Resource selection of den sites for bobcats (Lynx rufus) in the Northern Great Plains, United States

1. What they did

A safe den is vital for bobcat kittens: the first months of life are when young carnivores are most likely to die. Yet before this study, nobody had looked at how bobcats choose den sites. The authors asked which features of the landscape and the den surroundings female bobcats select in the Black Hills of South Dakota.

They radio-collared 35 female bobcats and located them at least three times a week from March to August. When a female stayed within 500 m for four days in a row, she had probably given birth. If she stayed in the same area for one to four more weeks, the team approached to find the den.

After the mother and kittens had left, they measured the habitat at each den and at a paired random site nearby. They worked at two scales: the wider area around the den (a 100 m radius) and the den site itself (a 13 m radius).

2. What they found

• Dens found: 27 dens from 19 females (18 in 2020 and 9 in 2021): 6 natal dens, where kittens were born, and 21 maternal dens, used afterwards.
• Den types: 21 were in rock crevices, 4 in piles of downed wood and 2 in shrub thickets. Six of the rock crevices also had shrubs around the entrance.
• Wider area: bobcats chose areas with more horizontal cover (cover at eye level, from rocks, shrubs and the shape of the ground) and more rugged terrain than random sites.
• Den site itself: horizontal cover was again the only feature that mattered.
• Features that did not matter at either scale: canopy cover overhead, aspect (which way the slope faces), slope, distance to roads, distance to water, tree basal area and downed wood.

3. What they concluded

• Bobcats choose den sites that hide them from predators. Coyotes and cougars, the main predators of kittens in the Black Hills, both hunt by sight. Coyotes prefer less rugged ground, so rugged terrain may help keep them away.
• Overhead cover did not seem to matter, which suggests eagles are not a major concern. An earlier study in the area attributed only 1 of 32 kitten deaths to a golden eagle.
• Unlike some other wild cats, bobcats showed no preference for shaded north-facing slopes. Rock crevices already stay cooler and more stable in temperature, so aspect may not matter.
• Bobcats did not den farther from roads, even though humans are the main cause of bobcat deaths in harvested populations. Rugged ground may give enough protection, or the road network in the Black Hills may simply be too dense to avoid.
• In less rugged areas, cover from vegetation may matter more. There, forest management that increases tree and shrub cover at eye level may provide den sites.

6. Other technologies, skills or innovations

• Very high frequency (VHF) radio collars with a mortality switch (Advanced Telemetry Systems), located by triangulation from the ground
• Cage traps baited with scent lures and other attractants, and sedation for collaring
• Standard vegetation tools: a cover board, a canopy densitometer, quadrats, a clinometer, a compass and a 10-factor prism
• Map analyses in ArcGIS Pro using United States Geological Survey elevation and water data

7. Implications for conservation (from the paper)

• Bobcats are an economically valuable furbearer and a species of management concern across the United States.
• Den sites with good concealment may improve kitten survival.
• In less rugged landscapes, land management that promotes tree and shrub cover at eye level may provide the cover bobcats need for dens.