Wedge Rumped Storm Petrel

Oceanodroma tethys (Bonaparte)

GALAPAGOS STORM PETREL

PELAGIC DISTRIBUTION.—The Galapagos Storm Petrel is one of the few white-rumped species in the Pacific that can be identified with a fair degree of certainty under most field conditions. The extremely large, prominent, wedge-shaped white rump, and alternately fluttering and bouncy flight are good characters for distinguishing most individuals from Oceanodroma leucorhoa, the species regularly recorded with O. tethys over much of the latter’s pelagic range. When I first encountered tethys at sea, and before I learned some of its more subtle characters, I had difficulty making on-the-spot distinctions between it and equally small, exceptionally white-rumped O. l. socorroensis. Part of this difficulty arose from the fact that both forms were in extremely ragged plumage at the time. It may be noted that specimens of both races of O. tethys had medium-gray oil (uropygial) glands, as opposed to the creamy white oil glands of all races of O. leucorhoa and of O. castro.

Supposedly, the nominate race (O. t. tethys) breeds in the Galapagos from April through September(?). The POBSP pelagic specimens of this race are confined to August 1967 and March and April 1968. During the latter period concentrations were especially prominent in a broad band along the equator from about 3°N to 2°S between longitudes 97°W and 105°W. August specimens were taken at 12°N, 120°W. A June specimen in the San Diego Natural History Museum was taken at 15°N, 101°W. Huey (1952) reports a specimen taken by J. R. Pemberton in March 1938 at Roca Partida in the Revilla Gigedo Islands south-southwest of Baja California (ca. 19°N). In this same publication Huey reports a male of this race in nonbreeding condition (testes 3 × 2 mm) taken from a colony of Oceanodroma leucorhoa kaedingi on Guadalupe Island off Baja California in January 1950. The bird was found in a nest cavity with a downy young kaedingi. For whatever reason the bird came to the island, Hubbs (1960) is incorrect in regarding this as constituting a breeding record for the Galapagos Storm Petrel at Guadalupe. I have examined this specimen at San Diego and, except for a short, heavy bill and slightly small wing for O. t. tethys, it corresponds well to that form. This locality, at roughly 29°N, apparently marks the northern limit for the nominate race.

The second-named form of this species, O. tethys kelsalli (Lowe), breeds on the Pescadores and San Gallan Islands off the coast of Peru. Northernmost POBSP records of this race are of two individuals collected on 6 August 1967 at 23°03′N, 118°24′W. Moffitt (1938) reports 12 specimens collected at 22°30′N, 112°39′W on 22 July 1905 and another specimen taken at 16°45′N, 110°28′W on 22 September 1933. These birds had traversed northward through most of the recorded pelagic range of the nominate race. Our pelagic sight records (Figure 111) cannot be qualified to race as the two named forms are separable only on mensural differences. The pelagic ranges of the two races show considerable sympatry.

Other POBSP specimens of O. t. kelsalli were collected in the Gulf of Panama during August 1968 and June and August 1969. Large numbers of O. tethys were observed in the gulf from August through November 1968 and June through August 1969. Although only a relatively few specimens were collected, all are definitely referable to O. t. kelsalli. On this basis, it is reasonable to assume that most Gulf of Panama wintering O. tethys are of the race kelsalli. From November on, tethys is absent from the Panama gulf waters, when it is apparently replaced by the Black Storm Petrel and the Least Storm Petrel.

MOLT.—Considerable variation in molt is displayed in two series of O. t. tethys collected at sea. A series of 36 taken between 1 March and 2 April falls essentially into two categories:

1. Molting: As shown in Figure 112, 44 percent of the birds in this sample are molting in both primaries and rectrices. There was no exact correlation in the progress of molt between these two plumage areas. A few birds still molting the primaries had completed their rectrix molt and vice versa. Hence, although only 44 percent of the sample is involved in each case, the total number of individuals concerned in these combined categories equaled 50 percent of the sample. In practically every case, primary and/or rectrix molt was accompanied by body molt.

2. Nonmolting: The remainder of the sample (50%) not in primary or rectrix molt, was, for the most part, in old, but relatively unworn plumage, indicating that molt had occurred within the previous few months. Definition of plumage condition in rectrices or remiges of birds not in molt is somewhat subjective. When molting, a distinction is easily made between new and old feathers, primarily because the feathers being replaced are obviously old and show considerable wear and discoloration.

The majority of birds in both molting and nonmolting categories were adults, judging by gonadal condition. None was in breeding condition, although several may well be considered either to have begun gonadal enlargement or retrogression. The nominate race of O. tethys is supposed to have a complete postbreeding molt from July through October (Murphy, 1936:730). It appears from our sample that the duration of the molt cycle is much more extensive. If tethys has a molt cycle similar to races of O. leucorhoa, a large number of birds would be expected to terminate a molt just prior to the advent of a breeding cycle.

The small series of five birds collected in August are all in finishing stages of complete molt, or in very new plumage. A combination of immatures and adults is represented, judging by gonads. This series substantiates Murphy’s (1936:730) statement concerning a postbreeding molt from July through October, but half of the large series taken in March and April certainly do not correspond to this pattern. Obviously we need to know the ages and times of breeding cycles of individual birds before it will be possible to draw conclusions concerning the molt cycle of the species.

The wedge-rumped storm petrel (Hydrobates tethys) is a storm petrel. It breeds in the Galápagos Islands and on the coast of Peru. It was formerly defined in the genus Oceanodroma before that genus was synonymized with Hydrobates.

breeding on Galapagus