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Dietary Habits of the Endangered Okinawa Rail

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Dietary Habits of the Endangered Okinawa Rail Ornithol Sci 17: 19 – 35 (2018) ORIGINAL ARTICLE Dietary habits of the endangered Okinawa Rail Shun KOBAYASHI1,#, Yuya MORITA2, Yumiko NAKAYA3, Takashi NAGAMINE3, Manabu ONUMA4, Tsukasa OKANO4, Atsushi HAGA4, Ichihito YAMAMOTO5, Mizuki HIGA5, Tohru NARUSE6, Yasuyuki NAKAMURA7, Tetsuo DENDA2 and Masako IZAWA2 1 Graduate School of Engineering and Science, University of the Ryukyus, 1 Senbaru, Nishihara Okinawa 903–0213, Japan. 2 Faculty of Science, University of the Ryukyus, 1 Senbaru, Nishihara Okinawa 903–0213, Japan. 3 Conservation & Animal Welfare Trust, 308-7-205 Maehara, Uruma, Okinawa 904–2235, Japan 4 National Institute for Environmental Studies, 16-2 Onokawa, Tsukuba, Ibaraki 305–8506, Japan 5 Yambaru Wildlife Conservation Center, Ministry of the Environment, 263-1 Hiji, Kunigami, Okinawa 905–1413, Japan 6 Tropical Biosphere Research Center, Iriomote Station, University of the Ryukyus, 870 Uehara, Taketomi, Okinawa 907–1541, Japan 7 Ryukyu University Museum, Fujukan, 1 Senbaru, Nishihara Okinawa 903–0213, Japan. ORNITHOLOGICAL Abstract Many species of flightless rail are endemic to small islands and endan- gered, or have become extinct during historical times. However, our understanding SCIENCE of their basic ecological knowledge remains limited. The Okinawa Rail Hypotae- © The Ornithological Society nidia okinawae is a flightless species of rail endemic to Okinawajima Island in the of Japan 2018 Ryukyu Archipelago, Japan. In this study we have quantified the dietary components of the Okinawa Rail using gizzard contents classified by the frequency of occurrence and wet weight. The Okinawa Rail feeds on a wide variety of animals and plants throughout the year, but the frequency of occurrence and wet weight of animal mat- ter significantly exceeds those of plant matter. The number of items present in the gizzard did not differ among seasons, ages and between sexes. Grit was present in almost all individuals and the wet weight did not differ between sexes or among age groups. Although the diet was observed to be diverse, the primary dietary component of both sexes, all ages, and throughout the year appears to be land snails, indicating their great significance as a source of food. Key words Flightless rail, Gizzard contents, Hypotaenidia okinawae, Land snail, Ryukyu Archipelago The family Rallidae includes many flightless spe- biology of and implementing conservation strategies cies and also those that are endemic to small islands for endangered species. The diet of rallid species has (Taylor 1996). Most of these flightless endemic rail been analyzed qualitatively by direct observation as species are endangered (or already extinct) due to their well as via gizzard content analyses (Meanley 1956; small initial populations, predation by invasive spe- Carroll 1963; Horak 1970; Robert et al. 1997). Based cies, habitat destruction, and hunting (Taylor 1996). on these results, flightless rails and those capable of Conservation programs have been implemented for flight are considered omnivorous and opportunistic endangered rallid species such as the Weka Gal- (Taylor 1996; Taylor & van Perlo 1998). For some lirallus australis and the Guam Rail Hypotaenidia species in this family, unique food items such as mil- owstoni (briefly reviewed by BirdLife International lipedes, flatworms, fish, and rats have been recorded 2016a; BirdLife International 2016b), but basic eco- (Taylor 1996), although the importance of these food logical data are still lacking for most species. items for rails is unknown. In order to understand the Dietary habits are fundamental to understanding the dietary habits of rails and the importance of unique food items, quantitative surveys are needed. (Received 10 January 2017; Accepted 3 August 2017) The Okinawa Rail Hypotaenidia okinawae (spe- # Corresponding author, E-mail: [email protected] cies name was reffered to del Hoyo et al. 2014) is 19 S. KOBAYASHI et al. endemic to the northern part of Okinawajima Island additional specimens. The objective of this study was in the Ryukyu Archipelago, Japan (Yamashina & to describe the feeding habits of the Okinawa Rail in Mano 1981), and lacks the ability to fly (Kuroda et al. a quantitative manner and discuss the characteristics 1984; Kuroda 1993). Because of predation by inva- of their food habits. sive carnivores (Harato & Ozaki 1993; Ozaki et al. 2002; Jogahara et al. 2003), traffic accidents (Kotaka MATERIALS AND METHODS & Sawashi 2004; Mori et al. 2010), and habitat loss (Ikehara 1995; Ozaki et al. 2002), the population 1) Samples of this species had decreased before. Therefore, the A total of 189 Okinawa Rail gizzard samples Okinawa Rail is listed as endangered (EN) on the were collected from deceased rails, mainly killed in IUCN red list (BirdLife International 2016c). As a traffic accidents or by predation by invasive carni- result of the conservation program aimed at the exter- vores, in the northern part of Okinawajima Island mination of invasive carnivores, especially the small from March 2008 to August 2014 (Table 1, Fig. 1). Indian mongoose Herpestes auropunctatus, the num- The specimens include 140 adults, 31 young (already ber of Okinawa Rail is estimated to have increased independent from parents, but not sexually mature), from 717 in 2005 (Ozaki et al. 2006) to 1,500 in 14 chicks (still being fed by parents), and four of 2014 (Ministry of the Environment 2014). Some eco- unknown developmental stages (Table 1). Giz- logical characteristics of the Okinawa Rail have been zards were preserved in 70% ethanol or stored in a reported so far; males are larger than females (Ozaki freezer at the Yanbaru Wildlife Conservation Center 2009), the breeding season is estimated to last from (Okinawa, Japan) and National Institute for Envi- March to June using testis observation (Onuma et ronmental Studies (Ibaraki, Japan). Frozen contents al. 2011), and it shows a preference for roosting on were preserved in 70% ethanol for several days prior branches (Harato & Ozaki 1993). Previous reports to analysis. Contents were washed on a 1 mm mesh have described the dietary habit of the Okinawa Rail sieve and then identified as close to the species level based on direct observations and gizzard contents as possible. Species were identified by the following analyses (e.g., Ozaki 1982; Hiraoka et al. 2007; Pub- authors: bone fragments of vertebrates, Chilopoda, lic Association for Construction Services, Okinawa and Diplopoda by Y. Nakamura; Crustacea by T. Region 2009; Shichiri et al. 2016). Previous research Naruse; plants by T. Denda and S. Kobayashi; other has clarified that it feeds on various kinds of food, groups were identified by S. Kobayashi. Since many but has not addressed dietary habits in relation to sex, gizzard contents were fragmented and could not be age or season. identified to the species level, we refer to the smallest The dietary habits of birds are usually studied by taxonomic units of the contents as “item” throughout direct observation, scat analysis, and gut contents this study. Thus, we have referred to the number of analysis (e.g., Litvaitis et al. 1994; Yamagishi 1997). identified species as “number of species”, but the Direct observation is a good method when attempt- total number of identified and unidentified species ing to clarify the food habits of target animals, but as “number of item”. Although it is possible that a is difficult to apply to forest-dwelling and/or non- single species may be represented by two or more flying birds such as the Okinawa Rail. For such spe- items, this method can provide a closer number of cies, gizzard contents analysis allows the collection diet species when compared with an evaluation based of detailed dietary information for each individual on identified material only. The wet weight of each directly. The problem with gizzard contents analysis item was measured using a microbalance to 0.001 is that it requires the sacrifice of sufficient individuals g. In addition, the wet weight of total grit was mea- to obtain enough data for statistical analyses (Fukue sured, as birds generally consume grit in addition to et al. 2011), which makes the method inappropriate food to aid in digestion (Meinertzhagen 1964). for endangered species. In the case of the Okinawa Rail, many dead bodied from mainly road kills have 2) Measurements been collected under the conservation program by The size of food that a bird can consume is cor- the government and institutions concerned, which related with its bill size (e.g., Hespenheide 1971; has allowed us to investigate their dietary habit by Willson 1972; Jordano 1986). The width and height using gizzard contents without having to collect any of the shells of land snails that remained intact in the gizzard were measured in this study. The longest 20 Dietary habit of the Okinawa Rail Table 1. Age, sex, and cause of death of Okinawa Rails used for analysis in this study. Predation Age Sex Season Road kill Drowning Unknown Total Carnivora Birds Snakes Adult M Winter 6 2 1 0 1 0 10 Spring 32 6 0 4 0244 Summer 19 1 20 0 1 23 Autumn 5 0 00 0 16 F Winter 5 1 00 0 0 6 Spring 25 140 0 2 32 Summer 13 0 0 2 0 0 15 Autumn 2 0 00 0 2 4 Young M Winter 0 0 00 0 0 0 Spring 1 0 00 0 0 1 Summer 8 0 3 0 0 2 13 Autumn 4 0 00 0 0 4 F Winter 1 0 00 0 0 1 Spring 0 1 00 0 0 1 Summer 4 0 4 0 0 08 Autumn 3 0 00 0 0 3 Chick M Winter 0 0 00 0 0 0 Spring 4 1100 17 Summer 301 00 04 Autumn 0 0 00 0 0 0 F Winter 0 0 00 0 0 0 Spring 0 0 0 0 0 0 0 Summer 3 0 00 0 0 3 Autumn 0 0 00 0 0 0 Unknown M Winter 0 0 00 0 0 0 Spring 1 0 00 0 0 1 Summer 0 0 00 0 0 0 Autumn 1 0 00 0 0 1 F Winter 0 0 00 0 0 0 Spring 0 0 0 0 0 0 0 Summer 1 0 1 0 0 02 Autumn 0 0 00 0 0 0 diameter of each shell was used as an index of shell size.
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