Flexibility of diet of stoats on Fiordland islands, New Zealand

The eradication operations to remove stoats (Mustela erminea) from islands in Fiordland provided an opportunity to assess the diet of stoats in areas with no rodents or with only mice (Mus musculus) available as mammalian prey. The carcasses of stoats trapped on Chalky Island in 1999, Secretary Island and the adjacent mainland in 2005, and Resolution Island in 2008 were collected and their gut contents analysed. On rodent-free Chalky Island, most of the stoats had consumed birds, mostly passerines. Stoats on Secretary Island (rodent-free) and Resolution Island (mice present) preyed mostly on invertebrates, particularly wētā (Orthoptera). On Resolution Island, mice were probably at relatively low densities, and were consumed by only 12% of the stoats. While average consumption of birds and invertebrates was lower for stoats at the mainland site, the only significant differences amongst the sites were the high bird consumption and low invertebrate consumption on Chalky Island compared with the other sites. The diet of male stoats was similar to that of female stoats on both Secretary Island and Resolution Island. Chalky Island male stoats were heavier than those on the other islands, while the females on the various islands had similar body weights. The variability in diet of stoats from these islands may in part reflect the temporal and spatial differences between the samples. However, it demonstrates the adaptability of stoats, and their ability to survive without mammalian prey in different ways. It supports the hypothesis that differences in body weights of stoats are at least partly driven by variation in prey size and/or availability.