An average of 2,600 vehicles travel to and from the Boise area on Idaho 21 each day.
An estimated 90 percent of a herd of 5,000 to 8,000 mule deer and a herd of 300 to 1,800 elk cross the highway each fall, when they come down from the mountains to the Boise Front and the Lucky Peak area.
Some deer dart across the highway, dodging traffic. Others linger on and near the road.
Ed Bottum, a wildlife habitat biologist who has worked for Idaho Fish and Game for 25 years, recalled seeing a huge amount of blood on the highway after a crash a couple of years ago. It was the result of a motorcycle colliding with a deer — the pregnant doe was split in half. The motorcyclist spent the night at a Boise hospital but didn’t suffer any life-threatening injuries.
“I found little pieces of that motorcycle 100 feet away,” said Bottum, who urges motorists to slow down and drive cautiously when the deer are migrating, generally October through April.
To reduce such collisions, Fish and Game biologists worked with Idaho Transportation Department officials to create a safe passage under the highway — Southwest Idaho’s first wildlife underpass.
With $800,000 in federal stimulus money, the underpass was completed in October 2010. ITD also installed an 8-foot-tall fence with galvanized steel posts and wire along the highway to help keep the deer off the road.
Fish and Game biologists have installed additional fencing on the eastern slopes above the highway to guide deer to the underpass, but more is needed to make the underpass work best. They need to raise about $140,000 to finish the last half of the roughly 1.5 miles of fencing.
The location — past the Mores Creek Bridge, or High Bridge, at milepost 18.2 — is a collision “hot spot” where a large number of deer have been hit through the years.
From 1979 to 2001, cars killed 159 deer in the area of the wildlife underpass — about seven per year.
The number killed in that area this migration season: zero.
The biologists say other factors, including migration numbers, also likely contributed to the drop in collisions this year.
HOW MANY LIVES SAVED?
Counting how many deer are hit by vehicles along Idaho 21 northeast of Boise each year is relatively easy.
The annual tally ranges from 50 to more than 200 for the 33-mile stretch of road from the highway’s intersection with Warm Springs Avenue to Robie Creek.
But it’s difficult to determine how many deer (and humans) have been saved from violent deaths since the underpass was installed.
There is ample evidence that deer and other animals are using the underpass. Cameras have captured deer, elk and other wildlife passing safely under the highway, often in groups.
“There’s tracks all the time,” Bottum said. “It works. We know it works.
“I’m confident saying there are hundreds of animals that use it.”
Bottum and a fellow biologist had hoped to get a count of the animals from the still images, but that proved difficult because animals aren’t one-way travelers. They go back and forth and meander.
ITD officials are pleased with the results, even if savings in lives, injuries and damaged vehicles can’t be quantified.
“We believe the crossing to be a great success to date and expect that when the rest of the wildlife fence is up, it will only increase its effectiveness,” said Greg Vitley, ITD Southwest Idaho environmental planner.
He said it has generated enthusiasm for partnerships and other “vehicle/wildlife mitigation” elsewhere in Idaho.
The underpasses have been successfully used in Wyoming, Arizona, New Mexico, Colorado and Banff National Park in Alberta, Canada, Bottum said.
STUDYING THE HERDS
Fish and Game wildlife biologists spend two or three days each December assessing the migrating mule deer herd throughout the 36,000-acre Boise River Wildlife Management Area. They’re looking for male-to-female ratios and fawn-to-doe ratios.
Those ratios provide an indication of the biological health of the deer herd, said Michelle Commons Kemner, a regional wildlife biologist for Fish and Game.
For every 100 females, there are about 60 to 80 fawns (the more the better, for a stable to increasing herd). The number of bucks varies; an ideal ratio is 25 bucks per 100 does, but 15 bucks per 100 is more typical.
This winter, the count found just about one-third of the deer typically found in the lower elevations of the Boise River area.
The mule deer that did migrate came in November, about a month later than usual. There’s usually a four- or five-day period between Oct. 15 and Nov. 15 when they migrate in large numbers, usually in family groups of 12 to 20.
With mild winter conditions, many didn’t migrate as close to Boise as in years past. The animals’ survival strategy is to pack on as much fat as they can during summer and fall, and then try to conserve their energy resources.
If the snow is deep in the mountains and they can’t forage for food, they migrate to lower elevations.
“They’re not going to come down and stand near us if they don’t have to,” Bottum said.
Each year for at least the past decade, Fish and Game biologists have caught and put radio collars and ear tags on about 25 fawns and five to 10 does.
They’re weighed, measured and checked for body fat.
“The fatter, the bigger the (fawn), the more likely the animal is going to survive,” Kemner said.
LEARNING FROM ROADKILL
Fish and Game biologists and ITD workers regularly pick up deer carcasses off the highway. A large pit behind their Idaho 21 office at milepost 15 is used for carcass disposal.
There are good years and bad years.
Krista Muller, a senior wildlife technician, said in winters past she has spent a good two hours a day retrieving carcasses from the road. She uses a winch to load them into a Fish and Game pickup.
“It was really hard for me. I had to put a couple animals down that were hit,” she said.
A map on the wall of the Idaho 21 Fish and Game office shows how many deer and elk have been killed every tenth of a mile on Warm Springs and Idaho 21 in Boise out to Robie Creek for the past 33 years.
The numbers represent a minimum count, because sometimes carcasses are removed by animals or passing motorists. A law passed earlier this year makes it legal for people to pick up roadkill along Idaho highways; those who do so are required to notify Fish and Game.
Wildlife biologists examine the dead animals to determine age, sex, species, disease, parasites and more.
For example, they can tell how many offspring a doe has produced. Examination of the animal’s bone marrow indicates how healthy the animal was before it died.
“You can gather a lot of biological data from a carcass,” Bottum said.