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 verte brates are well documented for species such as burrowing petrels (Procellar iidae), but less is known about impacts on surface-nesting species. The sub -Antarctic Prince Edward Islands, comprising Marion and Prince Edward Islan ds, support an endemic form of lesser sheathbill (Chionis minor). Marked di fferences have developed over the last 20 years in winter habitat use and p opulation trends of sheathbills between the two islands. Currently birds at Marion forage almost exclusively along the coastline and in penguin coloni es during winter, but at Prince Edward they occur at higher densities and f orage throughout the coastal plain. Compared to the 1970s, sheathbills at M arion are now less abundant around most of the island, forage proportionall y less in the vegetation of the coastal plain, commence breeding with a low er body mass and lay smaller clutches. These changes are likely a result of a decrease in terrestrial macro-invertebrate prey, formerly an important w inter food resource for sheathbills at Marion Island. The main biological d ifference between the two islands is the presence of many more introduced s pecies at Marion, including house mice (Mus musculus) and feral cats (Felis catus) (now eradicated). We suggest that house mice are impacting the shea thbill population by consuming terrestrial macro-invertebrates, and that th is impact has been exacerbated by the removal of feral cats, by the massive reduction in burrowing petrels (which promote invertebrate populations thr ough manuring), and by climatic warming (which may be promoting higher dens ities of mice). This proposed web of interactions between sheathbills, intr oduced species, invertebrates and burrowing petrels needs to be further inv estigated, given the likelihood of further climatic change. (C) 2000 Elsevi er Science Ltd. All rights reserved.