On July 1, 1917, in Clay County, Nebraska, a nest with four young was found (Swenk, 1926:321). On July 29, 1939, an adult and five young were plowed out of the ground in Allegan County, Michigan; one of the two young males weighed 40.5 grams two days after capture (Allen, 1940:459-460). On August 12, 1932, ten young with the mother, were found in Roseau County, Minnesota (Swanson and Fryklund, 1935:125). September appears to have been the month of birth of a specimen, no. 8472 in the Carnegie Museum, taken on November 24 in Pittsburgh, Pennsylvania. In October, a young least weasel is recorded from Pennsylvania (Winecoff, 1930:313). Early December was the time of birth of a specimen, approximately 10 weeks old, no. 88077, University of Michigan, taken on February 21 in Allegan County. On December 25, 1927, in Washington County, Pennsylvania, "five full-sized, though young . . . animals were caught under the same pile of corn fodder" (Sutton, 1929:253). The first week of January seems to have been the time of birth of a juvenile, no. 88080, University of Michigan, taken in Livingston County, Michigan, on March 27, 1943, since the specimen is approximately seven weeks old. On January 15, 1929, in Washington County, Pennsylvania, four young with the eyes yet unopened were obtained from a nest (Sutton, 1929:254). On January 25, 1928, young, the eyes of which may not yet have been open, were taken from a den in Washington County, Pennsylvania, by Winecoff (1930:313), who records other young having been taken in the same month as well as in February. On March 10, a female from North Portal, Saskatchewan, gave birth to four young (Dunk, 1946:392). On April 18, 1916, four young, half grown, were taken in Nebraska (Swenk, 1926:317). On April 2, 1929, three young were found in Roseau County, Minnesota, according to Swanson and Fryklund (1935:125) who remark that: "The Pennsylvania and Minnesota records show that least weasels may be born any time from July to early February in the northern states." Now, with all of the above records available, it turns out that November, May and June are the only months in which young least weasels have not been reported. Of course some of the young, for which the ages were not specified, were born in preceding months. Even so, the data now available suggests that, in the United States, young least weasels may be born in every month of the year. The number per litter is 3, 4, 4, 4, 5, 5, and 10, yielding an average of 5.
The rate of growth of the young has not been studied enough to allow of judging if it differs significantly from that of other species of the genus. Allen (1940:459-460), however, tells us that of the three young females and two young males captured on July 29, 1939, in Allegan County, Michigan, one male that was killed on July 31, 1939, weighed 40.5 grams. The male remaining alive increased from 46 grams (August 5) to 62.5 grams on September 20, having eaten 63 mice while in captivity. The females in the period of August 5 to September 4 increased in weight as follows: 41 up to 49 grams; 44 to 50 grams; and 47 to 58 grams, having eaten, by September 26, 60, 64 and 65 mice.
Concerning a nest in which young were found, Sutton (1929:254) writes that on January 15, 1929, near Burgettstown, Washington County, Pennsylvania, an animal was seen to enter a small hole in a creek bank. After the observer dug in a distance of approximately six inches an adult, female least weasel was seen and obtained. Back of the animal, the hole, which turned sharply downward, was full of water. The weasel first seen was a female nursing young. A chamber, to the side of the hole, filled with dead grass, comprised a nest containing four young with the eyes yet unopened. Several nests occupied by adult least weasels or by least weasels that were old enough to shift for themselves have been found. Polderboer (1942:145-147) in the winter of 1939-40, on a 144 acre farm in Butler County, Iowa, found four least weasels living, singly, in burrows dug by moles and pocket gophers. The nests therein made by mice were used by the least weasels. Winecoff (1930:312-313) mentions one den in Pennsylvania that contained the remains of only mice, "and not a hint of a feather." Above, in the account of food of the least weasel, Criddle's (1926:199-200) account of the havoc wrought by least weasels among the meadow mice (Microtus ochrogaster minor) has been given. In this account he mentions the fur-lined nests of the weasels that had appropriated the homes of the Microtus. Criddle's (1947:69) later account of a nest at Treesbank, Manitoba, is as follows: "A Nest of the Least Weasel.—When a least weasel finds its way into a locality that has a large number of mice in it, it selects for its home one of their nests that has been made in a well concealed place. This it immediately starts to improve by lining it with hair plucked from its victims before eating them; and as long as sufficient numbers of mice remain in the district the weasel continues adding their hair to the nest, so that the thickness of its walls give one a good idea of the length of time it has been in use. The nest is not only used for sleeping in, as most of the food is consumed in it. Frozen mice are taken in to be thawed out and the weasel carries those it has recently killed in to prevent them getting frozen, or perhaps to have them warm for its next meal.
"On January 27, 1946, my son Percy called my attention to a nest that he had just uncovered in a clover stack that we were using. This nest had originally been made by a Drummond's vole, Microtus pennsylvanicus drummondii, and taken from it by the least weasel, Mustela rixosa, the tracks of which had been noticed about the stack yard since the first snow in early November.
"The nest had evidently been in use for at least three months and the continual additions made to its walls had been so great that they were nearly an inch thick of hair matted together so closely that it appeared to be felt. The hair alone weighed nearly 22 gm., so that with this for protection the weasel must have been warm and comfortable through the severest winter weather.
Fig. 28. Map showing occurrences and probable geographic ranges of the subspecies of Mustela rixosa in North America.