Norway rats (Rattus norvegicus) on the Noises and Motukawao Islands, Hauraki Gulf, New Zealand

The Noises and Motukawao Islands in Hauraki Gulf are small (maximum size 26 ha) and bush— clad, and none is permanently inhabited. Norway rats reached the Noises about 1956, but their history on the Motukawao group is unknown. Live and kill-trapping was carried out between August 1977 and December 1981, mainly on the Noises Islands. Trapping success was high initially but declined rapidly and remained very low after mid-1978. Rats travelled widely between consecutive captures in live-traps and three home ranges of males averaged 1.2 ha. Density on one of the Noises Islands was estimated to be 2.6-4.2 rats/ha. The age distributions of kill-trapped rats and skulls found on the Noises were both weighted towards older individuals. Adult males and females were generally similar in body size and condition, but Motukawao rats were in better condition than Noises rats. Males remained fertile throughout the year, whereas females were in breeding condition only between August and April. Histological examination of females showed that, compared with urban and rural Norway rats, maturity was delayed, litters were small and some females ovulated but failed to breed. The most common foods on the Noises were insects, seeds and fruits, and other plant material. Potential factors limiting rat numbers on the Noises are discussed, especially shortages of protein-rich foods and fresh water.