Importance of tree cavities as natal dens for fishers

We determined characteristics of natal dens used by radio-collared fishers in southcentral Maine. Twelve adult (greater than or equal to 2 yr old) females were monitored for periods of 1-5 yr each during 1984-1989. Estimated whelping dates of 12 litters were March 3-April 1, and females used 1-5 natal dens each until denning ended in early June. All 33 natal dens were in tree cavities on upland sites. Hardwoods composed 94% of the den trees, with aspens accounting for 52%. We recommend that forestry practices allow for a continued supply of mature and decadent hardwoods greater than 40 cm dbh to replace cavity trees that fall. Where cavity trees must be removed, cutting should be done outside the March-June denning period to avoid disturbance or mortality of fisher litters.