Selection of Sites for Winter Night Beds by White-tailed Deer in a Hemlock-Northern Hardwood Forest

1. What they did

White-tailed deer gather in sheltered wintering areas, often in conifer stands on level bottomland, to save energy and escape harsh weather. Where they choose to bed down within such an area had not been studied for night beds. The authors set out to find which features of the immediate surroundings (the microhabitat) deer favour for night beds in an eastern hemlock and northern hardwood forest in western Maryland.

• Finding beds. They laid out nine transects, each 35 m by 700 m (22.05 ha in total), across the wintering area. In the late afternoon they brushed snow into all old beds. At sunrise the next morning they searched the transects for fresh night beds in the snow and marked each one with an aluminium tag. Surveys ran one to three times a week from January to mid-March 1981 and in January 1982, while snow lay on the ground.
• Weather. At three weather stations they recorded overnight lowest and highest temperatures and five random snow depths before each survey.
• Measuring the sites. In late March and April, before the leaves came out, they measured 62 features at 140 night-bed sites and 100 random sites in the same cover type. These included tree basal area, canopy cover directly overhead, how dense the cover was in four directions at different heights and distances, and the distance, direction, size and species of the nearest tree in each quarter around the site.
• Comparing. They compared bed sites with random sites one feature at a time, then used principal components analysis (PCA), a way of combining many measurements into a few main patterns, to see where bed sites fell among the sites available.

2. What they found

When deer bedded there

• The surveys found 0 to 28 night beds per visit (average 5.6).
• Nights when beds were found were much colder (average low of -17.3 °C) than nights with no beds (average low of -8.9 °C).
• No beds were found on five visits in March, when the weather was milder.
• Snow was shallowest on the bottomland (average 11.2 cm) and deepest on the northeast-facing slope (average 21.7 cm, up to 42.0 cm).

What the bed sites were like

Compared with random sites, night-bed sites had:

• More cover directly overhead: canopy cover averaged about 94% above beds against 84% above random sites.
• More open views to the southeast: less low cover (from the ground to 1 to 1.5 m) to the southeast at 5 and 10 m. At 5 m there was more cover to the southeast between 2 and 2.5 m high.
• A tree close by: the nearest tree was closer (average 1.3 m against 1.7 m).
• Patchier tree spacing: a higher spatial heterogeneity index (52.1% against 42.5%).
• More hemlock to the north and west: eastern hemlock was the nearest tree to the north at 56.4% of beds (34.0% of random sites) and to the west at 52.9% (37.0%).
• Less red maple to the north and less black cherry to the south than expected.

There were no differences in tree basal area or in cover to the northwest. Overall, 8 of the 62 features differed between bed and random sites, besides the overhead canopy cover.

The overall pattern

Four main patterns from the PCA explained 72.5% of the variation among random sites. Deer beds clustered where low cover to the southeast was sparse and trees were close and patchily spaced: 68 of the 140 beds fell in that quarter of the available habitat, far more than expected. Even so, deer used a wide range of sites.

From tracks and trails in the snow

Following snow trails and tracks, the authors found that deer moved into dense hemlock cover in severe weather (very low temperatures with gusty, strong winds) and left once the weather eased, apparently to browse elsewhere.

3. What they concluded

• Although many sites seemed suitable, deer chose night beds with: (1) an open southeastern exposure; (2) hemlock trees to the west and north; (3) patchy tree spacing; (4) a tree close by; and (5) dense cover directly above.
• An open southeast side may let in morning sun, give an open view or offer an escape route. An open view to the southeast may matter most when the wind, carrying scent, blows from the west and northwest.
• Hemlock to the west and north would shelter deer from the prevailing west-northwest winds. Dense cover overhead would cut heat lost to a cold, clear night sky.
• The chosen sites apparently gave better conditions for survival than other sites available.
• The night beds were similar to those in a study in Ontario, Canada, though there most beds were high on slopes, especially north-facing ones, while here most were on nearly level bottomland. In both places, dense hemlock cover was a key factor.

4. How tracking was or was not used

The whole study rests on finding and identifying fresh deer night beds in the snow. Old beds were erased the evening before, so beds found at sunrise could be dated to the previous night. The authors also report that their observations of snow trails and tracks showed deer entering dense hemlock cover in severe weather and leaving when it eased. No trailing method is described.

5. Tracker or observer certification

Not reported. The paper does not say who found and identified the beds or what experience they had. Five people are thanked for help in the field.

6. Other technologies, skills or innovations

• A simple way to date beds: brushing snow into old beds the afternoon before each survey, so that every bed found the next morning was fresh from that night.
• Spherical densiometer (Model C) for canopy cover overhead; cover density scores after Nudds (1977) in four directions, at five heights and two distances; angle gauge for basal area; compass, steel tape or rangefinder for nearest trees.
• Maximum-minimum thermometers at three weather stations.

7. Implications for conservation (from the paper)

The paper does not discuss conservation or management directly. It links dense coniferous cover, especially hemlock, with shelter for deer in severe winter weather.