All weasels are intolerant of crowds, especially crowds of other weasels. Even siblings reared together in captivity, where food supplies are regular and warm nests supplied, have to be separated after they become independent and their squabbles begin to turn nasty (R.A. Powell unpubl.). At that stage or long before, wild-born youngsters will have gone their separate ways.

Most families break up when the young are 3 or 4 months old and fully capable of looking after themselves. Then the young leave the mother's home range to find places of their own to settle. The few observations recorded suggest that young of both sexes are capable of astonishingly rapid travels over long distances before settling down. Most young females remain nearby, usually less than 5 to 6 km away (Erlinge 1977b; Debrot & Mermod 1983). Sixteen of the 18 young females observed by Erlinge (1983) in Sweden stayed in the same area for life.

Young males, however, may travel extraordinary distances in a very short time. In New Zealand in the summer of 1979-1980, six of 65 young males caught in live traps and eartagged were known to have traveled at least 6 km, 8 km, 12 km, 15 km, 20 km, and 23 km. These were only the minimum, straight-line distances between known capture points: The real distances, running in and out and round about as stoats do, must have been much greater. Furthermore, these distances were covered remarkably quickly: 12 km in 27 days plus a further 3 km the next day; 20 km in 5 days plus a further 4 km in the next 2 days; and 23 km in 39 days (King & McMillan 1982). In the same study area a few years later, Murphy and Dowding (1994, 1995) radio tracked an adult male that moved at least 3.7 km in 3 hours and 15 minutes. They also eartagged a young female on December 20, 1990, and were amazed when the same animal (indisputably the same one) turned up in a kill trap outside a rearing facility for rare native birds, 65 km away, on January 13, 1991.

Similar long-distance travels have been recorded even in the smaller weasels. One male stoat tagged in Alaska must certainly have traveled much further than the 35-km straight distance recorded between capture locations in 6 months (Burns 1964). These long treks by young males are no doubt encouraged by intolerance from the established adult males, which tend to move about less once they are established on a home range providing plenty of food. The adult males tolerate the young females, though. The difference in attitude is simply a matter of what serves the best interests of the local resident adult male. He could benefit by driving out the young males but accepting the young females, who will be, next season, his potential rivals and mates.

Most such long treks are made in bumper rodent years when large cohorts of young weasels are born, but are possible any time. The mere ability of such small animals to travel so far explains how least and common weasel populations can spring up where none has existed for years (Chapter 10). So, when populations of any weasel species become extinct, recolonization from long distances away is quite possible. In New Zealand, where reducing stoat populations over large areas is critical to protect native birds, rapid immigration from surrounding uncontrolled populations is a huge problem (Chapter 13).

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