In April, a female deer heads east, leaving her winter range on the North Platte River in Carbon County. She’s heading for the high ground near Coad Peak, perhaps to give birth to a fawn, or in search of summer forage and timber cover.
She’s moving in an age-old pattern animals like her have done year after year, but she was also the subject of a recent Platte Valley Migration Corridor local working group meeting on Jan. 22.
“Deer 36 … wintered on the North Platte, which is a good winter range in that it remains open for a good chunk of the winter. Deer can continue to forage without having that constant energetic battle of finding food,” Embere Hall with the Game and Fish explained. “In late April, she makes her return across Highway 130 to her summer range on the south side of Coad. She stays near Coad in the summer, and if she had a fawn, this would be the time to be taking in a diversity of forage, spending time in timber that provides cover.”
The Platte Valley is home to 12,000 mule deer that winter in the valley and move to summer ranges in the Snowy Range, Sierra Madres and foothills of North Park, Colo. The region was designated an official migration corridor in 2018 by Gov. Mark Gordon, and a group of Wyoming residents are currently considering how best to balance ecological needs and development opportunities in the area.
According to Hall, animals collared in Hunt Area 79 covered an average distance of 38 miles in a single fall or spring migration, and animals in Hunt Area 81 averaged a migration distance of around 24 to 26 miles.
Jill Randall, the Game and Fish’s big game migration coordinator, said the state has been gathering data on mule deer movement for many years, prioritizing areas considered “stopovers and bottlenecks” for their importance in the migration corridor.
“A lot of the work shifted into threats, barriers, things that we needed to get a better handle on,” Randall said. “We have learned that there are different strategies for animals within one population that will use a corridor.”
Migration distance from animal to animal may vary from long, medium or short treks within one herd. All the animals in a single herd may use the same winter range, but each animal can employ different strategies to move across a corridor.
“Although they are all certainly very important, we recognize that those long distance migrants experience the biggest threats,” Randall said. “That may be extra roads, extra fences, extra development that they are exposed to because they are covering a greater landscape during their seasonal migrations.”
Animals that migrate longer distances sometimes leave the winter range up to three weeks before other animals, and the Game and Fish is studying their movements as individual animals and as a herd. The goal is to understand the different strategies and components of behavior within one herd, so as to best preserve an entire migration corridor. Three have been designated in Wyoming: the Sublette Mule Deer, the Baggs Mule Deer and the Platte Valley Mule Deer migration corridors.
Not all parts of a corridor are created equal, Randall said.
“We know high-use corridors, and the interior part of that corridor, are the most important and critical for conservation action. That helps us to prioritize some of the work we do on the ground,” she said.
The timing of migration is based on annual conditions, as weather and forage availability impact when an animal leaves its summer or winter range. But animal memory as opposed to instinct is what guides the deer year after year.
“They learn from mom in their first year. That path, that habitat selection they do throughout their life, is something they learn right off the bat and they continue to have that fidelity to the route,” Randall said.
The Game and Fish has also researched how a disturbance, whether it is a housing development or an energy field, affects movement.
“We have incorporated some of that into our land use decisions and permitting recommendations,” Randall said, citing a 2020 report by Hall Sawyer called “Migratory Disturbance Thresholds with Mule Deer and Energy Development.” That report suggested that declines in migratory use declined when surface disturbance from energy development exceeded 3%. Deer have been documented to move more quickly through significant levels of disturbance, missing potential stopover ground and high-quality forage, Randall said.
“They frequently will stall before they enter a disturbance, or a development area, and they rush through it. The result of that is really getting out of sync,” Randall said. “We know how important the foraging opportunity is during migration, being out of sync is causing nutritional detriments to those individuals over the long term.”
Rapidly expanding residential areas can have the same impact, she said.
“It is not just one type of disturbance that can affect the behavior of deer,” Randall said. “It is documented pretty well that less development is better for mule deer, but really the important thing is, what level is acceptable? We’re maintaining that priority of keeping our population healthy and abundant, but we also want to know how much energy development can occur and still have both. Energy is a requirement, and it has to happen in the state of Wyoming.”