Ornithol Sci 5: 157–163 (2006)

SPECIAL FEATURE Ecology and conservation of of prey What landscape elements are correlated with the distribution of wintering Grey-faced Butastur indicus in the Sakishima Islands, southwestern Japan?

Yinyin WU#, Go FUJITA and Hiroyoshi HIGUCHI

Laboratory of Biodiversity Science, School of Agriculture and Life Sciences, The University of Tokyo, Bunkyo-ku, Tokyo 113–8657, Japan

Abstract The Grey-faced (Butastur indicus) is a typical migratory raptor ORNITHOLOGICAL in East Asia, and has suffered a population decline recently. In southern SCIENCE Japan, the wintering buzzards are widely distributed to the Sakishima Islands. The © The Ornithological Society mainly human activity on these islands is agriculture, and the natural environments of Japan 2006 are fragmented by such exploitative land-use that produces a characteristic landscape pattern. In this study, we describe the distribution pattern and habitat selection of the buzzards wintering in the Sakishima Islands, and focus on landscape elements in order to give a basic reference for further conservation study. Ten islands were inves- tigated during January to March 2005. We used the generalized linear mixed model (GLMM) on a set of landscape elements to model buzzard distribution in the 11km grid cell within each island, and studied the relationships between the numbers of buzzard and landscape elements among islands by using the generalized linear model (GLM). The results were similar on two spatial scales, and they both suggested that the area of farmlands (sugar cane, pasture, and rice paddy), and the perimeter of forests have significant correlation with the distribution and the numbers of the win- tering buzzards. Different from breeding sites where distribution was limited by criti- cal nest resource, distribution of Grey-faced Buzzards in wintering sites was simply related to foraging habitats. We suggest that further study should focus on measuring quantitatively the relationships of different farmland types with buzzard distributions in order to make more realistic predictions about distribution when habitat is lost or shifted from one category to another.

Key words Agricultural landscape, Butastur indicus, Distribution, Islands, Winter- ing sites

Understanding how species are distributed within (Muñoz et al. 2005). Understanding where and why the environment and identifying the factors influenc- species occur is a necessary precursor for schemes to ing such distributions are widely studied in biogeog- mitigate population declines and to create new popu- raphy and ecology. This knowledge can be crucial for lations through reintroduction (Rushton et al. 2004). many conceptual fields such as population dynamics Landscape-level approaches have been used to (Gaston & Blackburn 2000), biodiversity monitoring study the distribution of species among environ- (Brown 1984; Sergio et al. 2004), and applied conser- ments. Landscape elements not only provide re- vation (Sergio et al. 2004; Balbontin 2005; Muñoz et sources utilized by species, but also form boundaries al. 2005; Whittingham et al. 2005). Recently, the to control abiotic substance flow, and both of these modeling of species distributions has been increas- processes affect species distributions directly and in- ingly used to identify suitable habitat and to predict directly (Forman 1995; Bissonette 1997). Therefore, potential distributions in a particular region, espe- investigation of landscape elements offers informa- cially in species that are endangered or threatened tion that can be applied to assess the impact of envi- ronmental changes or landscape management on the (Received 30 March 2006; Accepted 12 September 2006) conservation of target species. In addition, many eco- # Corresponding author, E-mail: [email protected] logical interactions and processes have differing in-

157 Y. WU, G. FUJITA and H. HIGUCHI

fluence on the species distributions at different spatial developing agricultural landscapes could be of prime scales (Brown 1984; Wiens 1989; Mackey & Linden- importance for establishing more efficient conserva- mayer 2001). The identification of the scale at which tion planning. landscape elements operate becomes another funda- mental topic. The highly mobile character and large METHODS homeranges of large vertebrate predators, such as birds of prey makes it necessary to characterize their 1) Study area distributions and selection of habitat features at mul- The study area covered the Sakishima Islands tiple spatial scales (Newton 1979; Widén 1994; Mar- (24°30N, 124°12E. Fig.1), located in southwestern tinez et al. 2003; Sergio et al. 2003). It especially re- Japan with a semitropical climate and mean annual quires studies on distribution concerning with land- rainfall over 2000 mm occurring principally during scape level through multiple spatial scales. summer typhoons. The field work was carried out on The Grey-faced Buzzard (Butastur indicus) is a Miyako, Irabu, Tarama, Ishigaki, Taketomi, Kohama, typical migratory raptor species in East Asia. It Kuro, Iriomote, Hateruma, and Yonaguni Islands, is- breeds in northeastern China, the Korean Peninsula lands area ranging from 5 to 289 km2. Farmlands, and Japan, winters in southern Japan and southeast broadleaf primary forest, small residual woodlands Asia (Kugai 1996; Higuchi et al. 2000; Ferguson- and windbreaks that located in fields comprise the Lees & Cristie 2001), and feeds on frogs, snakes, in- typical landscapes of the Sakishima Islands. Pterido- sects, songbirds and small mammals (Ferguson-Lees phytes are the main species in the forest, while Pinus & Cristie 2001). Its population has declined rapidly luchuensis, Casuarina equisetufolium and Pandanus in recent decades (Morishita & Higuchi 1999; odoratissimus are the main tree species of wind- Kawakami & Higuchi 2003; J-IBIS 2004), and one breaks. suspected reason is habitat loss at breeding sites due to human activities (Deng et al. 2003; Momose et al. 2) survey methods 2005). However, to conserve migrant species such as Buzzards were surveyed from the end of January the buzzards, the information from breeding, to early March of 2005. We conducted surveys by re- stopover and wintering sites are all necessary and im- ferring to geographic maps of scale 1 : 25000 and the portant (Martin & Finch 1995). Currently, there are Global Position System. The surveys were performed few studies of buzzard-habitat relationships in win- while moving along all paved roads and unpaved tering regions and many aspects of the wintering tracks so that an entire island was covered, except for ecology of the buzzards remain unknown (Higuchi et those areas without roads, especially the mountainous al. 2000; Kugai 2003). areas on Ishigaki and Iriomote Islands. Therefore, the The Sakishima Islands are one of the important following data analysis did not include the unreached wintering area for the buzzards in Japan. The major area. Each road transect was surveyed only once. human activity on these islands is agriculture, and Moreover, by taking photos to record appearance of main crops are sugar cane, pasture, and rice (Re- each bird, we recorded the locations of confirmed in- gional and Outlying Islands Promotion Division dividual buzzards. Such a procedure allowed us to 2002). The natural environments on the islands are avoid pseudo-replication and duplicate counting. This fragmented to various extents by such exploitative census survey has proven to be a reliable method for land-uses, producing different characteristic land- estimating raptor populations (Viñuela 1997). scape patterns in the islands (Regional and Outlying Islands Promotion Division 2002). 3) Landscape elements data and assessments There are several Sakishima islands, and such nat- Within Islands ural fragmentation provides a material for studying The location of buzzards was incorporated into a the effect of landscape elements on the buzzard’s dis- Geographic Information System (GIS) using the tribution at multiple spatial scales. Our purpose is to UTM 1-km2 grid cells within each island. From our understand what landscape elements affect the distri- prior observations, we knew that the home range bution of wintering Grey-faced Buzzards in the Sak- sizes of wintering buzzards were about 0.25 to ishima Islands, and to carry on this study at two spa- 0.75 km2 (Yinyin Wu et al. unpublished data). There- tial scales: among islands and within islands. Identi- fore, using 11 km cells as the habitat selection unit fying potential causal elements within these rapidly seemed reasonable.

158 The distribution of wintering Grey-faced Buzzards in the Sakishima Islands

Fig. 1. Study area, the Sakishima Islands.

Six landscape elements were measured in each 1- sity per cell was used as the dependent variable in the km2 cell: area of forest, crop field, pasture, rice paddy analysis. The relationships between the distribution and town, and the perimeter length of forests (Table of buzzards over the grids and the landscape elements 1a). These data were obtained from maps produced were analyzed using a generalized linear mixed by the National Survey on the Natural Environment model (GLMM) procedure and model selection was GIS Data (J-IBIS 1999). based on Akaike’s Information Criterion (AIC) and AIC differences (Burnham & Anderson 2002; Gibson Among Islands et al. 2004). GLMM is an extension to a conventional The large-scale landscape analysis focused on en- logistic regression that allowed adding random ef- tire islands as survey units. These among-island sur- fects to the linear predictor, and expressing the ex- veys used the same categories of landscape element pected value of the response conditional on the ran- as those used within islands (Table 1b). dom effects. In our data, a problem concerning the analysis is that islands’ areas vary over a wide range, 4) Data analysis and statistical modelling from 5 to 289 km2, which means that the bigger the Within Islands island, the more cells it has. Such a bias may result in Presence or absence of birds rather than their den- spurious associations and misleading inferences. Re-

159 Y. WU, G. FUJITA and H. HIGUCHI

Table 1. List of landscape elements used as potential ex- Table 2. Number of Grey-faced Buzzards observed in each planatory variables of Grey-faced Buzzard distribution on the of the Sakishima Islands. Sakishima Islands of Japan. Island area (km2) Numbers of buzzards Variable Context (unit) Iriomote Is. 289.27 19 (a) Within islands Ishigaki Is. 222.56 75 Miyako Is. 164.78 98 CA Crop field areas (ha) Iramu Is. 38.59 30 PA Pasture areas (ha) Yonaguni Is. 28.84 9 RI Rice paddy areas (ha) Tarama Is. 19.75 26 TO Town areas (ha) Hateruma Is. 12.77 1 FR Forest areas (ha) Kuro Is. 10.02 16 PF Perimeter lengths of forest (km) Kohama Is. 7.84 2 Taketomi Is. 5.42 5 (b) Among islands

CA Crop field areas (ha) for a wide range of response data and explanatory SC Sugar cane field areas (ha) variables. In our case, both the amount of buzzards PA Pasture areas (ha) and the habitat area of each island had widely ranging RI Rice paddy areas (ha) distributions. When sample sizes are small, AICc is FR Forest areas (ha) PF Perimeter lengths of forest (km) recommended for model selection rather than AIC (Burnham & Anderson 2002). AICc is calculated by adding AIC to 2K(K1)/(nK1), where n is sam- cent work suggests that GLMM deals properly with ple size and K is the number of selected variables in such data (Milsom et al. 2000; Crawley 2002). There- each model (Burnham & Anderson 2002). DAIC is fore, we fitted every island as a random term in calculated using AICc values. The same as the GLMMs by using the R 2.1.0 package (Ihaka & Gen- within-island analysis, the R 2.1.0 package was used tleman 1996) to process the statistical analysis. in this procedure. AIC is calculated for a suite of models and the best fitting one has the lowest AIC value, namely zero. RESULTS AIC differences (DAIC) are calculated relative to this minimum, so for model i the AIC difference is calcu- From the end of January to early March 2005, we lated as: D i AICi AICmin. recorded 281 individuals on the 10 islands. The great- It is easy to interpret and allow a quick comparison est number (98) of buzzards was on Miyako Island, and ranking of the relative support of candidate mod- and the smallest number (1) was on Hateruma Island els. Burnham and Anderson (2002) suggested that the (Table 2). lower the DAIC, the higher the level of empirical support of that model. AIC differences from 0 to 2 in- 1) Buzzards and land use at the 1 km2 scale dicate substantial support for a model, and differ- The model selection procedure suggested that four ences from 4 to 7 indicate considerably less support. models could be considered as plausible models (i.e. Models with DAIC10 differences have essentially DAIC4) (Table 3a). These models included areas of no support and might be omitted from further consid- crop fields, pastures, rice paddy and the perimeter of eration, or at least those models fail to explain some forests, and this suit of variables was positively asso- substantial explainable variation in the data (Burn- ciated with the presence of the buzzards. The other ham & Anderson 2002). We ignored such models in variable, area of forest, showed a negative but rela- our results. tively weak relationship (Table 3a).

Among Islands 2) Buzzards and land use at the island unit scale At the among-island scale, we used generalized Prior to our analysis, we noted that there were high linear models (GLM) to investigate the relationships autocorrelations among the areas of crop fields, sugar between the amount of buzzards and the area of dif- cane fields and perimeter of forest. This correlation ferent landscape types on each island. GLM allows was also shown among the areas of pasture, rice

160 The distribution of wintering Grey-faced Buzzards in the Sakishima Islands

Table 3. (a) GLMM models for probability of presence of Grey-faced Buzzards at a 1 km2 scale, using binomial error and lo- gistic link. (b) GLM models for using explanatory variables related to landscape elements of wintering Grey-faced Buzzard num- bers among islands. All predictors from Table 1 were included in the modeling process, but only the models in which AIC differ- ences (D i) with the lowest model were smaller than 10 were selected. Parameter coefficients are presented and variables not show- ing a coefficient value were not selected in the model analysis.

(a) Models at the within islands scale (n716)

Variable Intercept CA PA PF RI FR TO AIC D i

AIC best 3.6937 0.0290 0.0325 0.0002 0.0577 653.20 0.00 3.6524 0.0292 0.0322 0.0002 0.0571 0.0084 654.90 1.70 2.5578 0.0295 0.0328 0.0003 0.0566 0.0021 655.00 1.80 3.6363 0.0289 0.0322 0.0002 0.0567 0.0023 0.0090 656.80 3.60 3.5773 0.0292 0.0342 0.0002 660.20 7.00 3.5322 0.0294 0.0339 0.0002 0.0107 661.60 8.40 2.5360 0.0293 0.0340 0.0003 0.0024 661.70 8.50 2.4701 0.0294 0.0335 0.0003 0.0029 0.0113 663.10 9.90

(b) Models at the among islands scale (n10)

Variable Intercept SC PA AICc D i

AIC best 2.337 7.342104 2.924104 104.754 0.000 paddy and forest. For focusing on detailed landscape 1) Importance of landscape elements that showed element relationships, we chose the area of sugar positive correlation in models cane fields rather than the area of crop fields and the Grey-faced Buzzards usually hunt in pastures and perimeter of the forest. In order to have a more gen- sometimes in short sugar cane fields where there are eral application across the islands and to concentrate high abundances of grasshopper prey (pers. obs.). Al- on regional cultivated situation, we chose the area of though tall sugar cane fields might be considered as pasture rather than rice paddy and forest. The final less optimal habitat for hunting by the buzzards, model selection suggested that only one model could sugar cane fields were harvested from October for be considered as plausible (Table 3b). The set of vari- about four to five months. The investigation was con- ables that were contained in both areas of sugar cane ducted within this period when the area of tall sugar field and pasture was better than those contained in cane field was continually decreasing. Moreover, we either variable alone. The areas of sugar cane field also found that mowed fields would likely make prey and pasture both showed positive relationships with conspicuous and, therefore, become suitable for for- the number of wintering Grey-faced Buzzards among aging. This is probably why the area of crop fields the islands. also showed a strong positive relationship with buz- zards’ distributions even though the height of crop DISCUSSION field vegetation was not defined. The areas of rice paddy field showed a positive re- In the small-scale analysis, we did not discriminate lationship with the distribution of the buzzards in the sugar cane fields from crop fields due to lack of de- within island scale analysis. Rice paddy field is an tailed land-use information. However, more than 80 important foraging habitat in the breeding areas (Mat- percentage of the area of crop fields consisted of suura et al. 2005; Momose et al. 2005). In the Sak- sugar cane fields (Regional and Outlying Islands Pro- ishima Islands, we observed that the buzzards often motion Division 2002). Because it is plausible to treat appeared near rice paddy fields during rainy days. Al- the area of crop fields as the area of sugar cane fields, though there are only four islands that have rice the landscape elements influencing the buzzards’ dis- paddy fields, this habitat may function as an impor- tribution at both small (within islands) and large tant foraging site on these islands. Further study will (among islands) scales were likely to be similar. be needed to clarify the importance of this landscape

161 Y. WU, G. FUJITA and H. HIGUCHI element in determining the abundance of the buz- breaks. This allows us to restrict the extent of further zards among the islands. survey areas (Martinez et al. 2003; Sergio et al. Because woodlands and windbreaks provide perch 2003). sites preferred by the buzzards, the longer the length Moreover, today, the environment on the Sak- of woodland edge, the more prey was available (Pre- ishima Islands is rapidly being exploited. The propor- ston 1990). This is considered as the function that the tion and area of sugar cane fields and pastures and perimeters of forest offered. Such an influencing ele- other kinds of crop all have a tendency towards dif- ment on the distribution of buzzards was also indi- ferent alterations on different islands in the near fu- cated at breeding sites (Matsuura et al. 2005; Mo- ture. Our study should be continued by measuring mose et al. 2005). For raptors, such as Grey-faced quantitatively the relationships of different habitat Buzzards, which rely on a pause-travel foraging tac- types with buzzard distributions in order to make tic (Andersson 1981), elevated perches to hunt from more realistic predictions about the wintering distri- are an important habitat requirement. Although the bution of buzzards when habitat is lost or shifted perimeter of forest was not included in the among is- from one category to another. lands analysis, it is suspected that this landscape ele- ment may be positively correlated with the abun- ACKNOWLEDGMENTS dance of wintering buzzards among the islands. We thank Y. Sakiyama, T. Oka, K. Kugai, M. Fujita, E. Hiraoka, S. Moriguchi, Y. Matsuoka, W. Kitamura, YW. Lu, 2) Scale effects and YF. Cheng for field assistance; Taketomi, Tarama, and As we mentioned above, in our results, the ele- Irabu Town offices, and Miyako and Ishigaki City govern- ments of landscape that were related to the distribu- ments for helping with land-use maps; HM. Liao, JL. Kuo, tion of wintering buzzards were similar at both and PH. Chen for assistance in GIS analyses. We are indebted within- and among-islands scales. This result sup- to T. Amano, and T. Grubb for providing valuable comments ported the view that the collective outcome of small on the earlier drafts. This study was financially supported by scale individual habitat choices can be linked with the Sasakawa Scientific Research Grant from The Japan Sci- the regional population density level (Mackey & Lin- ence Society, and a scholarship from the Rotary Club of Hsinchuang Central, Taiwan. denmayer 2001; Sergio et al. 2004). However, if we intend to predict precise population sizes based on such a bottom-up approach, the analysis had better REFERENCES integrate quantities related to specific individual be- Andersson M (1981) On optimal predator search. Theor haviors, quantities such as territoriality, dominance Popul Biol 19: 58–86. hierarchies and density-dependent habitat selection Balbontin J (2005) Identifying suitable habitat for dis- (Sutherland 1996; Martinez et al. 2003; Sergio et al. persal in Bonelli’s : an important issue in halting 2004). Including these variables may help avoid mis- its decline in Europe. Biol Conserv 126: 74–83. leading results and so promote the accuracy of the Bissonette JA (1997) Wildlife and landscape ecology: models. Our results indicated an interesting pattern effects of pattern and scale. Springer-Verlag, New that is worth further study. As Sergio et al. (2004) York. suggested, small scale models are not the final aim of Brown JH (1984) On the relationship between abun- an investigation, but become tools for further hypoth- dance and distribution of species. Am Nat 124: 255– esis testing and for deeper understanding of the 279. processes behind the observed distribution patterns. Burnham KP & Anderson DR (2002) Model selection and multimodel inference: a practical information- theoretic approach. 2nd ed. Springer-Verlag, New 3) Conservation implications York. Different from breeding sites where distribution Crawley MJ (2002) Mixed effects models. In: Statistical was also limited by a critical resource, namely the computing: an introduction to data analysis using S- nest site (Deng et al. 2003), distribution of Grey- plus. pp 669–708. John Wiley & Sons, New York. faced Buzzards in wintering sites was simply related Deng WH, Wei G & Guang-Mei Z (2003) Nest and to foraging habitats. Results of this study testify to roost habitat characteristics of the Grey-faced Buz- the value of monitoring human agricultural activities zard in northeastern China. J Raptor Res 37: 228– for wintering Grey-faced Buzzards, especially those 235. on farmlands and remaining woodlands and wind- Ferguson-Lees J & Cristie DA (2001) Raptors of the

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