The efficiency of yarding behaviour by white-tailed deer as an antipredator strategy

1. What they did

In the north of their range, white-tailed deer gather each winter in traditional wintering areas called yards, criss-crossed by trails. Yarding is usually explained as a way to save energy: shared trails and shallower snow make moving cheaper, and conifer cover and south-facing slopes help keep deer warm. The authors asked whether yarding also protects deer from coyotes.

Over two winters (1976-1977 and 1977-1978) in southern Quebec, they compared where deer lived, where coyotes travelled, and where deer were killed, in and around a 36 km² yard holding about 630 deer.

• Mapping deer density. After fresh snowfall they counted deer tracks and runways (trails with four or more tracks) along 111 km of logging roads, bush trails and waterways, and divided the network into four classes: outside the yard, low, medium and high deer density. Pellet group counts in 352 small plots gave deer density for the whole winter around each kill.
• Mapping coyote use. Two observers on snowmobiles drove the network whenever the weather allowed, recording every coyote track crossing and how many coyotes made it (single, pair or pack). They also compared these results with radio locations of radio-collared coyotes.
• Finding kills. When radio-tracked coyotes stayed in one small area, the team followed their tracks to find the kill. Six more kills were found while patrolling. A deer counted as a coyote kill only with signs of a chase or tooth punctures to the neck.
• Following chases. Along 438 km of coyote trails followed in the snow, they recorded every deer chase and whether it succeeded.

2. What they found

Deer distribution

• Low, medium and high deer density areas averaged 2.9, 7.4 and 14.5 deer tracks per km of trail, and 0.4, 3.5 and 7.5 runways per km.
• About 73% of the deer wintered in high-density areas, 15% in medium and only 12% in low-density areas (including the outlying pockets).

Where coyotes travelled

• The trail network was patrolled 131 times, and 1468 coyote tracks were recorded: 43% singles, 34% pairs and 22% groups of three to five.
• Coyotes did not use high-density areas more. Pairs and packs used low-density areas significantly more than expected; pairs used high-density areas less, and packs used areas outside the yard less. Single coyotes showed no preference.
• Radio locations gave the same picture: pairs and packs favoured low-density areas and avoided high-density ones, while singles showed no preference.
• Coyote numbers inside the yard did not build up as winter went on (apart from a short rise in December).

Where deer were killed

• Of 23 deer definitely killed by coyotes, 18 were in low-density areas, 3 in medium and 2 in high. With the two bobcat kills, 20 of 25 kills (80%) were in areas where only about 12% of the deer wintered.
• Pairs and packs made 17 of 18 known kills. Solitary coyotes mostly scavenged deer carcasses (56% of their winter diet) and ate snowshoe hares (27%).

Chases

• Of 13 chases seen in the snow, 10 succeeded (9 in low, 1 in medium and none in high-density areas) and 3 failed (one in each class).
• Coyotes killed deer by cornering them in deep snow until they were exhausted, or by a throat grip. Successful chases averaged 80 m and failed ones 150 m.

3. What they concluded

• Within a yard, deer are much safer where deer density is high. The authors believe the main reason is the dense web of runways, which gives deer a quick escape route; better predator detection and confusing a pursuing predator may also help, but they think these matter less.
• Coyote pairs and packs seem to hunt more effectively where deer are sparse, even though they meet deer less often there.
• Territorial coyotes could not crowd into the yard, so deer in the yard also gained from a lower coyote to deer ratio (a dilution effect). Two coyote families had only limited access to the yard. For wolves, whose territories usually include one or more yards, this benefit is probably smaller.
• Yarding is an antipredator strategy as well as an energy-saving strategy.

4. How tracking was or was not used

Tracking was central to this study:

• Track counts in snow: deer tracks and runways were counted 20 to 28 hours after snowfall to map deer density, and coyote tracks were counted from snowmobiles, noting how many coyotes travelled together. Repeat crossings by the same animals within 200 m were counted once, and tracks along a ploughed road were left out because they were hard to see or partly wiped out.
• Trailing: the team followed coyote tracks to find kills, and followed coyotes for 438 km in the snow, recording each deer chase they found in the tracks, how far it ran and whether it ended in a kill.
• Signs: deer pellet groups were counted to estimate deer density, and kills were judged from signs of pursuit and tooth punctures to the neck.
• Bobcat abundance was judged from tracks (8 bobcat tracks against 1468 coyote tracks).

5. Tracker or observer certification

Not reported. The paper does not say who read the tracks or how experienced they were, beyond two observers covering the network by snowmobile.

6. Other technologies, skills or innovations

• Radio telemetry of coyotes, used both to locate kills and to compare coyote space use with the track surveys.
• An aerial survey each winter to outline the yard, and an aerial photograph mosaic on which tracks were mapped.
• Snowmobile track surveys on a gated network of trails, with the time since snowfall recorded so that counts could be compared.

7. Implications for conservation (from the paper)

• Predation might be reduced by encouraging large yards with high deer density.
• Deer can lower their risk by resting in dense, protected areas and moving to areas with less competition for food to forage. So it matters not only how much browse there is, but where it lies in relation to stands that give protective cover.