Prior to human arrival, New Zealand was dominated by birds that had evolved in the absence of mammalian predators. Early Polynesian settlers brought with them the kiore (Rattus exulans), which subsequently decimated many native species. Following this, Europeans brought ship rats (Rattus rattus), Norway rats (Rattus norvegicus), house mice (Mus musculus) and an array of other mammalian predators causing even more damage to the native flora and fauna. Present day conservation strategies seek to control or eliminate invasive predator populations to give the native birds a chance to recover. At Maungatautari – a pest-fenced reserve in the Waikato, New Zealand, mice remain as the sole invasive mammal following extensive eradication programmes. When left on their own, mouse populations are known to greatly increase. On some offshore islands in the South Atlantic and sub-Antarctic, their diet has been shown to include bird eggs and chicks, and on other islands they have used resources that were previously unavailable to them (by predator or competitive exclusion). Therefore, the aims of the present study were to examine how mice; (1) use vertical space in the presence and absence of other mammalian predators and (2) impact bird nesting success when they are the sole predator. Chapter Two examines how mice use vertical space in the presence and absence of other mammalian predators while also quantifying how other small mammals use vertical space. Tracking devices (that included chew tags) were placed at different forest heights within two separate patches of bush, one with only mice present (Maungatautari Sanctuary) and one where all pest mammals were present (Te Tapui Reserve). Mice were found to frequently use the lower (<5m) levels of the native bush; however, they were not detected in the canopy. In contrast, mice were detected much less frequently at Te Tapui and only on the ground. Rats and possums were detected at all forest levels. The presence of other mammals clearly constrains mouse activity through predation and/or resource competition. Chapter Two also examined how mice use the hood of the predator proof fences at Maungatautari in the presence (external) and absence (internal) of other mammals. Cameras and tracking card were installed within the hood of the fence with tracking card also installed at the base of the fence in two areas (i.e. one with mice only present and one with all small pest mammals present) at Maungatautari Sanctuary. Mice were only found to use the base of the internal fence and were never detected in the fence hood. However both rats and mice were detected in the external fence-hood. The hood of the predator-proof fence likely provides invertebrate resources as well as a source of cover. Chapter Three considered the impact of mice on bird nesting success when mice are the sole mammalian predator present. Bird nests were located and monitored until either chicks were fledged or they failed. Nests that were located with eggs already present were monitored for 30 days with a camera capturing any activity. These cameras were set to take an image every 5 seconds nocturnally or could be triggered at any time by movement. Of 17 nests that were found, only 2 were successful. 4 nests were found with eggs that were already deserted and 11 were abandoned during the building stage prior to egg laying. One mouse was filmed on one occasion at a song thrush (Turdus philomelos) nest, but it was not seen to interact with the nest contents. Chapter Four summarises the results from the previous chapters and also makes recommendations for future research. The results presented in this thesis are consistent with the prediction that mice will occupy different forest spaces when they are the sole mammalian pest than when other mammalian predators were present. However no further evidence for active mouse predation on bird nests was found.