Montana biologists believe good weather and possibly the cyclical fluctuation of populations may be responsible for a 73% increase in the state’s 2020 estimate of greater sage grouse numbers compared to 2019.
In an Aug. 18 report to the Legislature, Montana Fish Wildlife and Parks put the 2020 population estimate at 77,977 birds. The agency estimated there were 44,867 greater sage grouse in the state in 2019, the report states.
The increase of 33,110 birds marks a jump of 73% between the two years. Biologists and others surveyed 805 breeding-ground leks during the spring mating season, counted the number of strutting males, and plugged those numbers into a mathematical formula to arrive at the estimate.
Montana holds about 11.6% of the world’s greater sage grouse. Other western states with significant greater sage grouse populations reported much smaller increases this year, if any at all.
Wyoming, which holds an estimated 38% of all greater sage grouse, averages the number of strutting males seen on leks statewide, then graphs that figure to see whether numbers have increased or decreased. In 2020, the state saw a slight decline in the average number of males seen strutting on active leks and reported a “leveling out” of a three-year slide. Lek attendance “held steady,” the agency said.
Montana’s population explosion may be due to favorable weather in 2019, the report states.
“The lack of widespread drought or extreme weather events (e.g., hail, flooding) during this period may have positively influenced late summer food resources and led to higher survival and recruitment,” the report reads. “Data from FWP’s sage-grouse research project in central Montana suggests nest success and hen survival were comparatively high in spring and summer 2019.”
If the central Montana data reflected conditions in the rest of the state, “they could explain the increase in the number of sage-grouse attending leks in spring 2020,” the report reads.
The annual compendium suggests “natural fluctuations” may also have some influence on the increase. “Sage-grouse population numbers oscillate over a period of 8-10 years across large scales,” the report reads. “It is important to consider long-term patterns over time and not make management decisions based on one or a few years of lek counts, especially at broad scales.”
Although biologists debate the concept of grouse population cycles, one peer-reviewed study found that 11 of 15 greater sage grouse populations “were cyclic at some point in a 50-year time series (1965–2015).” The cycle patterns “varied over both time and space,” authors wrote.
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The 2020 estimate of the total population “look[s] to be just above the long-term average,” Catherine Wightman, wildlife habitat coordinator with Montana Fish, Wildlife and Parks, told WyoFile just before the report was published.
The Legislature also charged her agency with reporting information about leks themselves. The number of confirmed active leks dropped from 1,019 in 2019 to 998 in 2020. The number of leks confirmed extirpated held steady at 66, the report says.