North Fork to summer there. Others traveled across the divide into Canada. Another doe on the Rocky Mountain Front traveled from the front, over the Continental Divide and the Bob Marshall Wilderness to summer near Seeley Lake, noted the study’s lead author, Nick DeCesare.

In the Cabinets, of the 40 deer collared, 33 were migratory. In the Whitefish Range, of the 44 collared, 31 were migratory.

Spring migration typically began in May, but with some deer, started as early as March.

In the fall, the return to winter range usually started in October and November. Summer home ranges were larger than winter range.

Perhaps most interesting were the Cabinet-Salish Range deer. Deer on the east side of Fisher River rarely crossed it to the west and deer on the west side rarely crossed to the east.

The quality of summer range is critical for mule deer so they can survive the opposite season _ winter.

‘Accessing high quality forage during summer is critical for mule deer because it strongly influences over-winter survival,” the study notes.

The drive for quality food and habitat outweighed other risks, of which there were plenty. But the greatest risk wasn’t wolves or bears or even people, it was mountain lions, the study found.

“In all study areas, mountain lion predation was the leading known cause of mortality, imposing 6-11% annual mortality upon adult females across regions,” the study noted.

Of the 26 predator-caused mortalities in the study areas, 21 were caused by mountain lions, four by wolves and one by coyotes.

The hunting season for mule deer in Northwest Montana is limited to bucks only in most areas, so hunting wasn’t a factor in doe mortality.

The greater scope of the study isn’t so much about predators, as it is about habitat. Biologists drilled down to the specific plant species and type of habitat the deer were using in the summer and winter.

In winter, it’s more about maintaining energy. Mule deer like wind-blown areas, where the snow isn’t as deep.

In summer it’s about quality forage. Mule deer particularly like wild strawberries. It was 14% of their summer diets. They also feast on shrubs and forbs. Shrubs were the greatest part of the diet (53%) in the Cabinets and Whitefish ranges, while forbs were the bigger part of the diet (57%) on the front.

Two wooded areas are good at providing those —logged areas, and relatively recent areas of wildfires.

While FWP doesn’t manage the lands in the study, it does consult with agencies like the Forest Service, timber companies and the state on land management decisions.

The overarching goal of the study is to not only identify the habitat and movements of mule deer, but to eventually improve them over time, the study’s authors noted.

For example, land management agencies might harvest timber in such a way to promote shrub growth, or do a prescribed burn in an area of summer range for a similar effect.

The migration routes and range locations of deer are passed down through families from generation to generation.

“It’s kind of neat that they do the same thing over and over,” DeCesare said.

But it also poses challenges, because deer could show up to historic ranges, only to find the habitat drastically altered.

That’s been an issue in Wyoming, with oil and gas development.

In that case, deer avoid drill pads and the development associated with them in their winter range. As a result, populations dropped by 46% in some places, according to studies in that state.

The study is still ongoing. A final report is expected to be completed by the end of the year.