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Photo of deer.

Bounce Back

About 20 percent of deer in local herds died in parts of northeastern Minnesota this past winter. But a groundbreaking study of does' reproductive capacity shows that our north woods could soon be teeming with whitetails again.

By Gustave Axelson

During a typical winter, Tom Rusch finds about four dead white-tailed deer while snowshoeing in the woods in northern St. Louis County. This past winter Rusch, the Department of Natural Resources area wildlife manager in Tower, found 21 wolf-killed deer -- the most he's seen since the winter of 1995-'96. Not coincidentally, the 2008-'09 winter was the harshest for deer in northeastern Minnesota since a pair of back-to-back severe winters in the mid-1990s. The deer herd near Tower may have suffered a winter mortality rate as high as an estimated 20 percent.

Should deer hunters be concerned about the future of deer in our north woods? Not according to an exhaustive 15-year research study of white-tailed deer conducted by DNR wildlife research biologist Glenn DelGiudice.

Over a decade and a half, DelGiudice's project monitored 477 female whitetails outfitted with radio collars in and around north-central Minnesota's Chippewa National Forest. DelGiudice's project showed how and why the white-tailed deer population can thrive over time in northern Minnesota despite harsh winters, wolf predation, and hunting. And as the longest-running designed field experiment ever conducted on deer, it has helped to rewrite the book on what wildlife biologists know about deer survival and reproduction.

deer graph

Deer trends

Within the firearm permit areas where DNR researcher Glenn DelGiudice's study occurred, the deer population and harvest levels followed the same trends as the rest of the northern deer herd. By the early 1990s, deer numbers were booming. The severe winters of 1995-'96 and 1996-'97 caused a crash. Mild winters the following three years, and the suspension of antlerless permits from 1996 to 1999, helped the deer herd bounce back.

Illustration by Matt Kania

A Couple of Killer Winters.

The study was launched in Cass and Itasca counties in 1991. The northern deer population had been steadily rising for most of the 1980s, while the amount of conifer thermal cover -- cedar, fir, spruces, and pines -- had remained about the same. The limited conifer habitat for more and more deer looking to escape the cold and deep snow during winter was a primary management concern for the DNR. So DelGiudice designed a study to investigate how deer use conifer cover in winter and how much cover deer need.

Most wildlife studies run two to three years, but DelGiudice structured his project to run beyond the short-term vagaries of weather. The study's first winter in 1990-'91 was the mildest in 40 years. Five years later, the fiercest winter (1995-'96) in 40 years arrived. The DNR's winter severity index of cold temperatures and deep snow, which registered 61 the previous year in Grand Rapids, soared to 195 in winter 1995-'96.

"Thirty percent of the does in our study died that winter," DelGiudice recalls. "Many bald eagles didn't migrate to their winter ranges. There were so many dead deer, collared and non-collared, that when we checked the remains as part of our investigation, we'd sometimes see three or four eagles on or above the carcass. It was extremely unusual."

The following winter (1996-'97) was the second harshest winter in the previous 20 years. But deer mortality in DelGiudice's study was only 7.8 percent.

"There weren't as many deer competing for food, so that helped improve survival rates," says DelGiudice. "And not many does gave birth to viable fawns that survived following the first severe winter, so most of the summer there was no lactational drain on the does nutritionally since they were not nursing fawns. So the does were able to recover their nutritional health more quickly. Consequently, they were in better condition coming into the second severe winter.

"And the weak deer, the old and the very young, had already died, so only the strongest deer made up the population in that second severe winter."

Trap, Collar, Monitor

The DNR's 15-year deer study aimed to monitor the movements and survival of female deer, while also monitoring the health and survival of their offspring. Researcher Glenn DelGiudice's study employed new technologies and tremendous numbers of people and hours.

To capture does, DelGiudice and his team (field biologist Barry Sampson, volunteers Richard Nelles and Rod Schloesser, interns, and several graduate students including now-DNR wildlife disease coordinator Michelle Carstensen) used a trap-line of about 55 Clover traps. Clover traps are collapsible steel-bar framed box-traps, with a mesh door and sides. When deer walked through the opening and triggered the trip-line, the net door dropped.

Some deer were also captured with nets shot from helicopter, but Clover traps were the most economical means for capturing deer and collecting data throughout the winter. The researchers anesthetized captured deer, took blood and urine samples, and weighed them and fitted them with radio collars. Then to identify pregnant does, the researchers used a portable, field-sized ultrasound instrument to sensitively probe over a small, shaved area on the doe's abdomen and listen for the fast-beating hearts of fetuses.

To enable tracking of newborn deer, Carstensen and DelGiudice implanted a transmitter in each pregnant doe during winters 2000-'01 and 2001-'02. While giving birth, the doe would expel the transmitter, which would emit a more rapid homing signal when it hit the cooler air. This study marked the first use of this kind of transmitter in pregnant free-ranging deer. It increased the success of locating newborn fawns, sometimes within hours of birth, and significantly reduced the search time. It frequently allowed immediate, precise identification of the birth sites.

When the transmitters signaled a birth during the springs of 2001 and 2002, Carstensen and the interns rolled out to radio collar, blood sample, and weigh the fawns. The team monitored the overall health, nutritional condition, and survival of the newborns, as well as causes of death when they occurred. Bears and bobcats were the greatest predators of fawns, though wolves preyed on some young deer as well.

Over 15 years, 145 graduate wildlife biology interns from across the nation and other countries worked on the study. They gained real-world experience in capturing deer and monitoring their movements (from the ground and by airplane). They also learned to evaluate the physiological status, food habits, and survival of deer.

"One of the side benefits of this study was the field experience afforded to recent college graduates," DelGiudice says. "All of these interns had textbook knowledge. But this study introduced them to the realities of conducting a comprehensive field study. The study also provided higher-level research opportunities to University of Minnesota graduate students."


About 140,000 of an estimated 360,000 deer in northern Minnesota died during those back-to-back severe winters 12 years ago. Public concern about the decline in the deer population was so high that the Minnesota Legislature passed a bill mandating the DNR to provide emergency feed to deer in severe winters.

The following winters from 1997-'98 to 1999-2000 were three of the mildest consecutive winters on record. The does in DelGiudice's study responded with fawn production so prolific it surprised him.

Conventional wisdom among wildlife biologists held that deer fertility hit its peak at about 8 years old, but DelGiudice and graduate student Michelle Carstensen documented does as old as 16 years giving birth to twins. During winter captures, they recorded pregnancy rates of 97 percent for does 2 to 7 years old, and 99 percent for does 8 to 15 years old. Most does annually gave birth to twins.

These booming reproduction rates helped explain how northern Minnesota's pre-fawning deer population rebounded from a low of 221,000 in 1997 to 366,000 by 2000.

DelGiudice and other wildlife biologists had long contended that deer did not require the feeding program instituted by the Legislature. DelGiudice believes his study results confirm that contention.

"The legislatively mandated winter feeding in 1995-'96 likely had a positive impact on small numbers of deer in small local areas where the supplied food was easily and consistently accessible. But across the landscape, the feeding operation had a negligible effect on deer at the population level," he says. "Our study and population modeling did show that by restricting the number of antlerless permits issued over the next several years, the high survival during winters 1998 to 2000 and high reproductive capacity of these adult does allowed the population to recover within just a few years."

Another couple mild winters ushered in the new millennium, and DelGiudice's study continued to document the thriving fecundity of does. Across the northern third of Minnesota, the pre-fawning deer population ballooned to 454,000 by 2003. In spring 2009 the northern Minnesota pre-fawning deer population was approximately 437,000.

Wolves Come Home.

Winters of deep snow weren't the only deer-killers in DelGiudice's study. Seven wolf packs lived within the study area and preyed on some of the radio-collared deer. When DelGiudice began the study in 1991, wolves had just recently returned to the area, after having been extirpated from the woods around Grand Rapids in the 1960s. DelGiudice's team trapped and radio-collared wolves to determine their pack territorial boundaries, so they could ascertain which packs were killing which deer.

The ensuing data toppled several myths about how wolves affect deer populations. One myth held that wolves hurt deer populations by engaging in surplus-killing (taking more prey than they can eat, a behavior documented among carnivores worldwide). During the severe winter of 1995-'96, DelGiudice indeed documented incidences of surplus-killing by wolves, with some wolves taking down four deer within spans of 100 yards. But as he examined the fat reserves on the deer carcasses -- and evaluated their nutritional fitness from urea nitrogen in urine samples -- DelGiudice found that most of the wolf-killed deer were seriously undernourished and about to die anyway. The wolves were just finishing off deer on their deathbeds. Relatively little surplus-killing by wolves was observed during the following winter of 1996-'97, and none during any of the other 13 winters.

"Surplus-killing by wolves of healthy deer rarely occurred in our study," DelGiudice says.

His study also clearly demonstrated that wolves do not depress the female segment of the white-tailed deer population that hunters harvest in fall. While wolves and hunters each accounted for 24 percent of collared female deer mortality over the entire study, the two mostly killed different types of does at different times. The average age of a doe killed by hunters was less than 5 years old, whereas the average age of does taken by wolves was 8 years old. Age-specific analyses of the study clearly showed that compared with wolf predation, hunting had the greatest impact on younger does. Hunter-kills occurred during the fall hunting season, while wolves tended to kill deer in winter -- when deer are naturally undernourished, struggle in deeper snow, and are easier prey for wolves and their pack hunting strategy.

"Hunters tended to kill young deer, while wolves served the role of culling the herd of older deer or those that were weaker," says DelGiudice. "Hunters and wolves help to keep deer close to the carrying capacity of their habitat. And by ensuring that only the fittest deer survive, wolves help the deer herd to be stronger over the long term."

In the presence of seven wolf packs, the deer numbers in the region of DelGiudice's study rebounded. Between 1997 and 2003, hunters quadrupled their harvest in that area. By 2003 the annual harvest greatly exceeded pre-crash levels of the mid-1990s.

"These study results will help inform the DNR [when] Minnesota assumes responsibility for wolf management," says DelGiudice. Wolves in the western Great Lakes region are currently listed as a federal endangered species, though wolf populations are recovered and the U.S. Fish and Wildlife Service has been working to transfer management back to state jurisdictions. "[The study results] will also help Wisconsin and Michigan as they contemplate deer management in an expanding wolf range. And for any states considering the reintroduction of wolves, this study tells them how deer might respond."

Eat, or Be Safe?

DNR wildlife research biologist Glenn DelGiudice's study of radio-collared deer also shed light on winter migration to deer yards. In winter deer travel an average of five to 10 miles to areas where conifer stands are prevalent. Among the conifers, the snow isn't as deep, food is more accessible, and deer can tramp down trail networks for escape from wolves.

As DelGiudice monitored the deer yards, he noticed the deer soon had tough choices to make about finding more food.

"During severe winters, once deer overbrowsed the browse patches available around the edges of the conifer shelter, they had to make decisions about venturing out through the deeper snow beyond those shelters to access more distant feeding areas," he says. "Do they continue to spend most of their time within the dense conifers where the snow on the ground may be up to 45 percent shallower, conserving energy but with less and less available to eat? Or do they expend greater energy moving through the deeper snow to increase their dietary intake -- and increase the risk of being preyed upon by wolves? "It was an energetics balancing act. And [the decision to leave dense conifer stands or stay put] depended largely on the amount of fat the does had coming into winter and how quickly it diminished as winter progressed.

"Most apparent during severe winters was deer staying close to the dense conifer stands until the snow cover beyond had diminished significantly."

Preparing for Another Winter.

Nobody knows what Mother Nature has in store for Minnesota and its northern deer herd this winter. But the DNR will be conservative in managing the fall hunt nonetheless.

"We have a flexible deer permit system built just for these kinds of situations," says Rusch, the Tower area wildlife manager. "We can vary antlerless deer permits across our deer management units as conditions require."

Rusch says that six out of the seven deer management units around Tower will have more restrictive antlerless permit regulations this fall. Last year hunters could buy up to five antlerless permits, but this year the system will return to an antlerless lottery.

"The past five deer seasons have been the most liberal we've ever had in northeast Minnesota. The intent of the liberal seasons was to decrease the deer population," says Rusch. "Now the local deer population is at, or slightly below, goals in our area. As a result, we will reduce the antlerless harvest to allow the population to rebuild to goal levels."

Based on his 15 years of deer data and a planned conservative harvest this fall, DelGiudice is confident that the northern deer herd will be fine in the long run regardless of what this winter brings.

"A severe winter to us isn't necessarily severe to deer. They're a lot tougher than many think they are," he says. "We studied an area saturated by wolves, with high hunter access, and some historically severe winters, and yet white-tailed deer still thrived."

DelGiudice sees a similarity between the deer he studied and his own family history.

"Those 15-year-old does that were still twinning, their fawns might have had smaller skeletal structures and higher neonate mortality rates, but you've got to remember that the offspring of those old does still might have a genetic leg up," he says. "One of my grandfathers lived to be 99, and I gotta say, I like those genes."

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