Modest increases in densities of burrow-nesting petrels following the removal of cats (Felis catus) from Marion Island

Abstract

Introduced predators are one of the main threats facing seabirds breeding on oceanic islands. Cats (Felis catus) were introduced to subantarctic Marion Island (290 km2) in 1949, and by the 1970s some 2000 cats were killing about 450,000 seabirds per year, greatly reducing burrowing petrel populations. Cats were eradicated by 1991, but house mice (Mus musculus) remain. The densities of utilised petrel burrows were estimated in 2013 by systematically searching for their burrows in 741 10 × 10 m sample quadrats in the north-eastern sector of Marion Island, repeating the sampling design and methods used by Schramm in 1979. The mean burrow densities and 95 % CIs were compared between surveys by species for the different habitat and vegetation types, with non-overlapping CIs considered indicative of an increase in burrow density. With cats eradicated and the potential for immigration from nearby Prince Edward Island (free of introduced mammals), we could expect a multi-fold increase in petrel numbers over the last two decades; however, burrow densities at Marion have increased by only 56 % since 1979. White-chinned petrels (Procellaria aequinoctialis) showed the greatest increase, despite being listed as vulnerable due to incidental mortality on fishing gear at sea. The recovery of other summer-breeding species decreased with decreasing body size, and winter-breeding species showed even smaller recoveries, similar to patterns of breeding success at Gough Island, where mice are major predators of petrel chicks and eggs. Predation by mice is the most likely explanation for the limited recovery of Marion’s petrel populations.