山 階 鳥 研 報(J. Yamashina Inst. Ornithol.), 33:142-147,2002 Breeding Population of Streaked Shearwaters (Calonectris leucomelas) and the Effect of Norway Rat (Rattus norvegicus) Predation on Sasudo Island Kyung-Gyu Lee* and Jeong-Chil Yoo* Abstract. To estimate the breeding population of Streaked Shearwaters (Calonectris leucomelas) on Sasudo Island, Jejudo, Korea, quadrat surveys were conducted during the hatching period in 1998, the early rearing period in 1999 and the early incubation period in 2000. In 2000, burrow density was 0.15/m2±0.02 (mean±SE). Burrow density was high in the abandoned field, rocky area and take off site of the birds in the island, where 71.3% of 595 burrows were occupied. The breeding population was estimated to be 7,486 ±3,000 pairs (mean±SE). Only 36.4% and 19.8% of burrows were occupied in 1998 (n=353) and 1999 (n=140), respectively. Burrow occupancy seemed to decrease through the breeding stages. Breeding failures were recorded in 31 out of 54 burrows (57.4%) in 7-29 August 1999. Predation by Norway Rats (Rattus norvegicus) was possibly the cause of mortality of eggs or chicks in 26 of these 31 unsuccessful burrows. Norway Rat predation may be the main factor affecting the breeding success of Streaked Shearwaters on the island. Key words: Breeding population, Burrow density. Norway Rats (Rattus norvegicus), Predation. Streaked Shearwaters (Calonectris leucomelas). キ ー ワ ー ド:繁 殖 数,巣 穴 密 度,ド ブ ネ ズ ミ(Rattus norvegicus),捕 食,オ オ ミ ズ ナ ギ ド リ(Calonectris leucomelas). Introduction Streaked Shearwaters (Calonectris leucomelas) breed in the northwest Pacific Ocean on islands off Korea, China and Japan (Marchant & Higgins 1990). They lay one-egg clutches and have a long breeding period including the laying period (mid-late June), the incubation period (late June-early August) and the chick rearing period (middle August- early November) (Yoshida 1981). Usually they make burrow nest about one meter under surface. In Korea, Streaked Shearwaters are distributed on about ten islands and the Sasudo Island is believed a major nesting ground. The island has been surveyed previously on several short trips (Won 1970, Yoon et al. 1986, Park & Won 1993), however detailed research was not conducted. Norway Rats (Rattus norvegicus) were the only introduced predator on the island (Park & Won 1993). This paper presents an estimate of the Streaked Shearwater breeding population and some evidence of the impact of Norway Rat predation. Received 22 May 2001, Revised 9 October 2001, 2nd Revision 25 December 2001, Accepted 26 December 2001. Paper presented in a symposium held by the Japanese Seabird Group and Pacific Seabird Group (Lihue, Hawaii; February 2001). * Korea Institute of Ornithology , Kyunghee University, Seoul 130-701, Korea. 142 Breeding Population of Streaked Shearwaters off the Southern Korea Peninsula 143 Fig. 1. Sasudo Island showing the habitat types. Beyond the forest boundary there are steep slopes or rock cliffs, making nest inaccessible. Study area Sasudo Island (33°55'N,126°38'E), belongs to the Chujado archipelago, about 30 km south of mainland Korea (Fig. 1). It has an area of approximately 22 ha and a maximum altitude of 79m. Its south and east coasts are rocky, with steep cliffs, whereas the north has a medium slope. The island is covered with broadleaf evergreens, mainly dense Litsea japonica. The understorey is generally open with the main ground cover of leaf-litter and the some grasses, Ardisia japonica and Ophiopogonjaburan as the dominant species (Lee et al. 1986). A lighthouse was constructed in 1989, with no permanent residents. The island was inhabited by 2-3 fishermen approximately 100 years ago and a small area of the island was cultivated at that time. Currently 4-5 fishermen stay in a cabin for several months. The island is also known as a well-known recreational fishing area. Methods Burrows census was carried out during 9-14 August 1998, 13-24 August 1999 and 144 Kyung-Gyu Lee and Jeong-Chil Yoo 12-16 July 2000. All area of the island were searched systematically. Potential breeding areas were determined by map, foot searches, photos survey of the surrounding landscape and aerial film. Most potential breeding areas were within the forest. The south-western steep slope is covered by shrub and grass and no burrows were found. Quadrat sizes and numbers were 5m×6m (n=34) in 1998,10m×10m (n=18) in 1999 and 10m×10m (n=41) in 2000, respectively. Each of the quadrats were located in similar area during the three year period. In 2000, number of burrows in quadrats were counted about two weeks after the laying period. Burrows containing adults were regarded as "breeding burrows" in this year. In 1998 and 1999, the number of burrows with adults were counted during the hatching and early rearing period, respectively. Some breeders may have failed and nonbreeders may have occupied the burrows during this period, as found in Manx Shearwaters (Puffinus puffinus puffinus) (Brooke 1990). Therefore these occupied burrows were not always regarded as breeding burrows in these two years. So, we estimated breeding population only in 2000. The term 'occupied burrow' will be used when we discuss the result of the three year period variation of occupied burrow as a whole. To determine whether they were occupied or non-occupied burrows, the nest cham- ber was traced through the burrows in all quadrats, usually by extending an arm as far as possible up the burrow. Range of burrow lengths were from 20cm to 220cm (Lee unpubl.). Most burrows were so long that it was necessary to use a one or two meter branch as an extension. In vacant burrows, we could reach the back of the burrow with the stick. If burrows contained adults, the branch was attacked by adults and if burrows contained chicks alone, chicks were detected by their movement or sound. In 2000, the habitat preference was also assessed. Forty-one quadrats were repre- sented four main habitats: abandoned field (n=9), rocky area (n=5), take off site (n= 5) and forest habitat (n=22). Abandoned fields were small cultivated areas, not used for at least 30 years with uneven stepped surface ground. Take off sites indicate ground near coast or cliff used for launching. Launching birds were directly observed in take off sites. Forest indicates all potential breeding areas except the above three habitats. Breeding habitats were estimated by the GIS program (GeoMedia 3.0). To examine the causes of mortality, fifty-four burrows were monitored daily from 7 to 29 August 1999. At first, 54 burrows containing eggs (35 burrows) and chicks (19 burrows) were checked. Twenty-seven burrows were fitted with trap doors at the entrance for 12 days. The function of the trap door was to retain the adult in the burrow. It also allowed direct observation of a rat predation. Results Approximately seventy percentage (15 ha) of the island was estimated as potential breeding area in 2000. Estimated burrow density and total breeding population are summarized in Table 1. Average burrow density was 0.15/m2±0.02 (mean±SE). There is a highly significant difference among the average number of burrows in the four habitats Breeding Population of Streaked Shearwaters off the Southern Korea Peninsula 145 Table 1. Estimated burrow density and breeding population of Streaked Shearwaters on Sasudo Island during 12-16 July 2000. This period is about 2 weeks after laying period. Table 2. Mortality in 54 nests of Streaked Shearwaters in 7-29 August 1999. (Kruskal-Wallis test, X23=35.153, P<0.001). The burrow densities in the abandoned field, rocky area and take off site are significantly higher than those in the forest habitat (Mann-Whitney U-test, U9,22<198, P<0.001; U5,22<110, P<0.001; U5,22=109.5, P< 0.001, respectively). Percentage of occupancy was not significantly different among four habitats (X23=1.288, P=0.732) and the percentage of breeding burrow out of the total burrow surveyed was 71.3% (n=595). The percentage of occupied burrows in the post-laying period in 2000 (71.3%) was almost double that of the hatching period in 1998 (36.4%, n=353) and three times that of the early rearing period in 1999 (19.8%,n=140). In 1999, 31 out of 54 burrows with eggs or chicks failed breeding and 83.9% of failures were caused by Norway Rat predation (Table 2). Discussion High burrow densities were recorded in the abandoned field area, rocky area and take off site in the forest. These habitats are concentrated near cliff areas or coastal areas. Take off sites with many droppings without leaf-letter are apparently distinguished from the surrounding areas. The largest take off site (A) was located on the bottom of the 146 Kyung-Gyu Lee and Jeong-Chil Yoo abandoned field in the northern valley (Fig. 1). More than 1,500 launching birds were observed in August 1999. Other sites (B-E) had about 1-200 launching. The abandoned field area also allows birds to dig burrows more easily than on the steep western slope. There was no much grass. Also, the rocky area located near the cliffs may be used for launching. It is suggested that launching point has an important role in distribution of the Streaked Shearwater burrow on the island. In July 2000, 31 burrows were occupied by adults who had an egg. In August 1999, 54 out of 73 occupied burrows (74%) contained eggs or chicks. Nineteen burrows contained one (n=10) or two adults (n=9) without an egg or chick during daytime.