Habitat use as a predictor of nest raiding by individual hedgehogs Erinaceus europaeus in New Zealand

Predation by introduced hedgehogs Erinaceus europaeus is a significant cause of nest failure in threatened endemic wading birds nesting on the dry gravel beds of braided rivers in New Zealand's central South Island. Night-time movements of 10 hedgehogs (four male; six female) were investigated during the 2002 bird breeding season using spool-and-line tracking and GPS data recording. Nine of the 10 hedgehogs studied used river braids rarely and concentrated their activity in scrub and woodland habitat. One female foraged almost exclusively on the dry river braids. The ranked order of habitat preference (scrub>woodland>river braid) matched the ranked availability of hedgehogs' most common invertebrate prey, estimated by pitfall trapping, in the three habitat types. Male foraging ranges were generally larger than those of females, except for the female that foraged on the relatively resource-poor river braids, which had the second largest range overall. This animal had a much higher probability of encountering, and opportunistically raiding, nests than the others. Trapping programmes targetted at nuisance predators commonly aim to reduce overall predator abundances within a designated area using traps set along logistically convenient lines. This design reflects the implicit assumption that all target animals within the area represent an equal risk to threatened prey. Our results suggest that this assumption may not always be valid. To improve nest protection during the birds' breeding season, we suggest that traps should be focussed in and around the nesting habitat in order to target those predators that habitually forage in the area.