Campbell Island Teal (Anas nesiotis)are a critically endangered species. The wild population in its natural range may be less than 30, although a captive breeding programme has raised more than 60 birds and the species is safe from extinction in the short to medium term. Due to the isolation of, and the difficulty of access to, Dent Island in the Campbell Group, the sole surviving natural habitat, no detailed study of the behaviour and ecology of Campbell Island Teal in the wild has been undertaken. Although observations have been made of captive birds, the release of Campbell Island Teal onto Codfish Island (Whenua Hou Nature Reserve) provided the first opportunity to carry out a detailed ecological study of this species in the wild. Aspects of the ecology of Campbell Island Teal (Anas nesiotis) were studied following two releases onto Codfish Island in 1999 and 2000, in order to assess the success of the releases in establishing a new population. A programme was designed to monitor the attempt to establish a self sustaining population of teal on Codfish Island, and to record how captive bred birds adapted to life in the wild by observing characteristics and behaviours of the teal that allowed them to establish a successful population. This information will be used to increase the chances of any future release of this species onto Campbell Island succeeding. Birds were released at two sites on Codfish Island, with different sex and age ratios released in April 1999 and May 2000. The survival and dispersal of these birds was monitored using radio transmitters, with direct observations made on their behaviour and ecology. In these respects, Campbell Island teal were found to be most similar to the Auckland Island Teal (Anas aucklandica). During the three years of this study 24 birds were released, with individuals being monitored from one to three years. Forty two eggs were laid in 13 nesting attempts by eight different females, resulting in 36 ducklings hatched and 17 fledged. Only four adults are known to have died during this period although 11 other birds could not be located at the end of the study. The dispersal of the birds from their release sites varied greatly, probably influenced by habitat quality, although this was not quantified. While the inability to track some individuals may mean that dispersal was greater than recorded, of those birds which could be tracked, juvenile males dispersed the farthest with three individuals moving more than 3km after fledging. Hatching and fledging success were influenced by the age of the females and it appears that environmental factors, especially availability of suitable damp feeding areas for ducklings, were the main factors affecting fledging success. The dispersal of the teal, their habitat preferences and breeding success provide guidelines on where birds should be released on Campbell Island and the optimum age of birds to be released. The high adult survival rate and productivity of the captive bred birds indicates that, providing sufficient are released, the teal should establish rapidly on Campbell Island.