Ornithol Sci 4: 73–79 (2005) ORIGINAL ARTICLE The distribution of the Malayan Night Heron Gorsachius melanolophus in the Yaeyama and Miyako Islands, southern Japan Kazuto KAWAKAMI1,# and Masaki FUJITA2 1 Tama Forest Science Garden, Forestry and Forest Products Research Institute, Todori 1833, Hachioji, Tokyo 193–0843, Japan 2 Laboratory of Biodiversity Science, School of Agriculture and Life Sciences, The University of Tokyo, Yayoi 1–1–1, Bunkyo-ku, Tokyo 113–8657, Japan Abstract The Malayan Night Heron Gorsachius melanolophus is a threatened ORNITHOLOGICAL species that inhabits the Yaeyama Island group in the extreme southwest of the SCIENCE Ryukyu Archipelago, southern Japan. The species’ distribution in Japan, which has © The Ornithological Society been poorly known, was surveyed on the basis of sound and sight records, and inter- of Japan 2005 views. The species was confirmed to occur on Ishigaki, Iriomote, Kuroshima, Ko- hama, Taketomi, Hateruma, Yonaguni, Miyako, Kurima, and Tarama islands. This range is wider than expected and records were independent of the area of islands. The past underestimation of the species’ range was likely to have been the result of a lack of research effort and the unremarkable ecology and morphology of the species. The population density was found to be higher on Iriomote Is., covered with abundant for- est, than on Ishigaki Is. with more degraded forests. Despite the wider than previously thought range, its range is still not very extensive in Japan; furthermore, increasing predators and introduced competitors may well endanger the population in the near future. Key words Distribution, Gorsachius melanolophus, Malayan Night Heron, Popula- tion density, The Yaeyama Islands The Malayan Night Heron Gorsachius melanolo- Archipelago (Fig. 1, The Ornithological Society of phus ranges from India and Sri Lanka to Southeast Japan 2000). Though it has been observed on Miyako Asia, Southern China, Malaysia, Thailand, Taiwan, and Aguni islands, it was considered a winter visitor Indonesia, Philippine and Japan (del Hoyo et al. there (Kugai & Yamamoto 1981; Brazil 1991), how- 1992; Ornithological Society of Japan 2000). This ever, this distribution pattern seems to be unnatural, species prefers tropical and subtropical wet forests as because there are many other islands in the immedi- its breeding habitats (Hancock et al. 1978; del Hoyo ate vicinity. The Malayan Night Heron had even been et al. 1992). While it is a migratory bird in some observed in Tokushima Prefecture in 1984 and on areas (Hancock et al. 1978), it is resident in Japan Rishiri Island of the northwest coast of Hokkaido (Ornithological Society of Japan 2000). This heron is Prefecture in 2000 (Yoshida 1986; Kosugi et al. classified as a vulnerable species in the Japanese Red 2005), indicating that this species has the ability to Data Book (Ministry of the Environment 2002). disperse to other islands. There have, however, been few studies of its ecology The Malayan Night Heron is an unimposing and distribution, though such information is essential species because they prefer dense humid forests, do for its conservation (Kushlan & Hafner 2000). not move actively, seldom fly, do not have bright In Japan, this species has been reported to breed plumage, and call only before sunrise and after sunset only on Ishigaki, Iriomote and Kuroshima islands in (Hancock et al. 1978; Kabaya & Matsuda 2001). the Yaeyama island group, of the southern Ryukyu There was, therefore, the possibility that they might inhabit islands near Ishigaki, Iriomote and Kuroshima (Received 26 July 2004; Accepted 28 October 2004) islands, but had not yet been identified there. Thus, # Corresponding author, E-mail: [email protected] we investigated their distribution throughout the 73 K. KAWAKAMI and M. FUJITA Fig. 1. Location of the islands in the study area. Yaeyama and Miyako Islands. We also surveyed the a speed of 4 km/hour. The survey was conducted in population density of this species to assess their re- lowland forests in Kuroshima Is. (26°15ЈN, 124° cent status. 01ЈE, 10.02 km2), Kohama Is. (24°20ЈN, 123°59ЈE, It is difficult to investigate the species visually be- 7.84 km2), Taketomi Is. (24°19ЈN, 124°05ЈE, 5.42 cause of the reasons stated earlier. We therefore used km2), Hateruma Is. (24°03ЈN, 123°47ЈE, 12.77 km2) their calls to detect them. The species calls in the and Yonaguni Is. (24°27ЈN, 123°00ЈE, 28.84 km2) in early breeding season, mainly in March, before nest the Yaeyama Islands, and on Miyako Is. (24°47ЈN, building (K. Kawakami, unpublished). Thus, we were 125°20ЈE, 159.05 km2), Irabu Is. (24°50ЈN, 125° able to confirm their presence by surveying their call- 11ЈE, 29.05 km2), Ikema Is. (24°56ЈN, 125°25ЈE, ing activity. In this study we first checked when they 2.83 km2), Kurima Is. (24°43ЈN, 125°15ЈE, 2.84 km2) called most frequently. Then, their distribution was and Tarama Is. (24°39ЈN, 124°42ЈE, 19.73 km2) in investigated on ten islands in the Yaeyama and the Miyako Islands from 8 to 21 March 2003 and Miyako island groups. Finally, the population densi- from 13 to 21 March 2004 (see Fig. 2). On Ikema and ties on Ishigaki and Iriomote islands, were analyzed Kurima islands, surveys were conducted without to assess their abundance there. walking, because the forests there were small enough for all calls to be heard from one point (see Fig. 2). METHODS In addition, we checked for birds visually during the daytime on every island in March 2003, and on 1) Calling activity Kuroshima, Kohama, and Taketomi islands from In order to establish a research schedule, we 13–15 December 2003 during a non-breeding season checked when the species calls most frequently. survey. Interviews were also conducted with local Thus, surveys of seven individuals were conducted people on Kuroshima, Kohama, Taketomi, Hateruma, overnight from 1800 to 0730 on Ishigaki Is. (3 birds), Yonaguni, Miyako, Irabu and Tarama islands, in Iriomote Is. (2 birds) and Kohama Is. (2 birds) on order to supplement the distribution data. 15–17 March 2004. Their calling frequency was recorded every ten minutes. 3) Population density and size One phrase of their ‘call’ consists of repeated short In order to assess the abundance of Malayan Night syllables that sound like “pooh” (Kabaya & Matsuda Herons, we mapped the calling locations in montane 2001). We counted one phrase as one ‘call’, and forests in the Mt. Ban-na area and riparian forests recorded the number of phrases per ten minutes. The along the Miyara River of Ishigaki Is. in March 2002 calls could be heard from more than 500 m away in and in montane forests in the Ohtomi area of Iriomote calm weather. Is. in March 2003. With this data, nearest neighbor distances were calculated for each individual giving 2) Distribution an index of population density. These values were According to the above survey, the Malayan Night then compared both between islands and between dif- Heron calls most frequently between 1840–1930 ferent habitats on Ishigaki Is. using the Mann–Whit- in the evening and 0600–0630 in the morning in ney U-test. Since female Japanese Night Herons G. March. During these time periods, we recorded the goisagi, a closely related congener of the Malayan location of the calling birds on maps while walking at Night Heron, sometimes call like males (H. Uchida & 74 The distribution of the Malayan Night Heron Fig. 2. The survey routes and the locations where Malayan Night Herons were recorded. Thin black lines: sur- vey routes; black triangles: survey points; white circles: calling points in 2003; black circles: calling points in 2004; hatch marks: locations where individuals were observed in March 2003; gray circles: locations where indi- viduals were observed in December 2003. K. Kawakami, unpublished), there is the possibility population density by the area. that female Malayan Night Herons also call like males. Therefore, we excluded data within an area of RESULTS less than 100 meters in order to avoid overestimation. However, as the frequency of calling females among 1) Calling activity Japanese Night Herons is very low, it is assumedly All individuals frequently called from 1840 to the same with Malayan Night Herons. 1930 and from 0600 to 0630 (Fig. 3). They rarely Next, we estimated the population size on the two called during other time periods. islands. Firstly, the population densities were calcu- lated on the assumption that the territories of the 2) Distribution species are in a honeycomb pattern. In this calcula- Calls of Malayan Night Herons were recorded on tion, the mean distance obtained with the above Kuroshima, Kohama, Taketomi, Hateruma, Yona- method was applied as the distance between centers guni, Miyako, and Tarama islands in both 2003 and of neighboring territories. Given that the areas of for- 2004 (Table 1, Fig. 2), and on Kohama, Hateruma est habitats are about 80 km2 on Ishigaki Is. and about and Kurima islands only in 2004. Individual adults 250 km2 on Iriomote Is. (The Ministry of Agriculture, were observed on Kuroshima, Kohama, and Miyako Forestry and Fisheries of Japan, unpublished), popu- islands during the survey period in 2003. lation sizes were calculated simply by multiplying the During the non-breeding season, some adults were 75 K. KAWAKAMI and M. FUJITA Fig. 3. Calling frequency of Malayan Night Herons (From upper: 3birds on Ishigaki Is., 2birds on Iriomote Is, and 2birds on Kohama Is.) recorded at 10 minutes intervals on 15–17 March 2004. The frequency was counted as the number of phrases during ten minute periods. Sunset was at 1853–1854 and sunrise was at 0650–0652 on the survey days. also observed on Kuroshima, Kohama, and Taketomi forests of Iriomote Is., 20 individuals were located islands (Table 1).