The foods of least weasels have been sampled in the Arctic, at Barrow on the north coast of Alaska, and in the extreme north of European USSR (Figure 5.3a, b). There they live almost entirely on lemmings and voles when these rodents are abundant. In the years when the lemmings and voles disappear, life for least weasels becomes extremely precarious. Alternatives are few and fleeting, except in the short northern summer.
Every year the tundra is occupied by vast flocks of migratory birds that forage and nest on the ground. During summers of lemming crash years, these birds become unwilling providers of eggs and young for hungry least weasels. In the lemming crash year of 1969 at Barrow, the nests of sandpipers and Lapland long-spurs suffered heavily. In such straitened circumstances, least weasels also kill less favored alternative prey, such as shrews, and scavenge carrion of large animals they could not kill themselves or leftovers from kills made by large predators (Nasimovich 1949). Averaged over many years, as in the long collection from the Arkhangelsk region in the far north of European Russia, these various subsidiary resources give a false impression of variety in the diet of the northern least weasels (Figure 5.3b). By the following winter, the migratory birds have returned south, the seasonally active mammals have gone into hibernation, and subzero air temperatures freeze carrion solid and prevent all small mammals from venturing above snow for long. Then, least weasels have no choice but to search and search and search for the last few live lemmings or voles under the snow.
In North America, the range of the least weasel extends south of the Canadian border into the northern prairie states and down the Appalachian mountain chain. Here least weasels, still virtually confined to small rodents, share their only resource with many other, more generalist predators. This competition is hard on least weasels, and it means that they are usually scarce except in habitats or at times that are exceptionally good for small rodents (Chapter 10). For example, in the Southern Appalachian Mountains, least weasels are most often captured in orchards, where thick fescue grasses provide superb habitat for meadow and woodland voles (R. A. Powell, unpublished data). In northern Fennoscandia, Microtus voles also dominate the diet of least weasels, occasionally supplemented with bank voles and mice (Korpimaki et al. 1991). By contrast, in primary forest in Poland, bank voles and yellow-necked mice dominate the diet of common weasels (Jgdrzejwski & Jgdrzejwska 1993; Jgdrzejwski et al. 1995).
In the end, and despite the differences in their morphology and reproduction, least and common weasels have extremely similar diets and respond in the same way to changes in prey abundance. Common and least weasels are the most specialized of all weasels for preying on small rodents, especially voles (Jgdrzejwska & Jgdrzejewski 1998).
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