Leach's storm petrels Oceanodroma leucorhoa off southern Africa: breeding and migratory status, and measurements and mass of the breeding population

Abstract

Leach's storm petrel Oceanodroma leucorhoa have bred at Dyer Island since at least 1995; a review of earlier records in the light of this discovery indicates that breeding has almost occurred since at least the 1970s. The South African offshore islands on which Leach's storm petrel almost certainly breeds are Jutten, Dassen, Dyer and St Croix. The South African population is probably of the order of 25 pairs. The migrant component of the Leach's storm petrel population visiting southern Africa is mainly confined to deep ocean waters (generally 2000–5000 m deep) and occurs in particularly high densities around Agulhas Rings where ring-edges apparently provided excellent foraging opportunities. Leach's storm petrels are comparatively scarce over the shelf areas (defined as coastal water of less than 2000 m depth) of Namibia and western South Africa. Given their abundance at sea, their distance offshore, the seasonality of their occurrence and their moult status, these birds cannot be part of the South African breeding population, and must therefore be mainly from colonies of the north Atlantic Ocean. The Leach's storm petrels breeding in South Africa appear to be relatively large, comparable in size to populations breeding in the north Atlantic Ocean. Molecular techniques should be used to determine the most closely related population, and possibly the period for which the South African population has been isolated.