Effects of Human Disturbance on Breeding Least and Crested Auklets at St. Lawrence Island, Alaska

-We studied breeding success, chick growth, and diets of Least (Aethia pusilla) and Crested (A. cristatella) auklets on St. Lawrence Island, Alaska, in summer 1987. Least Auklets had higher breeding success on control plots (50-66%) than on disturbed plots (36%). Crested Auklets had a breeding success of 42% on disturbed plots. Predation by microtine rodents and weather accounted for most natural chick mortality. Least Auklet chicks grew at a maximum rate of 4.9 g/day, and Crested Auklet chicks at 12.8 g/day. Least Auklet chicks were fed mostly copepods (Neocalanus plumchrus), whereas Crested Auklet chicks were fed Thysanoessa euphausiids. Received 13 December 1988, accepted 30 November 1989. LEAST Auklets (Aethia pusilla) and Crested Auklets (A. cristatella) are the most abundant planktivorous seabirds in the North Pacific, and breeding populations coexist at many colonies in the Bering Sea (Sowls et al. 1978). Both species lay their eggs in natural crevices formed in the rubble of talus slopes. There have been few detailed studies of auklet breeding biology, probably because auklet colonies are remote, and breeding sites are difficult to observe. We studied Least and Crested auklets on St. Lawrence Island in summer 1987 to learn more about their basic breeding biology and to evaluate methods used to measure breeding success. After several years of study on St. Lawrence Island, Bedard (1969a, b) described the breeding habitat and diets of auklets, and Sealy (1968) reported on chick growth and development (Sealy 1973, 1981), factors that influence breeding chronology (Sealy 1975, 1984) and survival of eggs and chicks (Sealy 1982). Elsewhere, Knudtson and Byrd (1982) estimated Least and Crested auklet breeding success at Buldir Island in the western Aleutians, and Roby and Brink (1986) studied Least Auklet breeding biology during two summers at St. George Island in the Pribilofs. All previous investigators have noted the negative effect of human disturbance on auklet breeding success, but this factor has never been quantified. We attempted to measure the effect of human disturbance on auklet breeding success and to identify natural sources of egg and chick mortality. Selected Least Auklet study plots were subjected to three levels of disturbance. Because fewer Crested Auklets were found, they were studied at only one (high) level of disturbance. We also measured chick growth and collected food samples to determine the composition of auklet diets. STUDY AREA AND METHODS The study was conducted from 28 May to 2 September 1987 at Kongkok Bay (63?24'N, 171?49'W) near the southwestern cape of St. Lawrence Island (Fig. 1). The avifauna, climate, and seabird breeding habitats of St. Lawrence Island have been described elsewhere (Fay and Cade 1959, Bedard 1969a, Sealy 1975). We established sixteen 200-M2 auklet study plots on the southeast-facing (inland) talus slope of Owalit Mountain (Fig. 1). At least 20,000 auklets (2:1 Least/ Crested) breed on those slopes (Searing 1977, Piatt et al. 1990). All 16 plots were inspected 3-4 times over 13 days during the early egg-laying period to locate eggs, check the status of previously located eggs, and mark breeding sites. Some eggs were located after minor excavation, but most were located by a line-ofsight search with strong flashlights. All inspections were conducted during mid-day (1400-1800) when attendance by adults on the surface of talus slopes was minimal (Piatt et al. 1990). Least Auklet breeding sites on one set of eight "Intensive" plots were checked every day weather permitted through the hatching period (27 July-13 August), every 2-4 days up to early fledging (25 August), and every day possible thereafter until 2 September. We recorded weights and wing-chord lengths of Least Auklet chicks on Intensive plots only. To reduce disturbance, Least Auklet breeding sites in the other set of eight "Control" plots on Owalit Mountain were inspected only four times after the initial search for eggs (but see below). Least Auklet chicks were not handled on Control plots. The 342 The Auk 107: 342-350. April 1990 This content downloaded from 157.55.39.144 on Wed, 21 Sep 2016 06:22:33 UTC All use subject to http://about.jstor.org/terms April 1990] Breeding Biology of Auklets 343 final check of breeding sites on all plots (including those below) was especially thorough, and excavation was sometimes used to determine the status of chicks. On 23-25 July, in another set of eight control plots on the north-facing talus slope of Kongkok Basin (Fig. 1), we searched Least Auklet breeding sites for eggs. These plots were then checked twice before chicks fledged to determine the status of chicks. Observer disturbance in Kongkok was minimal compared with Owalit, where observers were on the slopes daily to check plots and weigh chicks. Least Auklet densities were higher at Kongkok, and total numbers were far greater (ca. 1 million birds; Searing 1977, Piatt et al. 1990) than at Owalit. Because we located fewer Crested Auklet breeding sites, we studied them on all 16 Owalit plots with the same level of intensity as the "Intensive" Least Auklet study plots. Crested Auklet chicks were weighed and measured in all 16 study plots. Although we tried to avoid disturbing Least Auklets in their breeding crevices, our work on Crested Auklets increased disturbance to Least Auklets on the eight "Control" Owalit plots. It is difficult to monitor the breeding status of Least and Crested auklets because neither builds a nest, and eggs or chicks are never in plain view. Detailed written instructions with reference to orange spray-painted marks and numbers on rocks were used to locate breeding sites again. When eggs and chicks disappeared between site-checks, their fate was difficult to assess. Many reappeared on subsequent checks, but those that never were located again may have been taken by predators, moved permanently, or died of