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Foraging on Moths by Bears

Bear activity while at a moth site is remarkably predictable, unvaried, and perhaps even tedious. Bears spend between 50 and 70% of their daylight hours feeding on moths and, of that moth-focused time, they spend roughly 55-65% of it eating and 20-30% digging. The remainder of the day they mostly spend dozing in beds, although a small fraction, <5%, is spent grazing.


This near-exclusive focus on consuming moths is reflected in estimates of ingested diet based on analysis of bear feces--i.e., scats--collected at or near moth sites . The graph immediately below shows estimated fractions of different items, averaging the results of analyses from three different studies--one from Glacier National Park, and two from Yellowstone's Absaroka Mountains. At 82%, moths are unambiguously dominant, with grasses and sedges and fruits (Glacier) or whitebark pine seeds (Yellowstone) a far distant second and third. Given that as much as 40% of Yellowstone's grizzly bear population visits Absaroka moth sites, all of this signifies a major dietary role for moths at a population level, on a par with whitebark pine seeds, cutthroat trout, and meat from ungulates such as elk and bison.

Bears spend only a small percentage of their time seeking out favorable microsites to excavate, but, when they do, they seem to be searching primarily with their nose. Once they find a promising spot, they roll back the overlying rocks to expose lethargic chilled moths, which they verify visually, and then lap up with long flexuous tongues. Most excavations are between 20 and 50 cm (8-20 inches) deep, which, at deepest, include the isothermal conditions sought out by moths during peak daylight warmth. The photos are right provide a good impression of the kinds of angular medium-sized talus favored by bears, presumably because it is favored by moths. All told, Don White estimated that a bear could eat 40,000 moths a day, comprised of 40-80% fat, and acquire roughly 18,000 kilocalories in the process.

When bedded during the middle of the day, bears utilize a variety of microsites. Most often they bed at the base of large boulders, but will also commonly bed in open talus, on ledges, or in nearby tundra and trees. During rare episodes of grazing, bears typically target patches of alpine clover or sedges and grasses in more productive wetter sites.

As to the kind of involved bears? In the northern US Rockies, nearly 100% of the observed bears are grizzlies. Black bears don't even bother to show up. Of the bears that are present, an average 32% are females with cubs or yearlings and 44% are lone adults, although these fractions range from 28-38% for family groups and 24-52% for lone adults. In the net, family groups are over- and lone adults under-represented. Despite the potential for inter-individual conflict, little seems to occur, which facilitates a surprising degree of crowding during certain days and on certain sites. 

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