Polar Biol (2008) 31:333–349 DOI 10.1007/s00300-007-0363-2 123 ORIGINAL PAPER Introduced mammals coexist with seabirds at New Island, Falkland Islands: abundance, habitat preferences, and stable isotope analysis of diet Petra Quillfeldt · Ingrid Schenk · Rona A. R. McGill · Ian J. Strange · Juan F. Masello · Anja Gladbach · Verena Roesch · Robert W. Furness Received: 26 April 2007 / Revised: 29 August 2007 / Accepted: 4 September 2007 / Published online: 16 October 2007  Springer-Verlag 2007 Abstract The largest known colony of Thin-billed prions Pachyptila belcheri has been coexisting with introduced mammals for more than 100 years. Three of the introduced mammals are potential predators of adults, eggs and chicks, namely ship rats Rattus rattus, house mice Mus musculus and feral cats Felis catus. We here determine habitat prefer- ences over three seasons and dietary patterns of the unique set of introduced predators at New Island, Falkland Islands, with emphasis on the ship rats. Our study highlights spatial and temporal diVerences in the levels of interaction between predators and native seabirds. Rats and mice had a preference for areas providing cover in the form of the native tussac grass Parodiochloa Xabellata or introduced gorse Ulex europaeus. Their diet diVered markedly between areas, over the season and between age groups in rats. During the incubation period of the prions in Novem- ber–December, ship rats had mixed diets, composed mainly of plants and mammals, while only 3% of rats had ingested birds. The proportion of ingested birds, including scav- enged, increased in the prion chick-rearing period, when 60% of the rats consumed prions. We used  13 C and  15 N to compare the importance of marine-derived food between mammal species and individuals, and found that rats in all but one area took diet of partly marine origin, prions being the most frequently encountered marine food. Most house mice at New Island mainly had terrestrial diet. The stable isotope analysis of tissues with diVerent turnover times indicated that individual rats and mice were consistent in their diet over weeks, but opportunistic in the short term. Some individuals (12% of rats and 7% of mice) were highly specialized in marine-derived food. According to the iso- tope ratios in a small sample of cat faeces, rodents and rab- bits were the chief prey of cats at New Island. Although some individuals of all three predators supplement their ter- restrial diet with marine-derived food, the current impact of predation by mammals on the large population of Thin- billed prions at New Island appears small due to a number of factors, including the small size of rodent populations and restriction mainly to small areas providing cover. Keywords Ship rat · House mouse · Invasive species · Stable isotopes Introduction Small ground-nesting birds on islands rarely survive intro- ductions of mammalian predators such as rats (e.g. Atkin- son 1985), because their dispersal opportunities are limited and because often such insular populations evolved in the Ingrid Schenk: deceased Electronic supplementary material The online version of this article (doi:10.1007/s00300-007-0363-2) contains supplementary material, which is available to authorized users. P. Quillfeldt (&) · J. F. Masello · A. Gladbach · V. Roesch Max-Planck Institut für Ornithologie, Vogelwarte Radolfzell, Schlossallee 2, 78315 Radolfzell, Germany e-mail: petra.quillfeldt@gmx.de I. Schenk · I. J. Strange New Island Conservation Trust, The Dolphins, Stanley, Falkland Islands R. A. R. McGill Scottish Universities Environmental Research Centre, East Kilbride, Glasgow G75 0QF, UK R. W. Furness Institute of Biomedical and Life Sciences, Graham Kerr Building, University of Glasgow, Glasgow G12 8QQ, UK