Field evaluations of thermal optics and tracking tunnels for small mammal surveys at airports and grass dominated habitats

Abstract

Assessments of small mammal (e.g., rodents) relative abundance provide important information to safely manage airfields, as small mammals attract larger mammals and raptors that are involved in collisions with aircraft. Snap‐trapping is the standard method for assessing small mammal relative abundance at U.S. airports, yet thermal scopes/optics or tracking tunnels may be efficient alternative methods. Thermal optics (used at night) convert objects’ far‐infrared energy (3–14 µm) into visible images. Tracking tunnels are baited ink cards placed inside plastic tunnels that allow the footprints of visiting animals to be identified. After suspecting thick and tall grass interfered with spotting mammals smaller than striped skunks ( Mephitis mephitis , ~1.8–2.7 kg) when using thermal optics, we assessed detectability by placing carcasses of heated mice ( Mus musculus ) of 2 sizes ( = 19.8 g and 33 g) within varying heights of grass (0–20.3 cm) and distances from observers (4–30 m; n  = 144 trials) when thermal units were held by an observer (1.7 m above ground) or when fastened to the roof of a truck (2.1 m above ground). Also, within airfields, we compared small mammal relative abundances using tracking tunnels to those using snap‐traps. We found that grass ≥10 cm tall consistently obscured thermal detections of both mouse carcass sizes, yet mouse carcasses were detected in ~50% of attempts in 8.9 cm tall grass and 78% of attempts in 7.6 cm tall grass. Most airports do not maintain grass to heights <10 cm. Detectability decreased with distance from the observer. There was no significant difference in detections between roof‐mounted and hand‐held thermal optics, nor did mouse carcass size significantly affect detectability. Small mammal detections using tracking tunnels significantly correlated with snap‐trap captures (best fits [R = 0.51–0.73] with 1 or 2 nights of tracking tunnels active). We concluded that the use of ground‐based thermal optics would not be a viable survey tool for mouse‐sized mammals (19–41 g) at airfields and other grassy habitats with thick grass unless grass height was maintained ≤7.6 cm tall. Tracking tunnels may be a viable alternative method to snap‐traps for assessing small‐mammal relative abundances and potentially saving one day of snap‐trapping labor.