FORKTAIL 31 (2015): 87-91 Population status and habitat ecology of Bristled Grassbird Chaetornis striata in Chitwan National Park, central Nepal PARAS BIKRAM SINGH & DAVID L. BUCKINGHAM A study to determine the population status and assess the habitat ecology of the Bristled Grassbird Chaetornis striata was carried out in Chitwan National Park, Nepal. Using a call playback method to detect Bristled Grassbirds, the total population was estimated at 4,570±1,270 individuals. Bristled Grassbirds were five times more likely to occur in grassland dominated by Saccharum spontaneum than in grassland where other grasses predominated, and their presence was negatively correlated with tree density. Grassland lightly grazed by wild herbivores was found to be three times more likely to hold Bristled Grassbirds than similar grasslands where grazing was assessed as medium, whilst more heavily grazed grasslands used by domestic livestock were avoided. The preference for grasslands at an early successional stage suggests that the loss of such grasslands to scrub could be a major threat to the Bristled Grassbird, which occurs sparingly in lowland grasslands in Nepal below 300 m. INTRODUCTION This study focused specifically on the population status and habitat ecology of the Bristled Grassbird in Nepal during the The Bristled Grassbird Chaetornis striata is endemic to South breeding season and assessed threats to its habitat. Asia, occurring in Bangladesh, India, Nepal and Pakistan (Ali & Ripley 1987, Grimmett et al. 1998). It has a small and declining population, and is classified as nationally threatened in Nepal and METHODS by IUCN as globally Vulnerable (Inskipp 1996, BCN & DNPWC 2011, BirdLife International 2015). It was formerly included with Study area the Old World warblers Sylviidae but its true affinity has not yet Chitwan National Park, 932 km2 in area, is located in the lowland been established. External morphology suggests it might belong in Dun Valley at 27.250-27.583°N and 83.750-84.967°E (Ligure the newly established grass warbler family Megaluridae (BirdLife 1). The park is a complex of ecosystems comprising Churia hills, International 2015) and it is now placed in the monotypic genus ox-bow lakes and floodplains along the Rapti, Reu and Narayani Chaetornis. rivers. It was recognised as a World Heritage Site in 1984 for the Very little is known about the Bristled Grassbird in Nepal, richness of its biodiversity, scenic landscape and the unique Tfiaru where the species was known from four protected areas up to 2001 Cultural Heritage (DNPWC 2013). It is home to many threatened (BirdLife International 2001), namely Royal Sukla Phanta Wildlife mammals, birds and reptiles. In total 540 bird species have been Reserve, Royal Chitwan National Park, Lumbini Sanctuary and recorded from the park (Baral & Upadhyay 1998). Kosi Tappu Wildlife Reserve, and has been recorded at a few other sites since. Bristled Grassbirds are reported to prefer densely Avifauna! surveys vegetated, medium to tall grasslands, mostly on dry soils but also in The study was carried out over six weeks between 31 March and moist areas (although it is absent from wet Phragmites grasslands) 15 May 2010. Pour grassland areas were selected: Khagendramalli, and with tall, unburnt reeds or scattered bushes/trees which are Sahuraha-Padampur and Bhimle-Meghauli on the Rapti River used as vantage points. The height of unburnt grass at all occupied floodplain, and Bankata-Madi on the Reu River floodplain. Sample sites was 2.5-3 m, although at some sites there were also grasses plots 75 m in radius were established at 200 m intervals along burnt in previous fires present which were up to 1.5 m tall (Baral transect lines laid perpendicular to the rivers at intervals of250 m. 1997, 2000b). The total of 105 sample plots comprised 18 plots in Khagendramalli, The major threat to this grassland specialist is thought to be the 29 in Sahuraha-Padampur, 30 in Bhimle-Meghauli and 28 in vulnerability of its small, rapidly declining population to loss and Bankata-Madi. At each plot, Bristled Grassbirds were counted degradation of its grassland habitat, primarily through drainage and habitat condition was assessed. Playback was used to maximise and conversion to agriculture (BirdLife International 2015). the probability of detecting birds (Gregory et al. 2002). Calls of However, invasive plant species, including Mikania micrantha, Bristled Grassbird were played from the centre of each plot three Lantana camara and Eupatorium adenophorum, are expanding times, followed by five minutes listening for responses. Each survey into Nepalese grasslands, with Mikania being the most serious was carried out between 07h00 and lOhOO in calm conditions. No invader in Chitwan NP (Sapkota 2007). These invasive species Bristled Grassbirds were detected during the first surveys between generally grow and spread rapidly, preventing light penetration 31 March and 16 April. A partial second round of surveys between to ground level. If not controlled they replace the grassland, 1 and 15 May involved 62 plots in the three Rapti floodplain sites resulting in the loss of Bristled Grassbird habitat. Many grasslands and six plots in the Reu floodplain. Plain Prinia Prinia inornata, of conservation value are included in protected areas but continue Grey-crowned Prinia P. cinereocapilla and Slender-billed Babbler to suffer degradation through mismanagement (Baral 1998). The Turdoides longirostris were surveyed at the same time using the protected area priority of managing grasslands for large herbivorous same methodology. mammals has resulted in the creation of short, young vegetative grass at the expense of taller, mature grassland (Baral 2000a). Habitat sampling More information is needed to reconcile the conflicting habitat The structure of the grassland habitat was measured within each 75 requirements of the wide range of threatened species occurring in m radius plot (Bristled Grassbirds were assumed to be detectable up Nepal’s national parks. to 75 m from the observation point). The densities of the dominant 88 PARAS BIKRAM SINGH & DAVID L. BUCKINGHAM Forktail 31 (2015) tree, shrub and grass species were measured together with overall Table 1. Explanatory variables used in logistic regression models with vegetation cover. Trees and shrubs were counted in 50 m and 25 summary data (mean and range for continuous variables, frequency of occurrence for categorical variables). m radius plots respectively, grass shoots were counted in lxl m quadrats and their densities were calculated separately. Vegetation Variable Variable type Description Summary data cover was recorded in percentage bands (no cover 0%, very sparse Tree density (TD) Continuous Number of trees within 50 m radius 9(10-110) < 25%, medium 25-75%, high > 75%). Grazing intensity was Shrub density (5D) Continuous Number of shrubs within 50 m radius 6 (0-50) measured on an ordinal scale: heavy, medium or light. Grazing Grass density (GD) Continuous Number of grass shoots per m2 78(0-90) by wild herbivores was considered to be light, whereas grazing by Vegetation cover (VC) Categorical Medium = 25-75%, dense > 75% M = 21,D = 42 domestic livestock was graded medium to heavy. The presence of Grazing intensity (Gl) Categorical Light, medium or heavy L = 35, M = 28, H = 0 potential habitat threats, such as forest roads/trails, forest fires and Dominant grass Categorical DS = Saccharum spontaneum DS = 49,0 = 14 colonisation by invasive plant species, was also recorded. All the species (DS) dominated, 0 = other species aformentioned parameters were also measured in plots where no grassbirds were recorded. with the other explanatory variables fixed at their mean value (or weighted means of categorical variables). When examining Data analysis continuous variables, observations were ranked by the continuous Using methods detailed by Rodgers (1991), Bristled Grassbird variable and divided into equal groups for which means (± standard numbers were analysed to provide mean population sizes and errors) were plotted as the observed values. confidence intervals for each surveyed grassland plot, using the plot-level densities from the second round of survey visits. Based on the total area of suitable grassland habitat, a population estimate RESULTS for the entire national park was made using mean plot-level density and pooling data from all the surveyed grasslands. Thapa (2011) Bristled Grassbird population estimates classified the grassland of Chitwan NP into five grassland types: A total of 60 Bristled Grassbirds was observed in 34 of the 62 sample floodplain, short, tall swampy, tall and wooded. The perennial Kans plots surveyed along the Rapti floodplain in the second round of grass Saccharum spontaneum alone or as the dominant species was visits. The estimated populations and 95% confidence intervals found only in floodplain and tall grassland. For this study, suitable are presented in Table 2. The estimated population for the whole grassland for Bristled Grassbird was defined by the presence of of the park, 4,570±1,270 individuals, was based on the observed Saccharum spontaneum, which was reported to occupy 84.64 km2 mean density in Saccharum $pontaneum-&om\x\2.x.t& grasslands, of Chitwan NP (Thapa 2011). The effect of habitat condition on Bristled Grassbird occurrence Table 2. Estimated population sizes of Bristled Grassbird in Chitwan NP (probability of occurrence in a sample) was modelled using logistic based on plot counts in three study grassland areas. regression. Six explanatory variables were tested (Table 1). The final multivariate model was built using a step-down approach, starting Population No. survey Bird density Potential estimate with the model incorporating all six variables and deleting the least Study grassland plots km2±95%CI habitat (km2) (individuals) significant variables, one at a time, until all the remaining variables Sahuraha-Padampur 20 54±31 4.75 257±148 were significant (Type III tests,p < 0.05). Bhimle-Meghauli 20 62±27 3.56 221 ±96 Model fit was assessed by plotting observed values against Khagendramalli 22 46±26 6.87 316±179 predicted values of the probability of occurrence. The effect of each Total for Chitwan NP 62 54±15 84.64 4,570±1,270 explanatory variable in the final model was examined separately, Forktail 31 (2015) Population status and habitat ecology of Bristled Grassbird in Chitwan National Park, Nepal 89 including pure stands and associations with other species of grass such as Narengaporphyrocoma. Plant species diversity Table 3 lists the main tree, shrub and grass species recorded in survey plots occupied by Bristled Grassbirds. Only five tree species were recorded in occupied survey plots, whereas 25 tree species were recorded from unoccupied plots. Similarly, nine shrub species and 10 grass species were recorded in occupied plots, compared with 20 shrub species and 14 grass species in unoccupied plots. Table 3. Tree, shrub and grass species recorded in plots occupied by Bristled Grassbirds in Chitwan NP. Figure 2. The relationship between dominant grass species and probability of occurrence of Bristled Grassbird Chaetornis striata. Bars Tree species Shrub species Grass species show predicted values from the logistic regression model and black Trewia nudiflora Clerodendron viscosum Saccharum spontaneum dots show observed probabilities (± 1 SE). Wendlandia coriacea Pogostemon bengaiensis Saccharum bengaiensis Dalbergiasissoo Ziziphus mauritiana Narenga porphyrocoma Acacia catechu Colergaesia oppositifolia Jhemeda arundinacea Bombaxceiba Cupressussp. (n=2) Imperata cylindrica Calotropis gigantea Phragmites karka Artemisia indica Ageratum conyzoide Lantana camera Erianthus munja Typha elephantina Eragrostis tenella Vegetation density Trees and shrubs were widely scattered in all grassland plots, but densities of both were markedly lower in plots occupied by Bristled Grassbirds. Average densities in occupied plots were 966 trees/km , 1,168 shrubs/km" and 85 grass stems/m", compared with 4,110 trees/km", 4,630 shrubs/km" and 72 grass stems/m' in unoccupied plots. Figure 3. The relationship between tree density and probability of Logistic regression model occurrence of Bristled Grassbirds. Bars show predicted values from the The final model containing three habitat variables (Table 4) was logistic regression model and black dots show observed probabilities (± 1 SE). highly significant (%2 = 48.5, df= 6,p < 0.001), and Nagelkerke’s R: of 0.71 indicated a moderately strong relationship between prediction and grouping. Table 4. Summary of the final logistic regression model describing the probability of occurrence of Bristled Grassbird in a sample plot. Parameter Parameter Wald test P-valuefor Variables Level SE statistic Wald test Intercept -0.62 1.27 0.23 Tree density -0.12 0.4 8.5 0.004 Dominant grass Saccharum spontaneum 2.45 1.18 4.4 0.03 Other species 0 - - - Grazing intensity 1.80 0.86 4.2 0.04 Bristled Grassbirds were more likely to occur at lightly grazed sampling sites with low tree densities, dominated by Saccharum spontaneum. There was close agreement between the observed and predicted relationships for all three explanatory variables (Figures 2-4). Dominant grass species were strongly associated Figure4.The relationship between grazing intensity and the probability of occurrence of Bristled Grassbirds. Bars show predicted values from the (multicollinearity) with lower tree densities at Saccharum logistic regression model and black dots show observed probabilities spontaneum-dom 'mMtd sample sites. This association was controlled (± 1 SE). for when calculating predicted values for the dominant grass species relationship (Figure 2), by using different mean tree densities for each type of grassland. Bristled Grassbirds were almost five times be largely absent from grasslands with more than 40 trees within more likely to be found in Saccharum spontaneum-&om'\r\a.tz& a 50 m radius circle. grassland than in other types of grassland. Lightly grazed sites, typically grazed by wild herbivores A density of less than 20 trees within a 50 m radius of the (livestock grazing was only noted at four lightly grazed sample sites), centre of each plot was strongly preferred, and the probability were three times more likely to hold Bristled Grassbirds than more of occurrence increased rapidly as tree density approached zero intensively grazed sites (Figure 4). Domestic livestock were mainly (Figure 3). High levels of occupancy were recorded for Saccharum responsible for cases of medium grazing intensity. The effect of spontaneum grasslands with low tree densities (over two-thirds of grazing intensity could, therefore, be due to differences in grazing sample sites). The model predicted that Bristled Grassbirds would patterns between wild herbivores and domestic livestock. 90 PARAS BIKRAM SINGH & DAVID L. BUCKINGHAM Forktail 31 (2015) Threats relationship between previous fire damage and Bristled Grassbird Overgrazing (affecting 90% of plots), fire (65%), forest roads densities. or trails (27%) and invasive plant species (16%) were the main I nvasive plants were only recorded on a small proportion of the potential threats recorded in grassland plots used by Bristled study plots and, at these occurrence levels, no impact on Bristled Grassbirds. However, a multiple regression model failed to detect Grassbird densities was apparent. any relationship between the occurrence of any of these threats and Bristled Grassbird densities. RECOMMENDATIONS Other grassland birds Plain Priniawas almost ubiquitous on the study plots (116 records), Tire impact of irregular and controlled burning on Bristled Grassbirds preventing presence-absence modelling. Grey-crowned Prinia (8 should be investigated to determine the long-term consequences of records) and Slender-billed Babbler (4 records) were recorded too the current intervention regime on the grasslands, and whether these infrequently to permit modelling. Both these species only occurred can be predicted. Furthermore, succession advancement should be on study plots where Bristled Grassbirds were not found. Based controlled by means of controlled burning, tree felling and rotational on limited samples. Grey-crowned Prinia occurred on forest-edge management by grazing and grass cutting. Finally, a strategic plots with higher tree densities than plots occupied by Bristled Conservation Action Plan that incorporates grassland management Grassbirds, while the Slender-billed Babbler records were in plots should be initiated to accommodate the needs ofBristled Grassbirds with higher shrub densities. alongside all the other grassland-dependent species inhabiting the Terai grasslands. The results of this research are based on data from the Rapti floodplain. It would be prudent to confirm our findings by DISCUSSION studying the species in the Narayani River floodplain, Koshi Tappu Wildlife Reserve and Suklaphanta Wildlife Reserve. Population estimation and habitat ecology Bristled Grassbirds were detected usingplayback, which provoked birds into responding to defend their territories during the breeding ACKNOWLEDGEMENTS season (Gregory et al. 2002), hence maximising the likelihood of detecting birds. Prior to using playback, only males displaying to We are indebted to the Oriental Bird Club for funding this research and attract mates were observed, so surveying without playback would extend our thanks to Carol Inskipp and Hem Sagar Baral for their valuable have underestimated occurrence and population density. suggestions. We will never forget Sunil Nepal for his contribution during There was no evidence that Bristled Grassbirds’ requirements fieldwork. We also thank the Department of National Parks and Wildlife were conflicting with management for conservation-priority Reserves for granting us permission to carryout the research, and the Chitwan herbivores. The lightly grazed areas preferred by Bristled Grassbirds National Park team for generous support with fieldwork. were principally grazed by wild herbivores including One-horned Rhinoceros Rhinoceros unicornis, Asian Wild Elephant Elephas maximus. Hog Deer Axisporcinus, Spotted Deer A. axis, Sambar REFERENCES Deer Cervus unicolor and Barking Deer Montiacus muntjak. In contrast, the observed levels of grazing by domestic livestock Ali, S. & Ripley, S. D. (1987) Compact handbook of the birds of India and were detrimental to Bristled Grassbirds as the resulting heavily Pakistan. Second edition. Bombay: Oxford University Press. grazed grasslands were avoided. Overgrazing by domestic livestock Baker, E. C. S. 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These findings illustrate that the habitat requirements of grassland birds in Nepal. PhDthesis, University of Amsterdam. different grassland specialist species may not coincide, so habitat Baral, H. S. (2000b) Notes on distribution of some grassland birds in Nepal management plans must take these differences into account. with reference to Suklaphanta. Danphe 9(3): 6-7. Grass diversity in preferred areas was relatively low, with Baral, H. S. & Upadhyay, G. P. (1998) Birds of Chitwan. Kathmandu: Bird Saccharum spontaneum, which grows up to 3 m in height after the Conservation Nepal & Department of National Parks and Wildlife monsoon flood retreats each year, being the dominant species. Conservation. Older reports indicate that other grassland types, ranging from BCN & DNPWC (2011) The state of Nepal's birds 2010. 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