Deer need the right types of food and shelter in order to thrive.
Mule deer have small stomachs, compared to their overall body size. This makes them much more selective in their diet. Instead of eating large quantities of low-quality forage, deer select the most nutritious parts of certain plants. These plants vary from season to season.
Springtime grasses and forbs
Grasses and forbs (leafy, flowering plants) make up the bulk of a deer's early spring diet. In early March, motorists often see herds of deer feeding on the succulent plants that grow alongside many Utah highways. (This is a good time of year to slow down and be especially careful on the roads.) As the snow melts — and the vegetation begins growing up the mountainside — the deer follow the foliage to the higher elevations of their summer range.
Summer forbs and fall shrubs
In the summer, mule deer continue eating a combination of grasses and forbs. Examples of forbs include dandelion, mountain bluebell, sawtooth butterweed, kochia, tailcup lupine, clover, broadleaf filaree, globemallow and many other types of broad-leafed vegetation. In late summer, as the grasses and flowering plants dry out, mule deer shift their diet to shrubs.
The condition and availability of summer and fall forage is critical. Mule deer use it to build up their fat reserves for the winter months.
During the winter, when most of the other vegetation is covered by snow, deer survive by feeding on exposed browse. Browse is the leaf or twig growth of shrubs, woody vines or trees. Examples of commonly eaten shrubs and browse include sagebrush, bitterbrush, mountain mahogany, cliffrose, rabbitbrush, scrub oak, serviceberry and willow.
The growth of Utah's deer populations is particularly limited by the amount of food available on winter range. If a winter-range area loses its sagebrush and other browse, deer populations will correspondingly decline.
A gradual transition
Deer don't abruptly switch from one seasonal diet to the next. Their digestive systems are sensitive and need time to gradually adjust to any changes. This is one reason why the DWR asks the public to avoid feeding the deer. Suddenly changing a deer's diet can easily lead to the deer eating food that it cannot digest. In these situations, a deer can become malnourished — or die — even if there's food available.
To find high-quality forage, mule deer migrate to the best range areas. This migration can be difficult because urban sprawl, vehicle collisions, predation and other factors interfere with migration. Although some deer herds may not migrate much at all, others travel more than 75 miles on an annual basis just to reach an area that has sufficient habitat.
Precipitation (or the lack of it) is the most important factor in the growth or decline of mule deer populations. The amount and timing of moisture can affect:
Most rangelands throughout the West do not receive great amounts of moisture. Utah is now rebounding from drought years that took a toll on mule deer populations. In a drought year, there are fewer plants, and they have reduced nutritional value. Does cannot produce enough milk to adequately nourish fawns, leaving the fawns too weak to survive their first year of life.
At the opposite end of the spectrum, too much precipitation — in the form of heavy winter snowfall — can also swiftly diminish herd numbers. If the snow is too deep for deer to find food, it is also very difficult for them to travel through. They will expend their limited fat reserves searching for forage.
For more information on how precipitation affects mule deer herds, see page 12 of Mule Deer: Changing Landscapes, Changing Perspectives — (PDF format).
Sagebrush is a critical food source for mule deer, especially during winter. Its leaves contain higher protein levels than other available shrubs and grasses.
Unfortunately, range management professionals have seen a common problem across many of Utah's sagebrush rangelands — they are suffering from the ecological equivalents of old age and malnutrition. Many of the state's critical winter-range areas are dominated by old trees and shrubs (40 or more years of age). These plants have lost much of their nutritional value and don't provide adequate forage for mule deer.
The best habitat for deer
Mule deer thrive in habitat that has all of the following characteristics:
To help mule deer herds recover in Utah, the DWR and other partners of the Watershed Restoration Initiative are focused on removing many older plants (sagebrush, pinyon pine and juniper trees) and restoring critical habitat — including young sagebrush and other shrubs — on winter range.
During the winter months, deer conserve energy by limiting their movement. They also seek shelter among evergreen trees and shrubs, including Utah juniper, pinyon pine, mountain mahogany and tall sagebrush plants.
Areas with large amounts of this vegetation shelter deer from the wind. These areas are also warmer than open areas because the plants absorb heat during the day and then radiate the heat at night. Whenever possible, deer try to find cover near exposed forage. That allows them to feed without using a lot of energy to search for food.
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