A tale of two islands: contrasting fortunes for Subantarctic skuas at the Prince Edward Islands

  • PG Ryan
  • PA Whittington
  • RJM Crawford

Abstract

Subantarctic skuas Catharacta antarctica are key predators of burrowing petrels at sub-Antarctic islands, and can be used to monitor the health of burrowing petrel populations. A survey of skuas at the Prince Edward Islands was conducted during December 2008, repeating a previous survey in December 2001. Prince Edward Island (46 km2) remains free of introduced mammals, whereas Marion Island (290 km2) had a feral population of cats from the 1950s to 1980s, and still supports a large population of introduced house mice Mus musculus. Breeding skuas were more widespread, occurred at greater densities and extended to higher elevations at Prince Edward Island than Marion Island. Prince Edward Island also supported twice as many non-breeding birds. Burrowing petrels comprised 96% of prey in skua middens at Prince Edward Island compared to only 22% on Marion Island where penguins are more important. The numbers of breeding pairs at Prince Edward Island increased from 2001 to 2008, probably as a result of better coverage in 2008, whereas the number of skua nests on Marion Island was barely half that counted in 2001, continuing an apparent decrease in this species at Marion Island since the 1980s. There is no evidence that removal of cats from Marion Island in the early 1990s has benefited the major native predator of burrowing petrels.

Keywords: burrowing petrels; Catharacta; diet; distribution; Marion Island; population size; Prince Edward Island

African Journal of Marine Science 2009, 31(3): 431–437

Author Biographies

PG Ryan
Percy FitzPatrick Institute, DST/NRF Centre of Excellence, University of Cape Town, Rondebosch 7701, South Africa
PA Whittington
East London Museum, PO Box 11021, Southernwood 5213, South Africa; Department of Zoology, Nelson Mandela Metropolitan University, PO Box 77000, Port Elizabeth 6031, South Africa
RJM Crawford
Marine and Coastal Management, Department of Environmental Affairs, Private Bag X2, Rogge Bay 8012, South Africa; Animal Demography Unit, Department of Zoology, University of Cape Town, Rondebosch 7701, South Africa
Section
Articles

Journal Identifiers


eISSN: 1814-2338
print ISSN: 1814-232X