Influence of snow depth on prey availability and habitat use by red fox

1. What they did

Deep snow shapes where animals live and what they do in winter, but its effects on carnivore behaviour had been poorly documented. The authors asked how snow depth and snow structure (soft powder, crust or wet snow) affect where red foxes travel and hunt, and how easily they can get at their prey.

Over two winters (1982 and 1983) in eastern Maine, United States, they:

• Followed fox trails in the snow. Starting from roads and trails, they followed or backtracked the first clear fox tracks found each day until wind, melt, new snow or older tracks made the trail impossible to follow. In all, 125 km of trail were paced and recorded by habitat.
• Read the signs along the trails. They recorded what foxes were doing (traveling, hunting or resting) from signs in the snow. Pounces and digs were counted as a measure of hunting for small mammals, rounded beds of melted and packed snow as resting sites, and crossings with tracks of other species (especially snowshoe hare) were counted.
• Measured the snow. Every 100 m along the trails they recorded the snow surface and how deep the fox’s feet sank. Snow depth was also measured weekly at stations in hardwood, softwood, mixed forest and an open area.
• Studied the diet. They collected droppings and prey remains along trails and elsewhere, and identified what the foxes had eaten.
• Compared use with availability. They compared the distance foxes traveled in each habitat with how much of that habitat was in the study area (from satellite data).

2. What they found

Snow in different habitats

• In 1982, snow lay from December to mid-April and peaked at 92 cm in early March. In 1983, snow came late (mid-January), thawed in early February, was gone by mid-March, and peaked at only 31 cm.
• Snow was deepest and softest in hardwood and mixed stands. Among the forests, softwoods gathered the least snow but melted slowest. Open areas always had the least snow; wind kept the large barrens shallow and packed them into crusts that could hold a fox’s weight.

How deep the foxes sank

• Sinking depths ranged from 0 to 24 cm and differed between periods of the winter. Foxes sank least under crust (2.56 to 4.77 cm on average) and most in powder (7.86 to 10.52 cm).
• Only 10 measurements along the trails were deeper than 15 cm, and only 3 deeper than 20 cm, the depth at which foxes are known to change to a wallowing walk or a tiring loping gallop.

Where the foxes traveled

• Foxes used every habitat. They preferred softwood stands and open areas, used mixed stands about as expected, and avoided hardwoods (hardwood made up 43% of the area but only 20% of fox trails).
• Foxes seemed to choose the routes with the firmest footing. In soft powder they traveled more in softwoods and open areas; on crust, travel was spread more evenly and use of hardwoods rose. As snow deepened, they spent more time in dense understory and young regrowth.

Prey and hunting

• Snowshoe hare tracks crossed fox trails most often in dense understory (35.69 crossings per km) and least where there was no understory (2.43 per km) or in the open. Foxes wandered through these thickets, crossing and doubling back on their own trails among mazes of hare tracks. All four hare kill sites found were in mixed stands with dense softwood understory.
• Snowshoe hare was the main food in the 128 droppings in both winters, and hare remains were the most common prey found along trails. Food was often carried away and cached in shallow holes dug in the snow; whole shrews were left at six pounce sites.
• In the deep-snow winter (1982), pounces and digs were spread evenly across habitats. In the shallow-snow winter (1983), foxes hunted mostly in open areas, especially those with grass and sedge, and small mammal tracks were about five times as common (3.39 versus 0.63 per km).
• Deeper snow meant more hare and fewer small mammals in the diet. In midwinter 1982, hare was the only food found in droppings.

Other animals

• Fox trails crossed the tracks of bobcats, coyotes and weasels. Of 39 fox and coyote crossings, 35 were in February. Urine marks were often found where the trails crossed, and several times coyotes turned to follow a fox trail for a short distance.

3. What they concluded

• Snow conditions clearly shaped where red foxes traveled and hunted, even in eastern Maine, where the coastal climate limits deep powder.
• The main effect of snow was on which prey the foxes could reach. Small mammals live under the snow in winter and become harder to catch as snow deepens or crusts form, so foxes turned to snowshoe hare and hunted in the dense regrowth where hares gathered.
• Beyond some critical depth, snow seemed to restrict small mammals equally in all habitats.
• Red foxes are usually adaptable generalists, but harsh winter conditions in northern forests force them to narrow their role. Harsh climate plays a significant part in how predators select habitat and use food.

4. How tracking was or was not used

Tracking was the main field method. The researchers found fox tracks from roads and trails, then followed or backtracked them through the snow, pacing distances by habitat. Along the trails they read the signs: pounces and digs (hunting), beds in the snow (resting), food caches, prey remains, kill sites, droppings and urine marks. They measured how deep each fox track sank in the snow every 100 m, and counted where fox trails crossed the tracks of snowshoe hares, small mammals, coyotes, bobcats and weasels.

5. Tracker or observer certification

Not reported. The paper does not say who followed the trails or how their skill was checked, although all the habitat use, hunting and prey findings rest on reading tracks and signs in the snow.

6. Other technologies, skills or innovations

• Habitat availability mapped from LANDSAT satellite data.
• Snow depth and snow layers measured weekly with five stakes at each of four monitoring stations.
• Sinking depth of fox tracks used as a measure of how hard traveling was.
• Chesson’s alpha index to measure habitat preference.