Response of a reintroduced bird population to rat reinvasion and eradication

Many endemic species on islands are vulnerable to predation and local extinction by introduced rats (Rattus spp.). As a result, the reintroduction of species to predator-free sanctuaries is a successful conservation strategy, especially in New Zealand. Nevertheless, reintroduced populations, even those that reach high densities, are still vulnerable to predation in the event of a rat reinvasion, and may also be susceptible to non-target poisoning during a subsequent eradication operation. We quantify for the first time the changes in population size and survival rate of a well-established, reintroduced species (Stewart Island robins, Petroica australis rakiura) following the reinvasion and eradication of Norway rats (Rattus norvegicus) on Ulva Island, New Zealand, in 2011. The robin population declined by nearly one-third (31.5%; 432 to 296 adults) in the breeding season following the rat reinvasion and eradication. The survival rate of robins prior to the poison operation was only slightly lower than expected, which suggests the growing population of Norway rats may have had a relatively minor negative effect on robin survival. In contrast, the majority of the decline occurred immediately following the poison operation. This suggests the robins were susceptible to non-target poisoning from the brodifacoum poison bait, although the robin population would have likely declined even further if Norway rats had not been eradicated. Our results indicate the importance of developing permanent surveillance systems on island sanctuaries to detect and kill rats upon arrival in order to avoid the potentially high rates of non-target poisoning associated with post-invasion, large-scale eradication operations.