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Natural and unnatural predators of ACAP-listed species range from sharks (notably of Laysan Phoebastria immutabilis and Black-footed P. nigripes Albatross fledglings in the North Pacific) to the introduced House Mouse Mus musculus of Tristan Albatross  Diomedea dabbenena chicks on Gough Island.  However, the most serious "predator" of albatrosses and petrels is arguably ourselves, indirectly through longline and trawl fisheries.

What is unexpected is a recent report of a passerine bird as a predator of ACAP species.  The Tristan Thrush Nesocichla eremita is endemic to the Tristan da Cunha group of islands, as are the ACAP-listed Atlantic Yellow-nosed Albatross Thalassarche chlororhynchos and the Spectacled Petrel Procellaria conspicillata.

All three species occur on Inaccessible Island in the Tristan Group, where Peter Ryan and Rob Ronconi spent part of the austral summer of 2009/10.  In a paper recently published in the journal Ardea they report an observation of a Tristan Thrush successfully breaking open a deliberately exposed albatross egg and then consuming its contents.

They also conjecture that Tristan Thrushes were responsible for removing eggs of the Spectacled Petrel from burrows shortly after laying, based on the large number of broken eggs found outside burrows during early incubation.  The authors also report that the thrush is a regular predator of eggs and small chicks of the Great Shearwater Puffinus gravis.

Reference:

Ryan, P.G. & Ronconi, R.A. 2010.  The Tristan Thrush Nesocichla eremita as seabird predator.  Ardea 98: 247-250.  (click here to view the abstract).

John Cooper, ACAP Information Officer, 21 October 2010