The black bear housing market in Alaska
On a crisp winter morning in Alaska, before the sun is even over the mountains, Craig Perham, a Bureau of Land Management (BLM) Alaska wildlife biologist, and two interns are out looking for arboreal black bear dens—or more accurately, for heat signatures coming from the dens. These heat signatures indicate bears’ body heat, a sign that the dens are occupied.
Armed with a thermal imagery camera, bear spray, and Perham’s knowledge of the area, the three of them are monitoring dens near the BLM's Campbell Tract Special Recreation Management Area in Anchorage.
Bear surveys are necessary work on the Campbell Tract, a recreation area that sees more than 500,000 visits a year and just happens to be home to black bears. Data from the surveys helps staff understand the bears’ denning behavior. It also ensures the safety of Campbell Tract visitors by giving an idea of how much bear activity to expect as the days get warmer, thus minimizing potential human/bear conflicts.
For example, by getting a clear idea of where black bears’ dens are in Campbell Tract, BLM and its partners—including adjacent landowners like the Municipality of Anchorage—can work on keeping foot traffic to a minimum in those areas, especially during the winter denning season and as bears and their new cubs emerge in spring.
Perham is no stranger to bear surveys. The two interns with him, Elizabeth “El” McKenzie and Jordan Oldenburg, are still learning the ropes.
They’re looking for black cottonwood trees, one of the largest trees in Campbell Tract and typically found along creek drainages. Large cottonwood trees—at least 10 meters tall with a diameter of 1.5 meters at breast height—are viable dens when the inner wood rots and the outer sapwood remains, leaving skeletal support of the tree but creating a cavity in the center of the tree. When large branches break off from the trunk, holes are created that a black bear can use to get in and out of the cavity.
There are multiple known den trees along Campbell Creek. These trees are monitored during denning season to record the heat signatures (or lack thereof) that come from inside the tree. Sometimes there are no heat signatures in the dens, and sometimes the den appears to be occupied throughout the winter denning season. In the spring, once the bears become active, they use the dens differently. Multiple bears seem to use the same den cavity but not at the same time.
“It’s like a bear condo,” explained Perham. “We’ve had reports where a specific bear was in a specific den, then came back later to hear that a different bear moved into the same den. These spots seem to be pretty high in demand, so sometimes the smaller, younger bears get pushed out.”
Even if a clear heat signature is found, possible dens can’t be confirmed as positive bear den sites until additional evidence of bear usage is recorded, typically in the form of a visual observation or remote camera footage of a bear at the site.
Thermal imagery cameras are ideal for finding and monitoring bear dens since they’re user-friendly and portable. They work best during the coldest time of the day (early morning), and during civil twilight and clear weather.
McKenzie said she loved the opportunity to “hone in on the natural resource element of [the BLM’s] work,” and that she’s very grateful to be involved in the survey. “It’s probably the coolest thing I’ve been a part of in quite some time,” she added.
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