Nest defence behaviour, an important component of parental care in birds, was studied in two endemic passerines, the Grey Warbler, Gerygone igata, and the South Island Rifleman, Acanthisitta chloris chloris, at Kowhai Bush, South Island, New Zealand. Theoretical models based on Triver's parental investment theory predict that parents will increase the risk taken to defend their offspring as offspring age. By testing nest defence response of Grey Warblers and Riflemen to a mounted Little Owl, Athene noctua, near the nest I found that both species took a higher level of risk while defending nestlings than eggs. However, I show that the methodology used to study nest defence can affect the results obtained. I repeatedly exposed Grey Warbler and Rifleman parents to the model owl. Their responses after several tests became less intense than for parents that were only exposed to the model once. The level of intensity of nest defence was not affected by either the sex of the parent or whether parents were responding together or alone. Within pairs, Riflemen had highly correlated levels of nest defence response, whereas this correlation was less evident with Grey Warblers. Riflemen were found to adjust their level of defence according to the type of predator threatening the nest. Nest guarding, a previously undescribed aspect of nest defence in male Grey Warblers, was found to be carried out by males only at second nests. I suggest that nest guarding is a response to the threat of brood parasitism by the Shining Cuckoo, Chrysoccocyx lucidus.