-Nest site characteristics, breeding phenology, productivity, and mortality factors were evaluated from 1974 to 1976 for the Crested Auklet (Aethia cristatella), the Least Auklet (A. pusilla), and the Whiskered Auklet (A. pygmaea), which nested together in a colony at Buldir Island in subarctic Alaska. These species were compared and data from Buldir were compared with those from previous studies at an arctic colony. The auklets actively competed for nest sites and differed in their choice of site substrates. Crested Auklets, the largest of the three, used the largest nest crevices. Breeding activities occurred on Buldir from mid-May to mid-August. Crested Auklets had a slightly longer nesting period than the other species. Reproductive success (fledglings/eggs) was approximately 50% for Crested and Least auklets, and apparently higher for Whiskered Auklets. Little predation on eggs or chicks was recorded, but avian predators were the major cause of adult auklet mortality. The three species of the genus Aethia nest primarily on islands in or near the Bering Sea and the Sea of Okhotsk (Fig. 1). Crested (A. cristatella) and Least (A. pusilla) auklets nest together in mixed colonies throughout most of their ranges (A.O.U. 1957). The Whiskered Auklet (A. pygmaea) has a more restricted range, but it occurs with the other two species at some colonies in the Aleutian Islands (Murie 1959). The biology of these three species of auklets is poorly known, except at St. Lawrence Island, Alaska, where Crested and Least auklets have been studied intensively (Fay and Cade 1959; Bedard 1967, 1969a, b; Sealy 1968, 1973, 1975; Searing 1977). Stejneger (1885) and Kozlova (1957) presented limited data on all three species of auklets from the Commander Islands. Murie (1959), Gabrielson and Lincoln (1959), Sekora et al. (1979), and Byrd and Gibson (1980) have provided information on the distribution of auklets in the Aleutian Islands, but few data are available from this region on breeding biology. During avifaunal investigations on Buldir Island from 1974 to 1976, certain aspects of the biology of Crested, Least, and Whiskered auklets were studied in a mixed colony including over 250,000 birds (Byrd et al., in press). In this paper, we summarize data on nest site characteristics, breeding phenology, productivity, and mortality of the three species and compare these data with those from previous studies at St. Lawrence Island in arctic Alaska. We compared breeding schedules at Buldir and St. Lawrence islands in order to test the hypothesis that climate dictates the timing and duration of the breeding cycle at high latitudes. We also tested the premise, developed at St. Lawrence, that Crested and Least auklets are segregated in the talus habitat by the sizes of boulders (hence, crevices). STUDY AREA AND METHODS Buldir Island (52021'N, 175056'E) is the westernmost island in the Rat Island group, Aleutian Islands, Alaska (Fig. 1). It is the most isolated island in the Aleutians, being 113 km west of the nearest island. Buldir (6.4 km long, 3.2 km wide) has an area of about 2,000 ha. Volcanic in origin, the island has three dominant peaks, the tallest being 655 m. The main island of Buldir has about 8 ha of talus slopes, and Middle and Outer Rocks, 2 km offshore, have a combined area of I ha of talus. All talus areas on the island are inhabited by auklets. The island has 20 km of coastline including 7.8 km (39%) of large-diameter boulders. This boulder habitat was used by auklets, but to a lesser extent than talus. The Main Talus colony, our primary study area, is located on the north-central side of Buldir Island (Fig. 1). This talus slope encompasses 4.3 ha and ranges from 0.5 to 4.0 m deep. Buldir is one of the few Aleutian Islands where no land mammals have been introduced. Twenty species of seabirds nest there, including 12 species of alcids (G. V. Byrd, unpubl. data). The most common birds nesting in the talus slopes with the three species of Aethia are Parakeet Auklets (Cyclorrhynchus
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