In the late 1980s the endangered Chatham Island oystercatcher (Haematopus chathamensis) (CIO) was estimated at less than 110 individuals. Endemic to the Chatham Islands, New Zealand, it was feared to be declining and, based on existing productivity estimates, in danger of extinction within 50-70 years. These declines were thought to be caused by numerous changes since the arrival of humans, including the introduction of several terrestrial predators, the establishment of marram grass (Ammophila arenaria) which changes dune profiles, and increased disturbance along the coastline. The New Zealand Department of Conservation has undertaken recovery planning and conservation management to increase CIO numbers since the late 1980s. Recovery planning raised some key research questions concerning the population dynamics, habitat selection, and breeding biology of Chatham Island oystercatcher (CIO), and the critical factors currently limiting the population. The objectives of this study were to collect and interpret data on: 1) population size, trends, and distribution across the Chathams, 2) basic breeding parameters, 3) recruitment and mortality rates, 4) habitat selection at the general, territorial and nest-site levels, 5) habitat factors that are correlated with territory quality, and 6) cues that elicit territorial behaviour in CIO.