In the Holarctic, predation by mustelids on birds is often linked to population cycles of rodents (especially voles and lemmings) because birds may be buffered against mustelid predation at high rodent densities. By contrast, interguild relationships between introduced mustelids and rodents can have very different consequences for native birds in ecosystems where mustelids have been introduced. Here, we consider the interactions between Stoats Mustela erminea, feral House Mice Mus musculus and native birds in New Zealand beech Nothofagus spp. forests. We conclude that buffering to protect birds from Stoat predation normally fails in these systems, because peak populations of mice in these forests are low by Holarctic standards, and mice usually do not become sufficiently abundant to distract increased numbers of Stoats from preying on birds. However, temporary buffering is possible during rare episodes of extreme mouse abundance.