Sato et al.: Swinhoe’s Storm-Petrel in Japan133

BREEDING OF SWINHOE’S STORM-PETREL OCEANODROMA MONORHISIN THE KUTSUJIMA ISLANDS, KYOTO, JAPANFumio Sato1, Kiyotaka Karino2, Akio Oshiro2, Hisashi Sugawa2 & Masashi Hirai2,3
1Bird Migration Research Center, Yamashina Institute for Ornithology,
115 Konoyama, Abiko, Chiba, 270–1145, Japan (sato@yamashina.or.jp)
2
Japanese Bird Banding Association, c/o Bird Migration Research Center, Yamashina Institute for Ornithology,
115,Konoyama, Abiko, Chiba, 270–1145, Japan
3
Kyoto Prefectural University, 74 Oji, Kitainayazuma, Seika, Kyoto 619–0244, Japan (mhirai@kpu.ac.jp)

Received 13 January 2010, accepted 21 September 2010

Breeding of Swinhoe’s Storm-Petrel Oceanodroma monorhis isonly known on coastal islands in Japan, Korea, far-east Russiaand China (Lee & Won 1988, Sato 1996, Kondratyev et al.
2000). Global population estimates range from 30 000 individuals(Boersma & Groom 1993) to 100 000 pairs (Birdlife International2009), far fewer than the closely related Leach’s Storm-Petrel
Oceanodroma leucorhoa (Boersma & Groom 1993). Most reportsof O. monorhis are from observations or captures of birds outside
breeding areas (Bailey et al. 1968, James & Robertson 1985,
Bretagnolle et al. 1991, Cubitt 1995), whereas studies of thebreeding biology of this species are very limited (Won & Lee 1986,Lee & Won 1988). In addition to having a limited population, somecolonies of O. monorhis are threatened by human activities andintroduced predators. Although several colonies are known to be
active in Japan, most contain only small numbers of individuals(Sato 1996). Yoshida (1981) noted breeding of Swinhoe’s StormPetrel on the Kutsujima Islands, Kyoto, Japan, but details of
this colony had remained unexamined. Because the size of theKutsujima Islands suggested the possibility of a large breeding
population, we surveyed the colony and estimated the size of thebreeding population.

Fig. 1. Breeding areas of Swinhoe’s Storm-Petrels in Japan.

The Kutsujima Islands (35°43′N, 135°26′E) are located in the Seaof Japan, 25 km north of the port city of Maizuru in Kyoto, Japan(Figs. 1, 2). The islands are composed of two rocky islets, surroundedby steep rocky cliffs. They extend 650 m from north to south, andtheir maximum width is approximately 150 m. The highest points
are 89 m and 74 m above sea level on the north and south islets,

Fig. 2. Map of the Kutsujima Islands, prepared from an aerial
photograph, showing approximate size and shape of the islands.
Hatched areas are evergreen forests; shading depicts grassland.
Remaining surface consists of bare rocky areas.

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Sato et al.: Swinhoe’s Storm-Petrel in Japanthe south islet was not fully examined, however. In our preliminary
survey during the breeding season of 2008, we found eggs, chicks
or incubating birds in six out of 10 nest burrows. Assuming anoccupancy rate of 60%, we estimated that there were 3 800 pairson the south islet (Table 1). That figure may increase substantiallywhen the north islet is surveyed. In contrast, the breeding populationof Streaked Shearwaters on the Kutsujima Islands was very small(Table 1).

respectively. Narrow ridges are covered by small forests, dominatedby an evergreen tree species Machilus thunbergii; the forests aresurrounded by grasslands. The Tsushima warm current flows nearby.
The breeding of Swinhoe’s Storm-Petrel on these islands was first
noted in 1965 by Yoshida (1981). Three other colonial seabird
species are known to breed on the Kutsujima Islands—Blacktailed Gulls Larus crassirostris, Streaked Shearwaters Calonectris
leucomelas and Japanese Murrelets Synthliboramphus wumizusume.
Kammurijima Island, about 4 km south of the Kutsujima Islands,
is heavily colonized by the Streaked Shearwater (Yoshida 1981)
but is not known to have any breeding Swinhoe’s Storm-Petrels.
Neither the Kutsujima Islands nor Kammurijima Island has any
human inhabitants. Landing on these islands, except for research or
religious purposes, is prohibited by local laws.

The breeding of Swinhoe’s Storm-Petrel has long been noted inJapan (Kuroda 1922, Kumagai 1935, 1936; Fig. 1, Table 2), butthere is recent concern about populations at historical breedingsites. Bentenjima Islet, Shiriyazaki, Aomori Prefecture, has beenconnected to the mainland for the mining of limestone, andbreeding there is believed to have ceased. A recent survey ofOshima Island, Nanatsujima, Ishikawa Prefecture, found no signof an active breeding population (Tachibana 1986). Hoshigamijima
Island, Oki, Shimane Prefecture, may have a breeding population,
but detailed surveys have not been conducted. Recent evidenceof Swinhoe’s Storm-Petrels breeding in Japan has been reported
for only three areas outside of the Kutsujima Islands—Kojine
Islet off Hachijo-kojima Island, Tokyo; Sanganjima Island, Iwate;
and Koyashima Islet, Fukuoka. Observers estimated no morethan 200 pairs colonizing in each area (Kumagai 1935, 1936, Sato1996). Although large colonies of Leach’s Storm-Petrel are known
on Hokkaido, breeding of Swinhoe’s Storm-Petrel on Hokkaido
has not been documented (Osa & Watanuki 2002). Therefore, thetotal population of Swinhoe’s Storm-Petrel in Japan is estimated tobe <5 000 pairs, and the breeding colony on the Kutsujima Islands
may be the largest.

We surveyed the south islet of the Kutsujima Islands during thebreeding season, on 25 August 2007 and 11 August 2008. Foursmall forests grow on the south islet, in the far northern, northern,
central and southern areas (Fig. 2). The estimated number of nests
was based on nest counts in sample plots set in the forests. The
southern forest is dominated by Machilus thunbergii, but otherevergreen trees, Camellia japonica and Ilex integra, also growthere. The central forest is also dominated by M. thunbergii. The
far northern and northern forests were comparatively bushy, withsmall M. thunbergii trees mixed with a vine species, Elaeagnus
macrophyllus. Vines made our nest survey difficult, and we could not
set sample plots in the far northern forest. However, we were able toset a small sample plot in the northern forest. In all, we established
one sample plot in each of three forest sections of the south islet
and counted burrows in each plot (4 × 15 m–25 m; Table 1). The
north islet of the Kutsujima Islands also has forest dominated by
M. thunbergii, but we could not reach the area because of the steep
cliffs that ring the islet. Therefore, the population of Swinhoe’s
Storm-Petrel on the north islet remains unstudied.

The breeding population on the Verkhovsky Islands, off Vladivostok,Russia, is estimated to be 8 370 pairs, with an additional 100 pairs
or fewer found on small islands nearby (Kondratyev et al. 2000).
As many as 2 500 birds were ringed on Dagong Island off Qingdao,China (Cai 1998), where half of the birds released in one year wereretrapped the following year. Although the details of the ringing
were not described, we consider there are at least 5 000 individuals
in that colony. In addition, it is estimated there are a minimumof 100 pairs at Wu Yu (Black Rock) off Nan’ao Island, Guangdong,China (Sibley et al. 1999), and approximately 7 900 to 11 890 pairs
in the colony on Chilbal Island, southwest of the Korean Peninsula

Nest burrows of Swinhoe’s Storm-Petrel were found under theevergreen canopy on the ridge, but were not found in the grasslands,
where root systems of the grasses may make digging burrows
difficult. The forest floor had poor ground cover and a scattering ofmany small rocks. Petrel burrows were mainly found under theserocks and were easily distinguished from those of the Streaked
Shearwater by their smaller size. The occupancy rate of the nests on

TABLE 1
Estimated numbers of Swinhoe’s Storm-Petrels and Streaked Shearwaters on the south islet, Kutsujima Islands, Japan
Observations

Forest areaSouthCentralNorth

Far NorthYear examined

2008

2007

2008

not examined

Estimated area of wood (m2)

1 650

1 200

350

300

4 m × 25 m

4 m × 20 m

4 m × 15 m

Nests in sample plot

163

236

14

Estimated total nests

2 690

3 540

82

Nests in sample plot

702

Estimated total nests

116

Size of sample plot

Total

Swinhoe’s Storm-Petrel
6 312

Streaked Shearwater
12Marine Ornithology 38: 133–136 (2010)

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Sato et al.: Swinhoe’s Storm-Petrel in Japan(Lee & Won 1988, Taoka et al. 1989). Finally, it is estimatedthat there are as many as 112 600 pairs on Kugul Islet, Sohuksan
Island, Korea (Won & Lee 1986). Although other Korean islands
may have colonies of this species, information on population sizes
is not available. Based on these surveys, we estimate there are aminimum of 130 000 pairs of Swinhoe’s Storm-Petrel worldwide.
However, that figure depends largely on the estimated breeding
population on a single islet—Kugul Islet, Korea. More detailed
surveys are needed to refine the estimate and to obtain information
on population trends.
Some colonies of Swinhoe’s Storm-Petrel are threatened byhuman activities, accidentally introduced predators, tourism andother factors. The breeding population on Verkhovsky Island isthreatened by recreational visitors in warm seasons (Kondratyev etal. 2000), and Dagong Island is also visited by tourists. The colony
on Koyashima Islet, Fukuoka, was decimated by accidentallyintroduced Norway Rats Rattus norvegicus (Takeishi 1987) andhas not recovered fully, despite subsequent removal of the rats.
Intense fishing occurs near the breeding areas of Swinhoe’s StormPetrels, which likely results in occasional landing by fishermen andincreases the risk of accidental introduction of rats to the breedingcolonies. Although we found no evidence of rats on the Kutsujima
Islands, and the possibility of their introduction seems low undercurrent landing restrictions, small numbers of amateur fishermen
may sometimes come ashore.
The Kutsujima Islands are rocky, and soil on the ridge appears shallow.
If soil were lost during heavy rain or a typhoon, the colony could beseriously affected. Competition for nesting sites between Madeiran
Storm-Petrels Oceanodroma castro and Streaked Shearwaters has
been reported on Hide-shima Island, Iwate Prefecture (Sato &Tsurumi 2003). Competition with Swinhoe’s Storm-Petrel does notseem to be an issue on the Kutsujima Islands, however, because therocky forest floor is not suitable for digging large burrows, which
the Streaked Shearwater requires. In contrast, nearby Kammurijima
Island, with a wide, rock-free area under its M. thunbergii forest, ishome to a large colony of Streaked Shearwaters.
Predation by Herring Gulls Larus argentatus is reported for Atlantic
breeding colonies of Leach’s Storm-Petrel (Stenhouse et al. 2000).
Although the breeding seasons of Black-tailed Gulls and Swinhoe’s
Storm-Petrels overlap in the Kutsujima Islands, Storm-Petrel
predation by this gull is not known.

135

Our study is the first to report details of a breeding colony of
Swinhoe’s Storm-Petrel in Japan. The breeding population on theKutsujima Islands appears to be an important and sustainable one.
The north islet of the Kutsujima Islands remains unexamined,
however, and the occupancy rate of nest burrows needs morestudy. The trend of the breeding population should be carefully
monitored, because the possibility of human disturbance in thecolony or associated foraging areas cannot be excluded. Outside theKutsujima Islands, Hoshigamijima Island, Oki, Shimane, may havea breeding colony of this species but remains unexamined. A surveyof that island is needed to document further the breeding populationof Swinhoe’s Storm-Petrel in Japan.
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TABLE 2
Breeding areas of Swinhoe’s Storm-Petrel reported in Japan
Recent status
Populationof breeding
estimate (pairs)
population

Reference

141°26′

Probably extinct

Sato (1996)

39°18′

141°59′Active<200

Kumagai (1935)

Tokyo

33°08′

139°42′

Active<200

Unpubl. data

Nanatsujima

Ishikawa

37°37′

136°54′

Breeding
unconfirmed

Tachibana (1986)

Oki

Shimane36°09′

133°04′

Probably active

Sato (1996)

Koyashima

Okinoshima

Fukuoka

34°14′

130°07′

Active<200

Takeishi (1987)

Kutsujima

Maizuru

Kyoto

35°43′

135°26′

Active

3 790

Current studyIsland

Location

Prefecture

Latitude (N)Longitude (E)Bentenjima

Shiriyazaki

Aomori41°25′

Sanganjima

Kamaishi

Iwate

KojineHachijo-kojima

OshimaHoshigamijima

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