ABSTRACT The Masked Owl Tyto novaehollandiae was introduced to Lord Howe Island in the 1920s to control an irruption of the Black Rat Rattus rattus . The owl is a threatened species in New South Wales but is regarded as a pest on the island due to the perceived risk it poses to endemic land birds and breeding seabirds. However, its main diet there comprises Rattus rattus and House Mice Mus musculus and because it is proposed to eradicate these rodents from the island in 2019, the owls are also scheduled for removal then due to the likelihood of their switching their prey base to endemic land birds and breeding seabirds. Simultaneous point surveys and radio-tracking of two female owls over 14 months in 2009–2010 indicated that owls occupied small overlapping home ranges with smaller discrete home range cores. The population of Tyto novaehollandiae on the island was estimated at between 20 and 30 pairs, almost ten times the density that the species occurs at in its natural range. Roost sites were mainly in the dense crowns of canopy trees and in hollows and crevices of large trees. Analysis of regurgitated pellets and pellet remains showed that owl diets were predominantly comprised of Rattus rattus (in 72% of intact pellets) and Mus musculus (in 28% of intact pellets) with the remainder mostly seabird species (in 21% of intact pellets). It is suggested that the introduction of Tyto novaehollandiae in the 1920s may have slowed the extinction rate of endemic terrestrial birds on the island, with the species now functioning as the top apex predator there and possibly limiting population numbers of Rattus rattus . If Tyto novaehollandiae is exerting mesopredator control, this cautions the removal of its population, as required to prevent it switching its prey base once rodents are removed. Removal of owls prior to the removal of rats could lead to mesopredator release, with further highly detrimental effects on the island's biodiversity, emphasising that owls should be removed concurrently with rodents.