An enclosure for protecting small mammal traps from disturbance

Disturbance of live traps by raccoons, opossums, skunks, or other animals is often a serious problem in small mammal studies. Even a low incidence of trap molestation resulting in sprung traps or escape, injury, or death of captured animals can significantly bias results, and the level of disturbance may become so severe that trapping has to be discontinued. Trap interference is more likely to become a problem in long term investigations involving periodic trapping at permanent stations, as the longer trapping continues the greater the opportunity for potential trap molesters to become aware of the traps and conditioned to them as a food source. An example of such an effect is provided by trapping results on three grids on the Archbold Biological Station, Highlands County, Florida, that have been trapped at monthly or quarterly intervals since 1968. The grids consist of 144 stations (12 by 12) at intervals of 15.2 m, with one large Sherman trap set at each station during 4-day trapping periods. During trapping periods the first year of the study, 1.2% of the traps were sprung as the result of rain, insects, overly sensitive treadle adjustment, or occasional disturbance by raccoons or other animals. Trap disturbance by raccoons increased in 1969 and became progressively greater in subsequent years. The percentages of traps disturbed each year from 1969 to 1977 were as follows: 1969, 3.8; 1970, 4.0; 1971, 11.6; 1972, 22.4; 1973, 24.7; 1974, 37.7; 1975, 30.3; 1976, 52.0; 1977, 45.9. The greatest amount of disturbance occurring during a single trapping period was 58.3% on one grid and 95.1% on the other two. As raccoons in the study area typically spend the day in the tops of tall pines (Worley, 1980), individuals residing on and in the vicinity of the grids were immediately aware of when traps were being set. Thus a considerable number of traps were often raided on the first night of a trapping period, although the level of disturbance was usually higher from the second night on. This continuing problem with raccoons led to the development of the trap enclosure described below. Design and construction.--The enclosure is constructed of 1.3 by 2.5 cm 16-gauge welded wire mesh, and consists of a 12.7 by 15.2 by 30.5 cm cage with hinged top housing the trap and a 7.6 by 7.6 by 22.9 cm entrance tunnel (Fig. 1). The trap cage has a floor, but the entrance tunnel does not. The dimensions of the trap cage accommodate a trap up to the size of a large Sherman trap (7.6 by 8.9 by 23.5 cm), but can be modified for larger-sized traps. The enclosure is comprised of five individual parts that can be cut from a 54 by 92 cm piece of material. The components are assembled with "J" clips. An L-shaped piece of galvanized metal is riveted to the inside back wall of the cage to prevent raccoons from reaching in with a finger and opening the back of a large Sherman trap. The cage lid is held closed by a latch formed from a 22-cm length of no. 12 insulated copper wire (Fig. 1). White insulation is preferred as it makes the latch more visible if laid on the ground when checking the trap. The insulation is stripped off one end, which is twisted into a spiral forming a loop with a diameter of approximately 1 cm. The loop is slipped around juxtaposed strands of the lid and anterior edge of the cage when the cover is closed. The other end is made into a handle. The lid can also be held closed by running a 25-cm long stick or wooden dowel through overlapping meshes of the cage and lid from one side to the other. When the enclosures have been in use long enough for raccoons or other animals to learn that they cannot get at the traps, the lids may not have to be locked closed. If animals begin to discover that the lids can be opened, they can be relocked until the trap molesters are conditioned to again ignore enclosures with traps. When installing the enclosure, the bottom of the cage should be forced down into the soil far enough to conceal the wire floor, which if exposed might tend to inhibit entrance by some small mammals. The enclosure is anchored in place by two stakes made of 0.95 cm steel reinforcing rod. The tunnel is held down by a U-shaped stake at the rear end where it joins the trap cage and the back of the enclosure is held in place by a stake with a hook attached to the bottom strand of wire at the center of the back of the trap cage. The length of the stakes depends upon the type of soil. Evaluation of effectiveness.-In the 9 years since the enclosures have been in use on four grids and eight