The Minnesota DNR recently completed a nearly four-year study looking at deer movement specific to southeastern Minnesota that will help in the agency’s management of chronic wasting disease while also providing hunters some interesting information from one of the state’s top deer-hunting regions.
The large study area centered around the town of Preston in Fillmore County. Prevalence rates for CWD remain low in the southeast (about 1%), but the disease has been most persistent in this area since it was discovered near Preston in 2016.
From 2018-2020, a total of 229 deer were captured, tagged, and collared with GPS units. The study focused predominantly on 1- and 2-year-old deer because that is the age group most likely to move the farthest as deer leave their birthplaces to establish new home ranges.
A total of 104 females and 104 males were captured for the study. An additional 21 adult bucks, ages 2 and older, were also collared in 2018.
A primary focus was to see how many deer dispersed from their birthplaces and how far they traveled to establish new home ranges. This gives managers up-to-date data on how large CWD management zones should be to best limit the spread of the disease.
CWD, an always-fatal prion disease in deer, spreads between animals through bodily fluids or indirectly through environmental contamination of soil, food, or water.
“When you’re trying to manage disease in wild deer, you want to have some control measures that could really impact the disease,” said Michelle Carstensen, Minnesota DNR Wildlife Health Group leader. “You don’t want to go too big because it’s really hard to manage things at a scale that’s beyond your resources or, really, public acceptance. You also don’t want to pick an area that might be too small and you’re not being effective in any type of control strategy.”
What they found
Wildlife researchers observed that 21% of female yearlings and 58% of male yearlings dispersed from their birthplaces to establish new home ranges.
On average, does moved about 12 miles from their birthplaces, while bucks dispersed 14 miles. Typical annual home ranges for these young deer were about 1-square mile, while 20% of females and 6% of males migrated between seasonal home ranges.
The DNR has been using a 15-mile radius for its management zones around areas where the disease has been detected.
“I think our initial designation of zone sizes in the southeast was appropriate for where we found the disease in the Fillmore County area and now in Winona,” Carstensen said. “However, now we’re seeing some movement of the disease and spread into the more periphery. We do have a control zone currently in place that is sort of a buffer around where we’re doing most of our management. We’re seeing some cases push into those boundaries now. They aren’t quite in the control zone, but we’ve had a couple near Rochester, for example.”
Carstensen, who is the current lead for the study, said she was a bit surprised by the average distances the deer traveled because of the good mix of habitat features in the southeast.
This is one of the highest deer-density regions in Minnesota due to a good mix of food sources and security cover. The study area was about 60% agriculture, 24% forest, and 7% developed, with the rest in water, grasslands, and wetlands.
“Often when there are some higher densities, deer feel the pressure to move farther,” Carstensen said. “They’ll feel they have too much competition in a local area even if we view the resources as being adequate. That balance between deer density and dispersal behavior can really impact how far they’ll move, even if they might traverse habitat that we think is quite good as they’re passing through.”
Long dispersers and western movement
Two interesting observations of the study came in a couple of long dispersers and the direction in which the deer chose to travel.
One doe in the study traveled 77 miles. The longest-ranging buck moved 54 miles.
“It’s really interesting to make those maps and show people how far (deer) can go,” Carstensen said. “For a juvenile female to be our long disperser was exciting and a little terrifying because if that happened to be an infected deer that moved from Harmony to Cannon Falls, we just moved the disease almost 80 miles in the state.”
LISTEN TO THE WHOLE INTERVIEW WITH MICHELLE CARSTENSEN
(Editor’s note: The study finished in August of 2022. Not August of 2020 as is stated at the beginning of this interview.)
From a management standpoint, Carstensen said, they look at these long dispersers as an anomaly.
“Statistically, they’re an outlier,” she said. “What we’ve been using is a 15-mile radius around new infected cases to help frame where we want to manage CWD the most. That did capture 82% of all the movements of deer in our study. If we would have went out to 95%, that would have expanded zones to 27 miles around individual deer. That would have been at a scale we probably couldn’t manage, but that’s what happens when you include those couple of outliers.”
The deer in the southeast study had a tendency to move in a westerly direction. Some moved into Iowa, but not a single deer in the study ended up in Wisconsin. That was noteworthy for managers, with CWD being persistent in the southern part of Wisconsin.
Carstensen didn’t have any firm theories as to why no deer moved east across the border.
“One would think that maybe the Mississippi (River) would be a barrier, but Wisconsin has also found deer that can move across these bigger rivers,” she said. “I don’t think it’s a solid barrier.”
The drivers of dispersal
What wildlife biologists are finding as more and more GPS-collared deer studies take place is that deer move differently on different landscapes.
A written report from the Minnesota study in 2021 compared and contrasted a similar study of deer movement from Pennsylvania. That study area featured heavier tree cover – about 62% forested – and only 26% agriculture.
Results from the Pennsylvania study showed 4% of female yearlings and 68% of males dispersed, but typically at much shorter distances than in the Minnesota study area. Deer dispersed about 2 1⁄2 miles on average in the more forested landscape of Pennsylvania, in an area that features fairly similar deer-density estimates as the Minnesota study area.
Carstensen said there is more work to be done to determine exactly what influences dispersal rates and distances the most in different landscapes. It’s likely dependent on multiple factors such as cover on the landscape, food availability, and deer population levels.
Carstensen pointed to a deer-movement study from the Grand Rapids area in northern Minnesota from almost 20 years ago that showed deer would delay seasonal movements based on how harsh the winters were. Mild winters delayed deer moving to new winter ranges.
“The timing of harvest and the type of crop cover being planted each year I think can affect movement, and it also can vary between years,” Carstensen said. “We do these projects, and we have a snapshot in time from one year, two years, or three years. You could really have different results in those years based on weather patterns, harvest and deer don’t always make the same decisions year after year.”
More long-term data is something Carstensen wishes they would have been able to better gather from the southeastern Minnesota study. Legal harvest of deer from hunting and vehicle collisions were the main causes of death for deer in the study.
Overall, 40% of collars used also failed within about a year of being on the animals.
“Unfortunately, that’s kind of a risk in all these types of studies using collar technology to track animal movements,” Carstensen said.
Poor collar performance on the 21 adult bucks in the study limited the data biologists had on how those older bucks may have expanded their movements in the fall.
The probability of infection increases with age in bucks and does, with adult bucks being about twice as likely to be infected with CWD, according to the National Deer Association.
“I would have loved to have more in the study and be able to really understand more of that (adult movement), but collaring growing animals is really a challenge,” Carstensen said.
What’s next to learn?
The next phase of the Minnesota study this winter is to further analyze the data to look at what might be barriers to or facilitators of movement.
“Trying to understand more about those travel paths that they’re choosing, because they could be leaving prions behind as they move,” Carstensen said. “Then they have temporary places that they might stay for a while before they make a new decision to travel farther. Are those travel paths then at high risk and can we predict what they might be? Those types of things are going to be interesting for us to understand so we know more about where CWD might spread.”
The next phase will mimic similar work being done in Wisconsin and Michigan. All three study areas from these states have different landscape features that should allow researchers to better understand what influences movement.
The Minnesota DNR will partner with the University of Wisconsin-Madison and the Wisconsin DNR in this upcoming phase of the research. Those studies from Wisconsin also will bring more movement data from adult-aged deer into the analysis as well.