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The field characters of both the thin-billed prion (Pachyptila belcheri (Mathews, 1912) ) and the Antarctic prion (P. desolata banksi (Smith, 1840) and P.d. alter (Mathews, 1912)) are described and a discussion of the differing feeding habits and food is given. The food of Pachyptila belcheri chiefly consists of the amphipod Parathemisto gaudichaudii which is taken nocturnally, while the “krill” Euphausia superba is the primary food source of P. desolata and is mainly captured by day. The thin-billed prion ranges far from its known breeding grounds at the Falkland Islands and Kerguelen, and is the most frequently encountered Pachyptila in the southernmost waters of the central Pacific. Birds of the year reach the Bellingshausen Sea from the Falkland Islands by mid-April, and disperse over much of the South Pacific during May. In early November, sub-mature birds gather in substantial numbers south of the Antarctic Convergence to take advantage of a rich food supply and undergo an early moult in December and January. The adults apparently begin the moult cycle in early February and are through by May. The distribution of both species of prion is discussed on the basis of field work and specimen records, and an attempt has been made to correlate this data with published information. A tentative distributional pattern for both species is presented.