Changes in population size, habitat use and breeding biology of lesser sheathbills (Chionis minor) at Marion Island:: impacts of cats, mice and climate change?

Impacts on the native avifauna of sub-Antarctic islands by introduced vertebrates are well documented for species such as burrowing petrels (Procellariidae), but less is known about impacts on surface-nesting species. The sub-Antarctic Prince Edward Islands, comprising Marion and Prince Edward Islands, support an endemic form of lesser sheathbill (Chionis minor). Marked differences have developed over the last 20 years in winter habitat use and population trends of sheathbills between the two islands. Currently birds at Marion forage almost exclusively along the coastline and in penguin colonies during winter, but at Prince Edward they occur at higher densities and forage throughout the coastal plain. Compared to the 1970s, sheathbills at Marion are now less abundant around most of the island, forage proportionally less in the vegetation of the coastal plain, commence breeding with a lower body mass and lay smaller clutches. These changes are likely a result of a decrease in terrestrial macro-invertebrate prey, formerly an important winter food resource for sheathbills at Marion Island. The main biological difference between the two islands is the presence of many more introduced species at Marion, including house mice (Mus musculus) and feral cats (Felis catus) (now eradicated). We suggest that house mice are impacting the sheathbill population by consuming terrestrial macro-invertebrates, and that this impact has been exacerbated by the removal of feral cats, by the massive reduction in burrowing petrels (which promote invertebrate populations through manuring), and by climatic warming (which may be promoting higher densities of mice). This proposed web of interactions between sheathbills, introduced species, invertebrates and burrowing petrels needs to be further investigated, given the likelihood of further climatic change. (C) 2000 Elsevier Science Ltd. All rights reserved.