UNDER-SNOW SHELTER FOR SMALL MAMMAL TRAPPING

The inexpensive, easily constructed shelter described allowed routine livetrapping of Microtus pennsylvanicus to continue successfully when temperatures fell as low as -35 C. It also provided sum- mer shade so that traps needed checking only once a day to maintain low trap mortality. Trap mor- tality rates are given. While making a long-term study of an old-field population of Microtus pennsyl- vanicus in southern Manitoba we needed an effective sampling method under winter conditions, where temperatures, during the December-March period, may reach -45 C. It is well known, however, that tempera- tures beneath several feet of snow remain within a few degrees below freezing. We therefore suspected that a Longworth trap, supplied with food and bedding material, placed in this subnivean space, the normal winter environment, should keep animals alive. Attempts to trap in this space proved fruitless, since the traps quickly fouled with snow, and were not readily found by the animals. We needed a shelter which would keep the traps snow-free and could be checked without disturbing runways. Two such shelters have been described in the literature. The system described by Fay