Estimating density of mountain hares using distance sampling: a comparison of daylight visual surveys, night-time thermal imaging and camera traps

Surveying cryptic, nocturnal animals is logistically challenging.
Consequently, density estimates may be imprecise and uncertain. Survey
innovations mitigate ecological and observational difficulties
contributing to estimation variance. Thus, comparisons of survey
techniques are critical to evaluate estimates of abundance. We
simultaneously compared three methods for observing mountain hare (Lepus
timidus) using Distance sampling to estimate abundance. Daylight visual
surveys achieved 41 detections, estimating density at 14.3 hares km-2
(95%CI 6.3–32.5) resulting in the lowest estimate and widest confidence
interval. Night-time thermal imaging achieved 206 detections, estimating
density at 12.1 hares km-2 (95%CI 7.6–19.4). Thermal imaging captured more
observations at furthest distances, and detected larger group sizes.
Camera traps achieved 3,705 night-time detections, estimating density at
22.6 hares km-2 (95%CI 17.1–29.9). Between the methods, detections were
spatially correlated, although the estimates of density varied. Our
results suggest that daylight visual surveys tended to underestimate
density, failing to reflect nocturnal activity. Thermal imaging captured
nocturnal activity, providing a higher detection rate, but required fine
weather. Camera traps captured nocturnal activity, and operated 24/7
throughout harsh weather, but needed careful consideration of empirical
assumptions. We discuss the merits and limitations of each method with
respect to the estimation of population density in the field.