Optimising the sampling of skinks using artificial retreats based on weather conditions and time of day

Abstract

Artificial retreats are a low-cost, low-impact and effective technique for detecting cryptic taxa, such as herpetofauna, and have potential as a monitoring tool. The success of using artificial retreats for monitoring is dependent on our ability to maximise detection and minimise variability in sightings within a sampling period. Because ectotherms use retreats for their thermal properties, it may be possible to identify weather conditions during which animals consistently use retreats and design a monitoring protocol based on optimal conditions. We investigated the use of artificial retreats by common skinks, Oligosoma polychroma, in mixed grassland habitat in southern New Zealand under various weather conditions and at a range of times of day. We recorded 1175 sightings of skinks during 1800 artificial retreat checks over a nine day period in early summer 2008. Ambient temperature, rainfall, humidity and time of day were the strongest predictors of skink sightings beneath retreats. Skink sightings were highest and least variable: (1) in an ambient temperature range of 12-18°C, (2) during light or no rain, and (3) at low levels of relative humidity. Under optimal weather conditions, skink counts were similar during the day (8 a.m. to 5 p.m. inclusive) but lower during the evening (8 p.m.). Observer variability, type of substrate beneath the retreat and retreat colour did not significantly influence the number of skinks sighted. We highlight the need to optimise protocols for checking artificial retreats on a species- and site-specific basis before attempting to evaluate their use as a monitoring tool for herpetofauna.