DEER WINTER NATURAL HISTORY

Teacher Background

This article focuses on the winter ecology of the white-tailed deer (Odocoileus virginianus) and the mule deer (O. hemionus).  Deer belong to the family Cervidae, which in North America also includes elk (wapiti), moose, and caribou.  The Cervidae are large two-hoofed mammals which shed their antlers annually.  They feed on vegetation, hence are ecologically classified as herbivores.  Because they possess a compound stomach composed of specialized parts (rumen, reticulum, omasum, and abomasum) they are able to digest woody browse, and are also referred to as ruminants.

Habitat Preferences

The distributional range of white-tailed deer includes eastern North America north to near tree line, south to equatorial South American and west to Oregon and Washington (Baker 1984).  The mule deer's range is restricted to western North America, from southern Yukon Territory in Canada south to northern Mexico.  Its eastern limits are the midwestern Great Plains states (Anderson & Wallmo 1984).  The two deer species do occur together, or sympatrically, in parts of the Rocky Mountain and Great Plains states.  Where they are sympatric, the mule deer occupies higher elevations and rougher topography, while the white-tails segregate in river bottom ravines, dense pines, and agricultural areas (Swenson et al. 1983, Baker 1984).

Both white-tailed and mule deer are extremely adaptable, and are thus found in a variety of biomes and habitats (Wallmo 1981, Baker 1984).  They can live in the mountains, prairie, and desert; deer habitats include agricultural fields, grasslands, forests, swamps, marshes, and bogs.  Obviously, some of these habitats are more favored and can support higher deer populations than others.

Severinghaus and Cheatum (1956) point out that the white-tailed deer's preferred environment, where it also produces the highest populations, is in ecotone or edge habitats.  In these habitats, fields and forests come together, producing an area where shrubs and low growing trees prevail.  Mature forests are relatively devoid of deer; the crowns of trees produce a closed canopy which shades the ground and prevents the growth of the low shrubs and saplings needed for food. In ecological succession, the highest deer population densities are reached during the shrub-sapling stage as shown in Figure 1.

deer diagram

Figure 1. Generalized diagram of changes in deer forage supply and population density during secondary succession (adapted from Wallmo and Schoen 1981)

 Habitat preferences vary seasonally.  During the coldest winter months (January-March), both white-tailed and mule deer may congregate in yards, especially in the northern parts of their range.  This yarding behavior may be an evolved response to predation, especially by wolves.  The choice of a yarding area is based more on its sheltering properties than on its food supply.  Conifer swamps with an otherwise ample white cedar food supply are avoided by white-tailed deer if they offer little cover and deep snow.  The important characteristics of a wintering area are (1) the presence of an overhead vegetative canopy that retains heat on cold nights and (2) vegetative understory dense enough to provide shelter from prevailing winds (Verme 1965). 

In regions of deep winter snows, both mule and white-tails seek lowland regions because these areas are sheltered and have less snow.  White-tailed deer in the north seek the heavy cover of conifer swamps containing cedar, spruce, balsam, and hemlock trees.  Often these areas are used by many deer populations, some migrating distances up to 88 km from their summer home range to the wintering area (Marchinton & Hirth 1984).  Farther south, white-tailed deer seek wooded valleys with mountain laurel and rhododendron, and, in hilly country with little woodland they congregate on the southeast slopes of hills.  Mule deer in the west select pine-dominated woodland or riparian areas in the winter (Swenson et al. 1983).

Life History and Feeding Cycles

While exact dates may vary, an important event in the yearly life history cycle of white-tailed deer is the rut or mating season, which occurs in autumn (late November-December).  Prior to the rut, deer spend more of their time feeding than in any other activity (Michael in Marchington & Hirth 1984).  Accumulation of fat reserves occurs over the summer, peaking in September.  In August or September, white-tails loose their red summer coat and acquire a grey-blue winter pelage.  This coat consists of stiff, hollow air-filled guard hairs which cover a thicker underfur of fine wooly hair.  The hollow guard hairs serve as insulation through a thermos-bottle effect.  This pelage persists until it is shed the following spring.  The deposition of subcutaneous fat along with the insulative winter coat are adaptations that reduce winter heat loss.

In preparation for rutting, the stag's testosterone levels increase, its neck thickens, its level of aggression increases, and it rubs off its antler velvet on shrubs and sapling trees during September-November.  During the rut, the stag's relative food intake decreases precipitously as it feeds little while concentrating on competing with other stags for females.   

Stags shed their antlers at the end of the rut and are antlerless from about mid-December until April, when new antler growth commences.  The size of the antler rack is related to the stag's age and to its past nutritional condition.  Large and/or older bucks have larger antler racks, are more dominant over smaller bucks, and obtain more copulations with does.

After the rutting season, the activity of deer in winter is vastly reduced.  Although they make a network of trails as they move around to feed, they sleep a great deal, surviving on their summer-accumulated fat reserves.  At night, deer avoid excessive heat loss by bedding down under dense canopy near the trunks of trees (Armstrong et al. 1983).  Occasional feeding bouts tend to occur during the warm midday hours.  The slow metabolic rates of deer in midwinter is reminiscent of some aspects of hibernation (McMillin et al. 1980).  Thus, even in the presence of adequate feed, deer lose weight in winter as part of their annual life history cycle (Moen & Severinghaus 1981).

Factors Affecting Winter Deer Survival

The duration of snow cover and severity of the winter are important factors determining the survival of deer over the winter (Verme 1968, Wallmo & Regelin 1981, Sauer & Severinghaus 1983).  Deer are able to remain in areas which accumulate up to 45 cm (18 in) of snow.  However, with increasing snowpack, the amount of available forage decreases rapidly (Wallmo & Regelin 1981).  For example, 30 cm (12 in) of snow renders 97% of the potential food unavailable to New York white-tails (Moen & Evans 1971).  In addition to limiting the amount of available food, snow places an added energy burden on deer moving through it to find food and escape predators.  A 45 kg (100 lb) deer expands 7 to 8 times as much energy walking through a 50 cm (20 in) snowpack as it does on bare ground (Mattfeld 1973 in Verme & Ullrey 1984).   At a temperature of -20oC it is estimated that a resting deer's energy demand exceeds 2,000 kcal (Holter et al. 1975).  Wallmo and Regelin (1981) note that it is unlikely that a deer could keep up with this energy demand, especially as it must feed on a diminished supply of browse over the winter.  Thus, deer have adapted to winter conditions by voluntarily restricting their intake of food, even when ample food is present; in large part, they rely on their accumulated fat reserves for winter survival (Verme & Ullrey 1984).

In the past it was believed that the availability of winter food was the single most critical factor in deer survival over the winter.  However, the survival of an individual deer in winter depends not only upon its food supply and cover, but also upon the nutritional condition of the deer in autumn.  Healthy deer can lose 30-40% of their fall weight during winter and still survive (Severinghaus 1981).  Deer living in areas where the summer food supply is poor will enter the winter at lower weight, will have a lower reproductive potential, and a higher probability of winter death than deer from areas with a rich food supply (Severinghaus 1979, Moen & Severinghaus 1981).

Winter Deer Food

In southern regions, herbivores can specialize by feeding on a few plant species during the year-round growing season.  In contrast, in northern regions with distinct growing and dormant seasons, food quality and availability varies considerably.  Herbivores like deer tend to be generalist feeders under these conditions (Geist 1981).

Following these principles, both white-tailed and mule deer have broad feeding niches. They eat a variety of kinds and parts of vegetation including acorns, fruits, fungi, lichens, grasses, weeds, and the leaves and twigs of woody plants.  Atwood (1941) lists 614 plant species eaten by white-tailed deer, while Wallmo and Regelin (1981) state that at least 788 kinds of plants are eaten by mule deer.  While habitat preferences of the two deer species may differ, food preferences in their area of overlap are quite similar (Mackie 1981). 

Although North American deer have traditionally been described as browsing herbivores which feed on woody vegetation, neither their feeding behavior nor their digestive physiology seems to warrant this restrictive description.  More recent studies have concluded that both the white-tailed (Verme & Ullrey 1984) and mule deer (Geist 1981, Anderson & Wallamo 1984) are intermediate grazer-browsers in feeding habit.  In many portions of their broad ranges, white-tailed and mule deer feed on such non-browse items as fruits, acorns, fungi, grasses and forbs during the winter.  In agricultural regions, 60-90% of the winter foods for white-tails are crops such as corn and soybeans (Mustard & Wright 1963).

Some woody plants may be important as deer food, not for their browse potential but because of their fruit production.  In Ohio, crabapples are an important winter food for white-tails; in Missouri, acorns form the balance of the winter diet; in North Dakota, the fruits of buckbrush are an important winter diet item for both mule and white-tailed deer.  However, in portions of their ranges with deep snows, the only foods available to deer may be the twigs and needles of woody plants.  Thus, in the northern portions of their range they may be exclusively browsers during the snowy winter months.  In fact, during the winter, 75% of mule deer diet in the north consists of shrubs and trees (Kufeld et al. 1973).

Deer have definite feeding habits and preferences.  Normally deer feed lightly on many different kinds of plants, favoring some plant species more than others.  They never depend exclusively on one plant species (Crouch 1981). 

Aldo Leopold (1933) was the first to propose a classification system of deer foods which reflects deer preference, food availability, and apparent quality.  His categories of preferred, staple, emergency, stuffing and pastime are modified for use in this background (Tables 1 & 2).  When the food supply is depleted, the preferred plant species become overeaten, and deer feed on the less preferred staple foods.  For example, white-tailed deer in northern states prefer northern white cedar, red maple, and staghorn sumac; mule deer over much of their northern range prefer bitterbrush, mountain mahogany, serviceberry and big sagebrush during the winter.  When all foods are in short supply, deer are forced to feed largely on the least favored stuffer foods.  White-tails in New York, which feed mainly on speckled alder, spruce, and paper birch, have clearly overeaten their winter range.

Table 1. Selected Winter Browse of White-Tailed Deer

Sources: 1Allen 1968, Martinka 1968; 2McKean 1954; 3Mosley 1956; 4Segelquist & Pennington 1968; 5Mustard & Wright 1963; 6Hammerstrom & Blake 1939, Dahlberg & Guttinger 1956, Beals et al 1960

PREFERRED FOODS

Montana1

North Dakota 2

South Dakota3

Oklahoma4

Iowa5

Wisconsin6

quaking aspen western     serviceberry snowberry sagebrush skunkbush sumac

buckbrush chokecherry

bearberry buckbrush chokeberry red-osier     dogwood

cat greenbrier saw greenbrier winged elm poison ivy      red maple lowbush     blueberry

buckbrush honey locust

white cedar     bigtooth aspen      red maple     lowbush blueberry apple             staghorn sumac black chokecherry oaks                    willows                 black cherry   witchhazel               yew                 Canada hemlock mountain ash       wild cranberry alt-leaved dogwood sweet fern

SECOND CHOICE FOODS

Montana1

North Dakota 2

South Dakota3

Oklahoma4

Iowa5

Wisconsin6

 

rose cottonwood Juneberry

paper birch       bur oak

 

staghorn sumac

basswood             black cherry             jack pine         Juneberry           mountain maple white pine             yellow birch         black willow           round leaved                dogwood               fly honeysuckle black current red-berried elder downy arrowood balsam fir             sugar maple           paper birch     beaked hazel          white ash             hairy honeysuckle red oak             quaking aspen           pussy willow

EATEN FOODS

Montana1

North Dakota 2

South Dakota3

Oklahoma4

Iowa5

Wisconsin6

cottonwood longleaf                   sagebrush Symphoricarpus

poison ivy willows               bur oak         quaking aspen yucca

 

tree                           sparkleberry American                 hophornbeam

apple oak black cherry elm maple

shadbush hazelnut jack pine quaking aspen

STARVATION / STUFFER FOODS

Montana1

North Dakota 2

South Dakota3

Oklahoma4

Iowa5

Wisconsin6

hawthorne greasewood rose Douglas fir ponderosa pine paper birch plains cottonwood willows dogwood

buffaloberry dwarf sagebrush skunkbrush sumac yucca junipers big sagebrush ironwood beaked hazel paper birch box elder elm currents

 

hawthorne blackjack oak red oak white oak post oak hickory pin cherry

red cedar rose linden maple

white pine red pine paper birch speckled alder Scotch pine black spruce red-osier dogwood pin cherry Labrador tea tamarack

Table 1 cont. Selected Winter Browse of White-Tailed Deer

7Howard 1937, Verme 1965; 8Eckert & Lang 1978, Jackson & Sarbello 1980; 9Hosley & Ziebarth 1935; 10Crawford 1982; 11Telfer 1972

PREFERRED FOODS

Michigan7

New York8

Massachusetts9

Maine10

New Brunswick and Nova Scotia 11

white cedar common juniper Canada hemlock staghorn sumac with-rod yew red maple sugar maple mountain maple

white cedar striped maple hobblebush apple staghorn sumac yew sassafras mountain maple alt-leaved dogwood basswood

red oak black cherry red maple hazel pasture juniper staghorn sumac Canada hemlock

Canada hemlock

white cedar hobblebush mountain maple red maple yellow birch lowbush blueberry withe-rod red oak sweet fern false holly

SECOND CHOICE FOODS

Michigan7

New York8

Massachusetts9

Maine10

New Brunswick and Nova Scotia 11

 

honeysuckle wild raisin silky dogwood round-leaved dogwood elderberry nannyberry cucumber tree highbush cranberry Canada hemlock highbush blueberry red-osier dogwood mountain ash some willows

apple mountain maple striped maple white oak quaking aspen sugar maple white ash wild raisin hickory American chestnut dogwood shadbush mountain laurel yellow birch witchhazel

white cedar red maple speckled alder

witchhazel blueberry sweet gale hazel red maple

EATEN FOODS

Michigan7

New York8

Massachusetts9

Maine10

New Brunswick and Nova Scotia 11

quaking aspen silver maple red oak black ash hazel Norway pine brambles paper birch willow black ash red-osier dogwood speckled alder barberry juniper ironwood

white ash sugar maple black cherry hazelnut maple-leaved viburnum some oaks yellow birch chestnut hickory chokecherry spicebush elm black chokeberry arrow-wood black walnut butternut shadbush black ash lowbush blueberry

paper birch smooth sumac pin cherry bush honeysuckle sweet fern hobblebush white pine Scotch pine American beech sassafras chokecherry basswood red pine willow red cedar highbush blueberry lowbush blueberry

balsam fir white pine

sugar maple blueberry balsam fir rhodora American beech witchhazel yellow birch Canada hemlock sheep laurel huckleberry paper birch

STARVATION / STUFFER FOODS

Michigan7

New York8

Massachusetts9

Maine10

New Brunswick and Nova Scotia 11

white pine bigtooth aspen rose balsam fir speckled alder tamarack black spruce white spruce honeysuckle balsam poplar

hawthorne American beech quaking aspen balsam fir speckled alder tamarack red spruce chokecherry pitch pine current buckthorn raspberry blackberry rhododendron white pine red pine paper birch red cedar pin cherry grey-stemmed dogwood black locust huckleberry

hawthorne jack pine Spiraea snowberry speckled alder

Allegheny blackberry American elm beaked filbert bigtooth aspen bunchberry dogwood grey birch paper birch quaking aspen

spruce white pine alder

Table 2. Selected Winter Browse of Mule Deer

Sources: 1Lovaas 1958, Wilkins 1957, Martinka 1968; 2Edwards 1942, Klebenow 1965, Crouch 1968; 3Mohler et al 1951; 4Smith 1952; 5Leach 1956; 6Hoskins & Dalke 1955; 7McKean 1954

PREFERRED FOODS

California 1

Oregon 2

Idaho 3

Utah 4

Montana 5

Nebraska 6

North Dakota 7

sagebrush bitterbursh curlleaf mt mahogany

red huckleberry bitterbrush cascara buckthorn willows snowbursh (Ceanothus sanquineus)

curlleaf mt mahogany

cliff rose bitterbrush

western serviceberry snowbrush common juniper rocky mt juniper

buckbrush

buckbrush

SECOND CHOICE FOODS

California 1

Oregon 2

Idaho 3

Utah 4

Montana 5

Nebraska 6

North Dakota 7

squaw carpet green leaf manzanita rubber rabbitbrush

Ribes Sanquineum Holodiscus discolor mountain mahogany Sierra juniper

bitterbursh myrtle pachistema buffaloberry

big sagebrush mountain mahogany

juniper horizontalis quaking aspen big sagebrush kinni klinick (bearberry) mountain maple

jack pine wild rose

chokeberry wild rose skunkbrush sumac yucca

EATEN FOODS

California 1

Oregon 2

Idaho 3

Utah 4

Montana 5

Nebraska 6

North Dakota 7

yellow pine western serviceberry rabbitbrush Sierra juniper California black oak incense cedar

bitter cherry douglas fir California hazel big leaf maple rabbitbrush big sagebrush snowbrush

chokecherry western serviceberry big sagebrush rose snowbrush hawthorn

gambel oak Utah juniper skunkbrush sumac

chokecherry rose snowberry creeping holly- grape pear Philedelphus lewesi big whortleberry

ponderosa pine red cedar poison ivy

cottonwood willows quaking aspen Juneberry dwarf sagebrush junipers

STARVATION / STUFFER FOODS

California 1

Oregon 2

Idaho 3

Utah 4

Montana 5

Nebraska 6

North Dakota 7

snowbrush ponderosa pine white fir willow chokecherry quaking aspen

Rosa gymnocarpa red alder vine maple

quaking aspen rabbitbrush snowberry dogwood douglas fir rocky mt maple willow

pinion pine Ephedra Gutierrezia

hawthorne birch black cottonwood plain cottonwood ponderosa pine douglas fir sitka alder sagebrush ninebark bitter cherry spiraea

willow chokecherry wild plum scotch pine

poison ivy bur oak big sagebrush ironwood beaked hazelnut paper birch currants box elder elm

While deer show definite preferences for some plants over others, determining preferences is a complex matter.  In order to assess food preferences one must determine what is available to be eaten, as well as what the animal is eating in its winter habitat.  Available food items may be sampled by standard plant ecology methods.  What is eaten by the deer may be determined from analysis of gut contents or by following a tame deer around as it feeds. 

One can also assess what is eaten from signs of browsing in the field, since deer browsed twigs are ragged at the stub, whereas hare and rabbit browsed twigs are cleanly cut (Telfer 1972).  For example, in a three year study using only signs of browsing, Horton (1964) determined that white-tails feeding in a fenced-in conifer plantation preferred jack pine over white pine and red pine, and never browsed white spruce.  Since the conifer species had been planted in nearly equal proportions, what the deer ate was not affected by the plant's relative scarcity, but reflected the deer's preference. 

Aside from the importance of a plant's relative abundance -- if the plant is rare it will not appear in the diet -- preference for a particular plant species as food seems to vary from one region of the deer's range to another as Tables 1 and 2 indicate.  Thus, paper birch may be regular fare for white-tailed deer in South Dakota, while in New York it is starvation food.  Furthermore, within a given region, the palatability of a given plant to deer may also vary seasonally.

Characteristics of Preferred Winter Browse

Penned deer artificially fed only low quality winter browse, such as quaking aspen, quickly lose weight, and would obviously starve to death on such a diet.  Weight loss is not as severe when deer are fed on high quality browse.  For example, the highest-quality browse for white-tailed deer in the Great Lakes region is northern white cedar; it is the only plant species which by itself can sustain deer through the winter.  In general, herbivores prefer certain plants because of their nutritional and energy content.  In addition, herbivores may be unable to eat certain plants, or plant parts, because some plants have evolved antiherbivory defenses.

Nutrition.  Certain food items may be included in the diet of deer because they contain high levels of essential nutrients.  For example, sodium is often in short supply but is an essential element in the cervid diet.  Aquatic plants are high in sodium content but low in energy content, while the reverse is true of land plants.  Thus cervids, such as moose, stock up on sodium during the summer by feeding heavily on aquatic plants.  In winter, when energy is a high requirement, they feed exclusively on twig browse (Belowski 1978).

Caloric Content.  Large herbivores such as deer require a great deal of energy for growth, maintenance, and reproduction.  For example, the maintenance energy for pregnant white-tails during Michigan winters is around 155-160 kcal/(kg body weight)0.75, which amounts to about 1.6-3.2 kg (3.5-7 lbs) of high quality browse per 45 kg of body weight (Verme & Ulrey 1984; Murphy 1970).  Although deer can fast for several weeks without visible harm (Verme & Ozaga 1970), deer will eventually starve in the winter if they are unable to obtain energy at a rapid enough rate from their food.

The rate at which deer are able to digest vegetation is important to their well being, in addition to the rate at which they ingest it.  Winter browse varies in its digestibility.  Preferred browse such as northern white cedar has a digestibility of 60% dry weight.  In contrast, filler foods such as big-toothed aspen and balsam fir have digestibilities of around 49% (Ullrey et al. 1964,1968).

The rate of digestibility is related to the activity of the bacterial and protozoan microflora within the deer's rumen, the portion of the digestive tract in which breakdown of the browse occurs.  Less digestible browse apparently is more difficult for the microflora to ferment, or contains chemicals which inhibit fermentation.  For example, big sagebrush is an important component of mule deer's winter diet.  However, captive mule deer fed only big sagebrush do poorly unless it is combined in the diet with other plants.  It has been estimated that physical deterioration and death will likely occur if the diet consistently is composed of more than 15-30% sagebrush (Wallmo et al. 1977).  Apparently, the essential oils within the sagebrush twigs exert an antibacterial affect on the rumen microflora (Nagy et al. 1964).  Conifer needles are also high in essential oils and difficult to digest.  For example, the essential oils in Douglas fir and junipers, two abundant plants in the mule deer's range, also possess antibacterial properties.  Juniper of more than 20% in the diet is as well harmful.  Dilution with other plants in the diet ameliorates the antibacterial affect (Klein 1970).  This explains in part why deer eat small amounts of many different kinds of plants, some of which are high in toxins and energy, others which are low in both.

Equally important to digestibility are the chemical building blocks of the browse -- lignin, cellulose, hemicellulose, carbohydrates, and proteins.  Lignin and cellulose are the most difficult to digest, while hemicellulose, carbohydrates, and proteins are the easiest.  Plant species vary in the proportion of digestible to indigestible components.  Deer forced to feed exclusively on high-lignin woody twigs lose weight and suffer starvation (Short & Reagor 1970).

Antiherbivore Defenses.  Plants may defend themselves from herbivore predation with physical defenses such as spines and thorns, and through various toxic chemicals.  Plants armored with thorns and spines are often quite palatable.  It is obvious, however, that any herbivore attempting to feed on such a plant may pay the price of injury.  Hawthorn, a plant which bears large thorns, may be a stuffer food partly for this reason.

However, many plants possess lower levels toxin as part of their defense against herbivory.  Studies of subarctic herbivores such as moose, snowshoe hare, and ptarmigan have shown in each case that food preference was based neither on the caloric nor nutritional value of the browse, but rather on its toxic content (Bryant & Kuropat 1980).  Mule deer avoid eating highly poisonous plants such as tansy ragwort (Verme & Ullrey 1984).  In general, plants and plant parts with high toxic content are either avoided, or eaten in the diet mixed with low toxic items.

Effect of Deer Browsing on Vegetation

When feeding on browse, deer eat the needles of conifers or the terminal twigs of deciduous woody plants.  A study of the average diameter of 100 browsed twigs of five commonly eaten species in Pennsylvania indicated that only the very tips of branches are eaten (Table 3).  Given this kind of information, wildlife managers can use a twig clipping sampling technique to assess the winter food potential of a habitat (Shafer 1963).

Table 3.  Twig diameter of browsed twigs (from Shafer 1963)

Species

Twig Diameter (mm)

Red Maple

4.2

Quaking Aspen

3.8

Northern Red Oak

3.5

Black Cherry

3.0

Gray Birch

1.3

 

Heavy continuous browsing on plant twigs can retard vegetative growth or even kill the plants.  Browsing may also cause subsequent re-growth to contain higher levels of toxic compounds.  Unbrowsed shrubs and saplings rapidly grow to heights out of the deer's feeding reach.  Above about 3 m (10-12 ft), deer are unable to reach available browse; a young woods that has grown mainly into pole-sized trees becomes a less suitable winter feeding habitat.  Thus, through their browsing deer retard the rate at which shrubs and saplings grow out of reach of their feeding range, and in so doing, maintain suitable habitat longer than if it were unbrowsed.

Studies which have artificially simulated deer browsing by clipping twigs, have shown that some food species maintain their production of new stems under moderate to heavy clipping regimes (Aldous 1952).  Such winter deer foods as mountain maple, paper birch, beaked hazelnut, pin cherry, willow, and black ash thrived under artificial browsing.  In fact, some of these species produced more browse when 100% of the previous year's growth was clipped than when unclipped, or slightly clipped (Aldous 1952).  Similarly, favored western browse species such as bitterbrush, sagebrush, serviceberry, Gambel oak, and mountain mahogany are able to maintain stem production with 40-60% browsing loss of stems (reviewed in Carpenter & Wallmo 1984).  These studies imply that deer browsing may, in effect, stimulate growth and maintain, or even increase productivity of some browse plant species.  In contrast, other deer browse species, such as mountain ash, red-berried elder and red-osier dogwood were unable to tolerate more than moderate amount of artificial browsing.

Sometimes the effect of deer browsing depends on the size of the plant.  Northern white cedars below 2 m (7 feet) in height can only tolerate 15-20% browsing of its annual foliage production.  Heavier twig clipping caused slowed growth and death in simulated browse studies.  In contrast, white cedars taller than 2 m could withstand nearly unlimited artificial browsing pressure.

Determining the Size of Deer Populations

"How many deer live here?" is a question often asked by wildlife managers.  Of the several methods that have been developed to estimate the number of deer inhabiting an area, five are discussed here.

Censuses, or direct counts, attempt to enumerate all of the individuals found in a given habitat.  A census can be done from the ground, in which case there is always uncertainty about whether all of the population members have been seen.  In regions where deer inhabit woodlands as well as fields, this method will result in large errors.  Censuses are often taken from the air.  Counts from flyovers are expensive and require that the deer can easily be seen in relatively open terrain.

A second census technique is a drive count.  This technique is most accurate and practical when deer live within an area enclosed by a game fence (McCullough 1979).  However, the technique can be used to count deer in an unfenced area such as a woodlot, if it is surrounded by roads or fields on all sides.  The technique requires that prior to the drive all deer tracks exiting or entering the habitat boundary are eliminated.  A line of drivers -- equally spaced and in visual contact with one another -- sweep across the habitat in one direction until they reach the other end.  Deer that pass through the line are tallied by a driver only if they pass to his right.  At the termination of the drive, the deer passing through the line are summed and added to the tracks of deer leaving the area.  This constitutes the deer census for that area.  Drive counts are costly in terms of manpower, generally requiring around 50 drivers.  They are also limited to relatively small areas that are bounded in some way.

For most deer populations these two direct censusing methods are impractical or unreliable in terms of determining the number of animals in an area.  Thus a number of indirect techniques have been invented that estimate the size of an animal population in a habitat. 

Capture-mark-recapture techniques, also known as the Lincoln-Peterson method, are based on the assumption that marked animals disperse into the general population randomly.  The technique requires that some deer be captured, marked, and released back into the population.  After the marked deer have been given ample opportunity to randomly mix with the rest of the population, a survey is made to record the number of marked and unmarked deer seen (Connelly 1981).  From the proportion of marked to unmarked deer in the recapture survey, and the total number of marked deer in the original capture sample, a population estimate can be obtained.  For example, if the population is large, the marked animals will be "diluted" within the population such that when a recapture sample is taken, only a few marked animals in relation to total recaptured animals will be taken.  On the other hand, if the population is small, there will be a high proportion of marked to unmarked animals in the recapture sample.

Mark-recapture techniques are the most commonly used methods for estimating animal numbers.  However, with large animals such as deer this technique is expensive and time consuming.  Additionally, small populations require that a high proportion (about 45%) of the deer be captured and marked in order to make reliable estimates of the total population (Bartmann et al. 1987).

  Although McCain and Taylor (1956) point out that all signs left by deer can potentially be used to estimate population numbers, track counts and fecal pellet counts have proved most useful to wildlife workers.

Track counts can serve as a good estimator of deer populations in situations when deer are moving in the same direction, such as during migration.  The counts are most accurate when the animals must cross an open area at right angles to their direction of movement.  Thus counts of individual animal's tracks are usually made as the population moves across highways, fire lanes, right-of-ways or other open areas (McCain & Taylor 1956, Connolly 1981).  Counters travel a fixed route of 1-3 km (about 1/2 to 2 miles) at a prescribed time of the day counting tracks along the trackway.  Tracks of deer are obliterated from the sampling area after they have been counted to allow for subsequent new track impressions.

Fecal pellet counts have been extensively used to estimate the population density of deer as well as deer habitat preferences.  When a deer defecates, it leaves a group of from 10-210 (average of about 70) dung pellets in a pile on the ground (Rogers 1987).  Deer fecal pellets range in shape from spherical to cylindrical with usually one pointed end.  The number of groups of such fecal pellets reflects the number of deer in a habitat.  Pellet counts have been popular with wildlife managers as a deer population estimation technique because they are comparatively accurate, efficient, and require less effort than other population estimation techniques (Connolly 1981).

Estimating Deer Population Size From Fecal Pellet Counts

Deer, such as mule deer, on the average, eat 14.1 kg (31 lbs) of dry weight forage per day per 45 kg of deer.  They assimilate about half of this forage intake and eliminate the rest as fecal pellet groups (McCain and Taylor 1956).  The average daily defecation rates of deer in 24 studies summarized by Neff (1968) varied from 8-23.1 pellet groups per deer.  The most often cited average was 12-13 pellet groups per deer per day (Neff 1968).  This means that about 13 pellet groups equals one deer in the habitat.

Most estimates of deer population numbers derived from fecal pellets treat defecation rate as a constant of 13 pellet groups.  However, the matter of defecation is not this simple.  In a recent study, Rogers (1987) determined the defecation rate of seven free-ranging female white-tailed deer throughout a year in northern Minnesota.  His results are summarized in Table 4.

Table 4.  Defecation Rates and Major Foods of 7 Female White-Tailed Deer Over Four Yearly Periods (from Rogers 1987).

Period

Fecal Pellet Groups per Day

Major Foods

 

Mean

Range

 

January-April

22.3

14 - 31

woody browse, lichens

May-June

27.0

21 - 33

new leaves, weeds

July-August

34.4

19 - 45

mature leaves, weeds

Sept-Dec

51.9

40 - 66

old leaves, weeds, mushrooms, lichens, woody browse, evergreen ground cover

Thus the defecation rate changes seasonally, probably with the deer's diet as reflected in Table 4 by changes in major foods eaten.  Seasonal changes in defecation rate also reflect changes in the deer's metabolic rate.  For example, stags feed little during the fall rutting season, and the metabolism of all deer slows down during mid-winter (January-February).  Furthermore, defecation rate, in a still unknown manner, varies with age and the sex of the deer (Rogers 1987).  Finally, the defecation rate of 12-13 pellet groups per day reported in captive deer fed on artificial diets (Neff 1968) is lower than that of the free ranging deer at all seasons (Rogers 1987).  The commonly used rate of about 13 pellets groups per deer per day probably overestimates deer population size.  At least in winter, 20-25 pellet groups per deer per day may be a more accurate measure of the deer population in a habitat.

Finally, the presence of other ruminants in the habitat with deer, particularly in western North America, introduces a source of error in the pellet count.  The fecal pellets of pronghorn antelope, as well as wild and domestic goats and sheep, are difficult to distinguish from deer fecal pellets.  Thus attention must be paid to the presence of other ruminants.

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