| Summary: | Six ship (roof, black) rats (Rattus rattus) were cage-trapped adjacent to a pest-proof fence and released with radio transmitters inside the 65 ha pest-free exclosure at Maungatautari, North Island, New Zealand, to mimic reinvasion. Unexpectedly, four of the six rats climbed back out of the exclosure and returned to their original home ranges after periods ranging from a few hours to seven days. All six rats travelled along the fence top at some time during follows, and only three of the six used tracking tunnels set on a 50 m grid inside the exclosure to detect invaders. The rats that remained inside the fence stayed within C. 100m of the release point for about three days, then made increasingly large (to 1100m) movements into the reserve. Resultant range lengths greatly exceeded those of four other rats radio-tracked outside the fence where rat density was higher. This behaviour is very similar to that reported for experimentally released house mice (Mus musculus) and Norway rats (R. norvegicus) on islands. These results suggest that a) some invading ship rats may themselves vacate a fenced sanctuary without encountering efforts to detect and remove them; b) rats at low density have much larger movements than occur in home ranges at typically higher mainland densities, and c) managers should target rat invaders with detection and killing devices within 100 m of a fence breach for at least three days, and some traps should be set on top of the fence.
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