2021 Field Results

Last summer, we completed the field portion of our project’s research by implementing a second season of helicopter surveys within potential grizzly bear foraging areas for army cutworm moths (ACM), courtesy of in-kind contributions from Two Bear Air Rescue. As well, and thanks to donors to our crowdfunding campaign (https://experiment.com/projects/why-do-grizzly-bears-climb-mountains-to-eat-moths), we also conducted a small, repeat ACM count-sampling survey at two close-proximity survey units located on different mountains. With the crowdfunding, we hired previous field technicians Danny Proctor and Burt Bjorling to implement the ground surveys. We are indebted to Danny and Burt for their sweat and dedication to the project over its three field seasons!

Technicians gathering data at a survey plot – Photo: Steven Gnam

Aerial Surveys

We conducted six rounds of aerial surveys, spaced approximately two weeks apart, from early July to mid September. In total, we surveyed 29 units containing potential grizzly bear foraging habitat for ACMs in Glacier, Waterton Lakes National Park, and Akamina-Kishinena Provincial Park. We documented grizzly bears foraging for ACMs in 9 of these units, comprising 65 groups and 99 individuals. Roughly 80% of our group observations occurred in just 3 of the 9 units.

A grizzly bear observed foraging for ACMs during an aerial survey of potential habitat

Females with young and lone adults made up 82% of our grizzly bear group observations. A period of abnormally hot/dry conditions during the latter half of July likely impacted our 3rd survey, when we observed a sharp, temporary decline in grizzly bear detections. Otherwise, our survey observations were rather consistent for July through the first half of August. Beginning in late August, our grizzly bears survey observations declined markedly.

Grizzly bear observations across survey units (green bars) & the number of surveys with detection per unit (brown circles) – 80% of grizzly bear groups were observed in survey units 5, 15, and 24

Grizzly bear observations across age/sex classes – 82% of grizzly bears groups were females with cubs and lone adults

Ground Surveys

Our ground sampling effort occurred in two adjacent survey units, whose mountains both experience frequent off-trail human use. By repeat sampling for ACMs across their season of talus slope occupancy, we wanted to gain insights into how their abundance varies over time. Would moth abundance vary at some locations, but not others? Are moths generally more abundant at certain survey units over others? By repeat sampling moths on multiple occasions can we begin to understand this better.

Army cutworm moths in flight at night

At the two survey units, each comprising a single mountain, we sampled three different talus slopes. Unit 1 had two slopes, unit 2, one slope. On each slope, we sampled 7 survey plots where we previously detected ACMs in summer 2020. We count-collected ACMs at all plots on five occasions, spaced two weeks apart. For each sampling, we dug talus and collected moths at adjacent, previously undisturbed locations in four meters apart from one another.

Count-sampling ACMs at a survey plot – Photo: Steven Gnam

At unit 1 plots we counted more than double the number of ACMs as unit 2 plots. Army cutworm moth occupancy at plots across sampling sessions in unit 1 was 0.7, compared to 0.4 in unit 2. This means that on average, we observed ACMs at survey plots in survey unit one 70% of the time, unit two, 40% of the time. Moth abundance over time also differed between survey units. At unit 1, ACM abundance generally increased across surveys, whereas at unit 2, moth abundance remained relatively constant before declining.

Moth abundance over time at two ground survey units

It’s an exciting time for us. Our project is entering its final stages. We are currently analyzing project data collected by our field technicians across three field seasons . We aim to complete our analysis, write and defend a thesis in the coming months. Publications stemming from this research will be completed at a later date.

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