The recent development of Oistance Sampling techniques (OS) has provided a promising device to
yield good population estimates with samples of small size (40-80 independent detections) with reasonable
survey costs. On this subject, during the past Roe deer Meeting (Portugal) were presented
interesting results suggesting that OS coupled with thermal imaging (IR) could be a suitable and costeffective
monitoring methodology.
We present the results of a 3-year project devoted to compare density estimates from OS & IR with
more conventional Mark Recapture (MR) estimates of an Appennines roe deer population. We worked
in an open hunting estate, where a long-term study on roe deer ecology includes radio-monitoring
and densities assessment since 1995. We tested the possibility to survey roe deer using a portable
thermo-camera, equipped with a laser rangefinder and an electronic compass, which allows the operator
to record ani mais presence (group size and number of groups) also in completely dark night
and to measure their distance and angle with respect to the observer. We walked 35 transects (total
effort 20.5 km) in 3 nights, from 7:00 PM to 5:00 AM, to survey an area of 523 ha. We made 2 survey
replicates each year (late winter/early spring) to compare the results with estimates of early spring
survey calculated by MR methods.
Night survey is thought to mitigate the problem of g(O) < 1 (which affect daylight ones when animals
f1y away reacting to the presence of an observer) because the use of an IR camera is supposed to
allow the detection of deer before being discovered. However, IR & OS survey design could be
biased because observers are forced to use paths and so transect positioning may not be random
with respect to ani mais distribution. We present an a posteriori evaluation of the survey design and
we compare the 2 survey methods in order to assess the presence of biases in IR & OS