Fawns Thrive During Mild Winter
On a snowy day in early January a helicopter hovers over a hilltop in southwest Idaho. A crew of biologists and technicians from the Idaho Department of Fish and Game waits on the ground amidst a series of nets. Suddenly a small mule deer bounds over the horizon, running from the chopper. The fawn races down a hillside straight into one of the nets.
The Fish and Game workers spring into action, binding the deer's legs and putting a blinder over its eyes. Then they test its health, looking for injuries, measuring its legs and weighing it. The information they gather provides clues about whether the animal will survive until spring.
"Probably the most significant determinant of whether a fawn is going to survive the winter is its weight and condition going into the winter," said Jon Rachael, Southwest Regional Wildlife Manager "So the higher the weight going into the winter the higher the likelihood that fawn will survive the winter."
This year the fawns were heavy thanks to abundant food late last summer.
"The weights are as high as we've seen, so we predicted we were going to have very high survival." Rachael said.
After they measure and weigh each fawn, Fish and Game workers fit it with a collar using surgical tubing that will eventually decompose allowing the collar to fall off. Throughout winter a small radio transmitter in the collar provides Fish and Game with valuable information.
Wildlife technicians spend a great deal of time each winter tracking the collared fawns. Each radio collar transmits signals on its own frequency. Technicians tune into the transmitters with radio receivers to track the movement of the fawns. They enter all of the information they gather into a database using laptop computers. Most of the work is done from the cab of a Fish and Game pickup truck, which might be parked in an open area or meandering down a road where the terrain is steep.
If a collar has not moved for more than four hours, the transmitter will put out a different kind of signal known as a "mortality signal." When a technician receives a mortality signal he assumes it is coming from a fawn that has died. At that point the technician must leave the road, connect a receiver to a directional antenna, and set out on foot to track down the collar.
If the technician finds a dead fawn he conducts a "field necropsy" to determine how it died.
"You'll be able to tell where the animal was taken from, by the neck or front quarter or rear hind. There are little puzzles you can figure out from tracks. Things like that," said wildlife technician David Collins.
Those pieces of the puzzle often add up to a very clear picture of how the fawn died. In some cases the clues show that it died of malnutrition. This is often the case in an area with a habitat problem or one that has been hit hard by winter. A carcass may show clear signs of being killed by a predator, and the trained technician is often able to determine what type of predator it was. Sometimes a technician honing in on a mortality signal will find the radio collar and nothing else. In this case he must also do some investigative work to determine whether a predator has moved or completely devoured the fawn or if the fawn has simply shaken the collar. In some cases there simply isn't enough evidence to draw any solid conclusions. In these cases the technician will record the cause of mortality as "unknown."
This year technicians have generally recorded very little fawn mortality.
"This year our survival rate has been between 87 and 96 percent in our (Southwest Region) study areas, which is about as high as we've had, so fawns made it through the winter very well this year and seem to be in pretty good shape," Rachael said.
The lowest survival rate has been 62 percent in Unit 36b in the Salmon Region, where 13 of 21 captured fawns are still alive. On the other hand one of the units with the highest survival rate is Unit 30, also in the Salmon Region where 24 of 25 fawns have survived.
Mule deer herds are showing varied results in the focus area of the Mule Deer Initiative. Study areas in the Southeast, Magic Valley and Upper Snake River regions reveal lower survival rates, probably due to more severe winter conditions than those found in other regions.
The statewide average among collared mule deer fawns is better than 80 percent survival, which is unusually high. That will likely mean good news for Idaho hunters because Fish and Game uses the information from the fawn monitoring program to set seasons that satisfy the desires of hunters while keeping Idaho's mule deer herds strong.