Tristram's Storm Petrel

Oceanodroma tristrami Salvin

SOOTY STORM PETREL

PELAGIC DISTRIBUTION.—The POBSP pelagic sightings of this species are from various positions around the Hawaiian Islands, with concentrations near the Hawaiian Leewards (breeding sites), as would be expected (Figure 116). A disjunct, high-density concentration was also recorded between 44° N and 45°N near 173°W. The latter records may pertain to tristrami populations from the Bonin or Izu islands, or possibly to Oceanodroma matsudairae from the Bonin Islands. It is extremely doubtful if tristrami could be differentiated from O. matsudairae in the field. No specimens are available to clarify which of the two species were seen here, but even if specimens were available, distinctions could not be made between the Hawaiian and Bonin Island populations of O. tristrami.

BREEDING BIOLOGY.—Sooty Storm Petrels have a disjunct breeding distribution, with populations at Torishima in the southern Izu Islands (Austin, 1952), the Bonin Islands, and the Hawaiian Leewards. The specific localities in the Leewards are Laysan, Pearl and Hermes Reef, and Midway. Richardson (1957:19) mentions the possibility of their breeding on Necker but gives no details. Bryan and Greenway (1944:96) record the species on Nihoa. Clapp and Woodward (1968:10) report recent POBSP records on Kure Atoll and French Frigate Shoals. The latter cite the collection of three specimens with definitely enlarged gonads on Kure (one each on 1 January 1964, 1 March 1965, and 12 December 1965). The January 1964 bird was one of two observed digging a burrow. Two other birds were observed digging a burrow on 2 April 1964, and one examined in the field between 30 December 1966 and 5 January 1967 had a completely bare brood patch. Others were seen on Kure between November and January during the years 1963 to 1967. Three specimens collected 1 to 3 January 1967 have greatly enlarged gonads. On 14 March 1967, one very small downy chick and another about three-quarters grown were found in shallow burrows under the grass on Whale-Skate Island, French Frigate Shoals. Amerson noted a fledgling here in early June 1967. I believe it very likely that the species will eventually be found nesting on Lisianski Island, which has an extensive habitat comparable to that which supports a sizable population on Laysan. From this latter island POBSP collected three specimens on 22 October 1966 with considerably enlarged gonads. Willett (1919) reports egg-laying on Laysan from 1 to 15 January.

Inadequate POBSP winter surveys in the Leewards, plus the very secretive nature of this storm petrel on the breeding grounds, make it difficult to obtain a clear picture of the breeding cycle. From available data it appears that the species is to be found on the breeding grounds during an extended period from at least October to May. Apparently December and January are peak egg-laying months; both young and adults are gone from the islands by mid-May.