Differential responses to habitat structure and degradation by two Grassbirds (Fam. Locustellidae) syntopic in floodplain grassland

doi:10.21203/rs.3.rs-2847011/v2

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Differential responses to habitat structure and degradation by two Grassbirds (Fam. Locustellidae) syntopic in floodplain grassland

https://doi.org/10.21203/rs.3.rs-2847011/v2

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Wet grassland birds world-wide face mounting threats due to habitat destruction especially in the Indo-Gangetic foodplains, where in addition to intense human use, primarily for agriculture, there is a paucity of baseline information to inform conservation and management. We examined the response of two co-occurring foodplain grassland specialists, the globally threatened Bristled Grassbird Schoenicola striatus (VU) and widespread but little-studied Striated Grassbird Megalurus palustris (LC), to habitat structure and human use in foodplain grassland at two sites, one on the lower Ganges River and one on the Brahmaputra River, Bangladesh through point counts combined with vegetation surveys. Responses from the breeding seasons of 2018 and 2019 were assessed by linear models and bird densities estimated using distance sampling. Results indicate that the occurrence of Bristled Grassbird increased with homogeneous tall Saccharum grass cover but decreased with increased crop cover, which may be the main driver of its decline. Striated Grassbird showed a more generalized pattern of positive association with tall grassland vegetation including taller crops and exotic plants, but was also positively associated with grass harvesting indicating its avoidance of homogeneous tall grassland. Although the density of Striated Grassbirds at the Brahmaputra site was 3–4 times greater than that of Bristled Grassbird, it was nearly absent from the Ganges site, revealing previously unappreciated micro-habitat preferences. Comparison with a more common species provided a clearer understanding of habitat and human infuences on the more threatened grassbird. Long-term conservation of Bristled Grassbird requires careful management of grass harvesting and cattle grazing.

Bristled Grassbird Schoenicola striatus

Striated Grassbird Megalurus palustris

grassland birds

grassland management

Indo-Gangetic plains

Globally, grassland birds have experienced major population declines in recent decades due to habitat loss and degradation (Marques et al. 2020, Rosenberg et al. 2019). Hence, efforts to stabilize or increase grassland bird populations require identification of species’ ecological requirements as a first step (Fisher and Davis, 2010). In addition, vegetation structure and spatial distribution in seasonally flooded grassland ecosystems found in wetlands may vary dramatically, depending on the extent of seasonal inundation (Winter et al. 2005). This adds another level of complexity to understanding how each bird species might respond to vegetation variables (Winter et al. 2005), as well as differing levels of human disturbance (Price 2008) and grassland use (Roth et al. 2005; Hossain & Li, 2020; Porensky et al. 2020). For example, seasonally flooded grasslands at the base of the Himalayan foothills and in the floodplains of the Ganges and Brahmaputra rivers in northern India, southern Nepal and Bangladesh (the Indo-Gangetic grasslands) support a distinctive bird community (Baral, 2001; Rahmani et al., 2016), with several bird species restricted to this system (BirdLife International, 2003 & 2015). However, high levels of human use, land clearance for agriculture, overgrazing, and grass harvesting (Baral, 2001; Gopi Sundar, 2011; Grimmett et al. 2021, Jahan et al. 2022) are likely reducing breeding success and survival of birds of this habitat region (Gopi Sundar, 2011). Livestock grazing occurs most of the year (October to June); grass harvesting for feeding livestock occurs year-round at a small scale, and at a larger scale in June, immediately prior to the onset of flooding, when sale of Saccharum sp., especially, serves as an important livelihood source for villagers. Ecologically, Saccharum sp. provides soil binding services, stabilizing riverbeds and reducing bank erosion (Chandran, 2015). Although some areas of floodplain grassland in the Indo-Gangetic Plain fall within protected areas in India and Nepal, no government policies have been implemented to protect wet grassland habitats in Bangladesh. Furthermore, there is a lack of data on habitat factors and their impacts on birds in the Indo-Gangetic grasslands that can support management actions.

The Bristled Grassbird Schoenicola striatus and the Striated Grassbird Megalurus palustris (Family: Locustellidae) are closely related genera (Alström et al. 2018), both reliant on parts of these Indo-Gangetic grasslands for part or all of their life cycle (Khan et al. 2015, IUCN Bangladesh, 2015, Madge 2020, Jahan et al. 2022). The Striated Grassbird is widely distributed throughout South-east Asia whereas the Bristled Grassbird is endemic to South Asia, and is patchily distributed in Bangladesh, India, Nepal and Pakistan (Del Hoyo et al. 2016). Bristled Grassbird is considered to be globally threatened (Vulnerable), while Striated Grassbird is regarded as common and widespread, with status Least Concern throughout its range (BirdLife International, 2022). Our paper seeks to answer two questions: what explains their differences in status and how might insights from this comparison inform grassland bird management in wetlands? Such comparisons are worthwhile because within bird communities, species sharing a similar degree of specialization to a particular habitat (such as the Bristled Grassbird and Striated Grassbird) co-occur more often rather than they do with broad generalists (Julliard et al. 2006). Understanding the associations and interactions between such similarly specialised, often closely related, species and their differential responses to human influences may enable more reliable predictions about the effects of habitat alterations on multiple species and potentially allowing pre-emptive action to reduce or mitigate threats to key populations and species (Okes et al. 2008, Han et al. 2020).

Here we tested the hypothesis that Bristled Grassbird and Striated Grassbird show different wet grassland vegetation structural preferences and different response to habitat degradation. We predicted that Bristled Grassbird would be limited to a narrow range of habitats supporting contiguous taller, homogeneous floodplain grassland cover, principally Saccharum sp. (Baral et al. 2014, Singh and Buckingham 2015, Chowdhury et al. 2022, Jahan et al. 2022). Striated Grassbird, on the other hand would occupy a wider range of vegetation types, occurring at higher density in more open, patchy, and heterogeneous cover (Round et al. 2014, Ali and Ripley 1987). We likewise expected that the impacts of human use (agriculture, livestock grazing and grass harvesting intensity) would affect the two species differently, predicting that Bristled Grassbird would avoid highly grazed (Singh and Buckingham 2015) and harvested floodplain grasslands (Chowdhury et al. 2022), unlike the Striated Grassbird. Finally, we predicted that the Striated Grassbird would be more abundant in the floodplain landscape given its somewhat broader habitat associations compared to Bristled Grassbird (IUCN Bangladesh, 2015).

Study sites and study species

The study islands (locally known as chars) were located in the Ganges-Padma River floodplain of Rajshahi (Char Majardia) and in the Brahmaputra-Jamuna River floodplain of Kurigram (Char Sakahati), Bangladesh referred to hereafter as Rajshahi and Kurigram (Fig. 1). The total area of the Rajshahi site covered 19.4 sq km, and was approximately 1.2 km from the mainland, while the Kurigram site covered 35.2 sq km and was about 3.4 km from the mainland, based on 2018 Google Earth data. The two sample islands are about 170 km apart. The study areas were chosen to represent conditions along the two rivers and particularly for the presence of Bristled Grassbird (Jahan et al. 2022). These sites are roughly near the eastern edge of the Bristled Grassbird range and roughly in the center of the mainland Asian range for Striated Grassbird (BirdLife International 2022). Both areas are subject to seasonal flooding during the monsoon (July–October). During the bird breeding season, when surveys were conducted (May to July), wet, inundated areas were scarce. Land-cover of both areas was characterized by floodplain grasslands containing a mosaic of patches of both short grasses (Cynodon sp.) and tall grasses (Saccharum sp.), with a mix of forbs and bushes. Forbs and bushes included Solanum sp., Tamarisk sp., Ipomoea carnea, Lippia nodiflora, Persicaria sp. and Xanthium strumarium, and were usually mixed with grasses. For the two woody species:

Ipomoea carnea, considered invasive to these tropical grasslands (Akter and Zuberi, 2009), was more abundant (based on individual plants/sample point) in Kurigram (0.58 stems per sample point) than at Rajshahi (0.07stems/point); while Tamarisk sp. was more abundant in Rajshahi (3.88 stems/point) than at Kurigram (0.75 stems/point). Ipomoea carnea can reach heights of up to 5 m and occurs in patches or clumps amid other vegetation. Tamarisk sp. also can reach heights of up to 5 m and occurs in extensive patches or small patches amid other vegetation. Saccharum sp. mostly grows up to 3 m tall and occur in dense and more extensive patches. Croplands and small villages were also characteristic of the landscape. Differences in crop varieties were noted between islands. In Kurigram site a wide range of crops were planted including rice, lentils, jute, maize, sesame, dhaincha (Sesbania bispinosa), berries (Ziziphus mauritiana), pointed gourd, bitter gourd, chili, and other vegetables; while in Rajshahi site mostly rice was planted with small areas of sesame and jute. Kurigram site supported year-round human occupancy, in villages located on the island, while Rajshahi site had no human occupants within the study area, although a village named Nobinagar was located at the south edge of the island, adjacent to the study area, close to the border with India (part of the island is located in India, Fig. 1). Livestock grazing (cows Bos indicus, goats Capra hircus, sheep Ovis aries, and water buffaloes Bubalus bubalis) and grass (Saccharum sp.) harvesting by local people takes place at both sites before the onset of the monsoon. Grazing intensity was observed to be higher in Rajshahi compared to Kurigram in 2019 (Fig. 1).

The Striated Grassbird Megalurus palustris toklao is typically present in grassy floodplains throughout the country and is considered to be a common and widespread resident (IUCN Bangladesh 2015). Bristled Grassbird Schoenicola striatus, formerly described as common in parts of Bangladesh in the nineteenth century (Ali and Ripley 1987), went un-recorded in the country for over a century. It was rediscovered in northeast Bangladesh in 2014, and considered to be rare (Haque and Tareq 2014, Khan et al. 2015). However, a recent study found that Bristled Grassbird was more widespread along the major rivers than previously recognized (Chowdhury et al. 2022). Both species breed during the summer and early monsoon season (March-July) (Jahan et al. 2022, Khan et al. 2015). While Striated Grassbird is a highly visible, year-round resident (IUCN Bangladesh, 2015), outside the breeding season Bristled Grassbird remains largely unrecorded probably due to a relative lack of surveys.

Survey methods

Abundance data for both grassbird species were collected during their breeding season, when birds were singing and most easily detected. Point-count surveys were conducted from 21 May to 2 July in 2018 and from 22 May to 16 July in 2019. A total of 382 point-counts were conducted (185 points in 2018 and 197 in 2019) by overlaying a grid of 250 x 250 m on 2018 Google Earth imagery of the study islands. The grid size was decided based on field surveys to minimize double counting the same birds in adjacent points. Based on image interpretation and field checking, grid points that fell on water, bare sand, woodland, forest plantations and human settlements were excluded, and the remaining points were in grassland patches and other open-country vegetation (bushes, reed, swamp, scrub, cultivation). Sample points were chosen randomly from among these, one per grid-square so that all sample points were ≥ 250 m apart. One observer (IJ) recorded individual Bristled Grassbirds and Striated Grassbirds seen or heard within 100-m fixed-radius point-counts (Hutto et al. 1986) of 10-min duration without a waiting period (Savard and Hooper 1995). Both species were noted as either heard, or seen in song-flight, or perching. Differences in song flight behaviour of both species were also recorded. Distances between the observer and birds were estimated directly using a rangefinder when birds were visible, otherwise in 5 m bands to 30 m, then 10 m bands to 100 m (Rosenstock et al. 2002). With points spaced 250 m apart, and birds only counted to a distance of 100 m at each point, the 50 m buffer minimized the likelihood of double-counting the same individual. Moreover, the observer also carefully noted the locations of individual birds seen at adjacent points to further avoid double-counting the same individuals. Surveys took place between 06h00 and 10h00 under good weather conditions (i.e., with no rain or strong winds). Weather conditions (wind and cloud cover) were also recorded. Wind velocity was estimated following the Beaufort scale, cloud cover (visual percentage approximation) and survey time and duration were recorded. Rajshahi was surveyed at 94 points which were sampled in the two survey years and an additional 12 points surveyed in cropland in 2019 (Fig. 1). Kurigram was surveyed at 91 points which were also sampled twice (once in each survey year). In total, the detection area of the survey points covered about 17% of the Rajshahi site and 8% of the Kurigram site.

Habitat measurements

In 2018 and 2019, plant species and height were recorded at 5 m intervals along a 100 m radius from each bird survey point in each of the four cardinal directions (see Jahan et al. 2022 for additional details). This gave a total of 80 vegetation samples per bird survey point. The plants were classified into four categories: Saccharum sp., Cynodon sp., forbs and bushes, or cropland. Bristled Grassbird and Striated Grassbird were observed using Saccharum sp., Ipomoea carnea, Tamarisk sp. forbs (> 3m height) and trees (> 7m heigt) as their perching substrate (Fig. 2). Of which Ipomoea carnea and Tamarisk sp., were tall and woody classed within the forbs and bushes category, were additionally classified as song posts. Crop and non-crop vegetation height (cm) was recorded as an average for each sample point within a 50 cm radius at 5 m intervals. Human use variables (grazing and harvesting) were measured only in 2019. Grazing intensity was based on a count of livestock dung (Sheidai-Karkaj et al. 2022) and grass harvesting intensity was based on a count of cut grass stems within a 1.5 m radius, every 5 m interval along the four cardinal direction sampling transects. Some changes in habitat were observed between 2018 and 2019: four points previously covered with Saccharum sp. or forbs and bushes in 2018 were bare land in 2019, and an additional five points previously covered with Saccharum sp. or forbs and bushes were inundated in 2019. Crop cultivation increased in 2019 (29 points) compared to 2018 (12 points). This comparison does not include an additional 12 points which were surveyed in cropland in 2019 in order to increase the sample of cultivated points. Vegetation heterogeneity was quantified based on the extent of different vegetation heights recorded at each bird survey point. Vegetation heterogeneity was summarized by calculating means and standard deviations of vegetation height measures within each bird survey plot (n = 80) and then averaged these across all plots (n = 382). Vegetation heterogeneity was considered as the extent to which short vegetation and tussocks of taller vegetation occur in proximity. Areas with a mix of both short and tall vegetation were considered to be more heterogeneous than areas with only tall or only short vegetation for example.

Analysis

Vegetation associations

Both male and female Bristled Grassbirds and Striated Grassbirds were recorded at each sample point. However, females of both grassbird species were relatively cryptic, whereas males were more readily detected, particularly in song flight, and when perched and singing. All the aural detections from song were also likely from males. Therefore, models assessing vegetation associations and density estimates were performed for only males of both species. To evaluate the occurrence of singing males in 100-m point counts of each species relative to habitat variables, we used binomial regression models in a generalized linear mixed models (GLMMs) framework, using R package glmmTMB (Brooks et al. 2017). Models were fitted as a binary response variable: detected = 1 and not detected = 0. Model sets were developed to conduct vegetation structural preference analysis and the effects of grazing and harvesting for Bristled Grassbird and Striated Grassbird combining both study islands and both years. We used the same set of randomly sampled points to replicate the survey in both years (apart from the 12 additional points in cropland in 2019). However, the impact of the differing extent of flooding and changes in vegetation on bird usage during the two years meant that these sets could reasonably be considered to be independent of one another. Two sets of models were developed: the first set of models included vegetation type with vegetation heterogeneity, height and song post, using data recorded in 2018 and 2019. The second set of models added grazing and harvesting intensity, these data were recorded in 2019 only. Vegetation categories were Saccharum sp., Cynodon sp., forbs and bushes, and cultivated crops. Use of tall, woody song posts (Ipomoea carnea and Tamarisk sp.) was analysed for both of the bird species. Both model sets contained each variable and combination of variables, in addition to null and global models. Collinearity among predictor variables was checked before analysis. Variables with correlations above 0.70 were not included in the same model per Dormann et al. (2013). AIC model comparison methods (Burnham and Anderson 2002) were used to evaluate the relative ability of each model to explain variation in occurrence for Bristled Grassbird and Striated Grassbird. All models were checked for the homogeneity of their residuals. Models were compared and ranked using Akaike’s information criterion corrected for small sample size (AICc) following Burnham and Anderson (2002). Models and their relative importance were also assessed using Arnold (2010) to avoid the inclusion of uninformative parameters. AICc weights (w) provided the relative weight of any particular model (Burnham and Anderson 2002). Model coefficients (β) with 95% confidence intervals (CI) from AICc selected models (Burnham and Anderson 2002, Arnold 2010) were reported, and predictor variables were considered to be strongly influential on response variables if the CI for β did not overlap zero (Bolker 2008, Arnold 2010). We also used model-averaged coefficients to make multi-model inferences when model-selection uncertainty was high (Burnham and Anderson 2002). All statistical analyses were carried out in the Program R version 4.1.0 (R Core Team, 2020) using glmmTMB, tidyverse, ggpubr, AICcmodavg, MuMIn, visreg packages.

Other habitat variables tested included the presence of standing water within 100 m of the survey point as an indicator of ‘wet’ habitat as a rough test of a hypothesis regarding the relative preferences for the two grassbird species for wetter versus drier habitats.

Grassbird density

Distance sampling was applied to estimate bird densities using Program DISTANCE (Buckland et al. 2001). Four estimators: uniform, half normal, hazard-rate, and negative exponential were employed to determine the detection model of best fit. Hazard-rate models were not considered when the output gave implausible shapes (Thomas et al. 2010). All detection models were run with the cosine adjustment. Models were evaluated using AICc (Buckland et al. 2001). The detection model with the lowest AIC score was considered the best fit and was used to estimate density of individuals within that specific habitat category (McCollum et al. 2018). Kolmogorov-Smirnov goodness-of-fit (GOF) test, were used to evaluated models with models having P > 0.05 considered well-fitted to the data (Buckland et al. 2001).

Detections

In 2018 and 2019 Bristled Grassbird was found on both study sites, while Striated Grassbird was primarily found at Kurigram and was largely absent from Rajshahi (Table 1). A significantly greater density of Bristled Grassbird males was detected at Rajshahi in 2019 versus 2018 (Table 1). No other significant differences in abundance were observed between years for either species.

Table 1

Detections and densities of Bristled Grassbird and Striated Grassbird in the sample floodplain grassland at two study sites. All selected detection models were run with cosine adjustment. A total of 185 sample points were surveyed, which covered 581 ha in total on the two islands: 295 ha in Kurigram and 286 ha in Rajshahi. Values reported are: number of birds (M = Male; F = Female), number of points detected, estimated density (birds/ha) with 95% confidence intervals (95% CI).
Study site	Year	No. of points surveyed	Bristled Grassbird			Striated Grassbird
			No. of birds	No. of points detected	Male density (birds/ha) (95% CI)	No. of birds	No. of points detected	Male density (birds/ha) (95% CI)
Kurigram	2018	91	19 (15 M; 4 F)	12	0.10 (0.04–0.24)	39 (39 M; 0 F)	23	0.32 (0.15–0.67)
Rajshahi	2018	94	33 (31 M; 2 F)	26	0.11 (0.06–0.19)	2	2	0.02 (0.01–0.07)
Kurigram	2019	91	26 (22 M; 4 F)	24	0.13 (0.06–0.29)	58 (58 M; 0 F)	34	0.72 (0.36–1.48)
Rajshahi	2019	106	64 (63 M; 1 F)	47	0.26 (0.16–0.42)	0	0	0

Bristled Grassbirds were mostly (n = 69) detected in song flight (displaying males), and less often (n = 42) perching on Saccharum sp. (Fig. 2). Most Striated Grassbird detections were of birds singing from song posts (n = 81). They mostly used Ipomoea carnea and other woody perches as their perching substrate (Fig. 2). Displaying Bristled Grassbird males made long duration song-flights (2–3 minutes) high (between 15-21m) in the air, returning to perch in tall Saccharum sp. for a period of about 30–50 seconds. Striated Grassbird males, on the other hand, sang and called while perched on song posts making only short (~ 50 seconds) song-flights during a short ascent and a “parachute” descent to the same or similar nearby perch.

Habitat associations

Vegetation species diversity differences (excluding differing crop types) were generally low between the two study areas, but changes in dominant vegetation and land cover types were observed between years (Fig. 1). Within our survey area, the area of Saccharum sp. cover at Rajshahi increased from 14.9% in 2018 to 19.8% in 2019. In Kurigram a near tripling of crop area (from 11.0% in 2018 to 30.8% in 2019) resulted in a concomitant decrease in the area of Saccharum sp. (from 23.1% in 2018 to 14.3% in 2019). The area of forbs and bushes mixed with other grass cover was also somewhat reduced, from 65.9% in 2018 to 53.8% in 2019. There was no increase in crop cover at Rajshahi between years at survey points sampled in 2018 and 2019, although the area covered by short grass, Cynodon sp., increased from 5.3% in 2018 to 14.2% in 2019, associated with grazing and harvesting. Considering the survey areas, both sites were used by local people for cattle grazing (34.1% in Kurigram and 53.8% in Rajshahi) and grass harvesting (11.3% in Rajshahi and 23.1% in Kurigram).

Patterns of vegetation association were broadly similar for both species, but the main difference was the relationship to crop cover, with Bristled Grassbird being negatively associated with crop cover, while Striated Grassbird was positively associated (Fig. 3). Model results for Bristled Grassbird (both study areas combined) ranked Saccharum sp. as the top model (Table 2) indicating that the occurrence of male Bristled Grassbird increased with increasing Saccharum sp. cover (Fig. 3). There was a positive, statistically signficant, beta coefficient for Saccharum sp. Crop cover and vegetation heterogeneity had significantly negative beta coefficients, while vegetation height had a significantly positive coefficient (Fig. 3, Table 2).

Table 2

Effect of vegetation structural associations and human use variables on the occurrence of Bristled Grassbird and Striated Grassbird in floodplain grasslands of the lower Ganges and Brahmaputra Rivers, data recorded in 2018 and 2019. Parameters of the average of top models for habitat selection with ΔAIC < 2. Values reported include coefficients (β) estimate, 95% confidence interval (CI), standard error (SE), LCI = lower confidence interval, and UCI = upper confidence interval. If LCI and UCI do not overlap with zero, the variable explains well the model.
Bristled Grassbird (2018–2019)					Striated Grassbird (2018–2019)
Models	β	SE	95%LCI	95%UCI	Models	β		SE			95%LCI	95%UCI
Saccharum sp.	0.650	0.117	0.421	0.879	Vegetation height	0.373		0.135			0.109	0.637
Vegetation height	0.428	0.113	0.207	0.649	Cynodon sp.	-0.448		0.186			-0.814	-0.083
Crop	-0.488	0.144	-0.771	-0.205	Crop	0.303		0.126			0.055	0.552
Heterogeneity	-0.266	0.122	-0.506	-0.027	Heterogeneity	-0.217		0.153			-0.518	0.085
Cynodon sp.	-0.115	0.117	-0.345	0.116	Forbs-bushes	-0.269		0.167			-0.597	0.060
SongPost	0.102	0.107	-0.109	0.314	Saccharum sp.	0.143		0.139			-0.130	0.416
Forbs-bushes	-0.041	0.115	-0.266	0.184	Woody songpost	-0.024		0.145			-0.309	0.260
Bristled Grassbird 2019					Striated Grassbird 2019
Models	β	SE	95%LCI	95%UCI	Models		β		SE	95%LCI			95%UCI
Saccharum sp.	0.912	0.165	0.589	1.236	Harvesting		0.563		0.154	0.261			0.865
Vegetation height	0.652	0.158	0.341	0.963	Cynodon sp.		-0.857		0.318	-1.482			-0.233
Crop	-0.561	0.182	-0.918	-0.204	Vegetation height		0.546		0.190	0.173			0.919
SongPost	0.388	0.156	0.082	0.693	Crop		0.365		0.171	0.030			0.700
Heterogeneity	-0.226	0.156	-0.533	0.080	Heterogeneity		-0.366		0.223	-0.802			0.070
Harvesting	0.231	0.145	-0.052	0.514	Saccharum sp.		0.356		0.184	-0.005			0.717
Cynodon sp.	-0.206	0.155	-0.509	0.097	Forbs-bushes		-0.148		0.208	-0.557			0.261
Forbs-bushes	0.122	0.145	-0.163	0.406	Grazing		-0.071		0.196	-0.457			0.314
Grazing	-0.093	0.151	-0.390	0.204	Woody songpost		0.031		0.184	-0.330			0.392

For Striated Grassbird, vegetation height was ranked as the top model (Table 2) with a significantly positive coefficient between height and occurrence (Fig. 3). Another indicator of vegetation height, the cover of Cynodon sp. (an indicator of short vegetation) also had a significant negative beta coefficent indicating that the occurrence of Striated Grassbird decreased with increasing Cynodon sp. cover (Fig. 3). Crop cover had a significant positive beta coefficent indicating a positive association between Striated Grassbird and crop cover. Considering the plants used as song posts as a potentially important explanatory variable, occurrence of Striated Grassbird was found to be strongly positively correlated with Ipomoea carnea cover (β = 0.435, 95% CI = 0.202 to 0.668) (Fig. 4). Ipomoea carnea was much more abundant at Kurigram than at Rajshahi. While there was no association of Bristled Grassbirds with Ipomoea carnea and Tamarisk sp. Regarding possible associations with wetter versus drier grasslands, Striated Grassbird tended to be detected at wetter patches (12 out of 59 points, ~ 20%), compared with Bristled Grassbird (11 out of 109 points, ~ 10%), but this difference did not quite reach statistical significance (χ² = 3.4, P = 0.065).

Effects of grazing and harvesting

Grazing and harvesting intensity had no significant effect regarding the presence of Bristled Grassbird when combining data from both study areas. For Striated Grassbird, harvesting appeared to have a significant positive effect, while no impact from grazing could be detected (Fig. 3, Table 2).

Density

In the site where both species were present (Kurigram), the density of Striated Grassbird was approximately three times that of Bristled Grassbird in 2018 and four times the density in 2019. This difference was significant in 2019. Bristled Grassbird densities were not significantly different between islands but were significantly different on Rajshahi between years (Table 1).

Bird-vegetation structure associations

Our results suggested that Bristled Grassbird and Striated Grassbird have similar grassland vegetation structural preferences for taller grassland vegetation, although clear differences with respect to Saccharum were noted. In addition, a key response to cropland was contrasting, with Bristled Grassbird avoiding cropland, while Striated Grassbird was positively correlated.

The strong association between Bristled Grassbird with Saccharum sp. cover matches well with previous studies (Baral et al. 2014, Singh and Buckingham 2015, Chowdhury et al. 2022, Jahan et al. 2022). It was also significantly positively associated with taller vegetation in general. Furthermore, it appeared to be negatively influenced by agriculture, avoiding cropland. The negative coefficient associated with heterogeneity further indicated this species was likely associated with more continuous cover of Saccharum, but its avoidance of cropland may be the largest reason for its conservation concern. On the other hand, Striated Grassbird also tended to be associated with taller grassland vegetation, but was more general in its preference and did show greater tolerance for cropland. The results also indicate that this species may benefit from the presence of the woody Ipomoea carnea (exotic species) which is used as sturdy song perches in somewhat contrast to the Bristled Grassbird, which largely makes aerial song-flights and appears not to require such perches.

Another reason for the observed differences between the species were differences in display behaviour. Whether their different song flight performances are related to territorial defence or mate attraction (or both), our observations indicated that while both species shared similar habitats they used different resources in display. These observations are also consistent with previous studies - Ali and Ripley (1987) that Striated Grassbird commonly perches on bush-tops and reeds. The differences in display and singing habits of Bristled Grassbird and Striated Grassbird, in particular their differential use of song posts, may contribute to different patterns of habitat use and occupancy.

Impacts of human use

Our results support the hypothesis that the two species are affected differently by human use (agriculture, livestock grazing and/or grass harvesting intensity). Bristled Grassbird presence was negatively correlated with cropland, and similar results were found in Bangladesh (Chowdhury et al. 2022) and Nepal (Singh and Buckingham, 2016). On the other hand, the Striated Grassbird had a significantly positive association to cropland suggesting at least some ability to utilize a wider range of tall vegetation, including croplands. However, Striated Grassbird may decline as agricultural intensification progresses as seen in Thailand (Angkaew pers. com. 2023).

There was no clear indication that grazing or harvesting affected Bristled Grassbird. However, Striated Grassbird was positively associated with harvesting which points to a lack of dependence on homogeneous grassland by the Striated Grassbird as more of a generalist compared to Bristled Grassbird. This fits previous observations that grassland bird species respond differently to harvesting and grazing practices as well as intensities (Coppedge et al. 2008, Kim et al. 2008, Sliwinski and Koper 2015), and the evidence from this study of two grassbird species supports this generalization.

Density

The estimated density of Bristled Grassbird was similar at both study islands in 2018. However, in 2019, density was higher in Rajshahi than Kurigram in 2019 and although there was a large increase in cropland in Kurigram in 2019, Kurigram densities did not decline compared to 2018, suggesting a more complex relationship with agriculture than a simple linear relationship with area. More study is needed to assess this. In both years, the density of Striated Grassbirds in Kurigram was three to four times greater than the density of Bristled Grassbird attained at either study area (Table 2), indicating a much greater ecological tolerance of the wide-ranging and relatively abundant species in relation to Bristled Grassbird.

As the micro-habitat requirements of both grassbird species differ, even though they occupy similar floodplain grassland habitats, so habitat management plans must take these differences into account, with greater emphasis on the scarcer habitat required by the more threatened species. For Bristled Grassbird in Bangladesh, Chowdhury et al. (2022) recommended protection from grass cutting and livestock grazing. However, we found no clear significant negative impact from either grazing or harvesting on Bristled Grassbird, although presumably over-grazing and over-harvesting would be a problem and the timing of these activities would also probably be important for sustainable management. Patches of tall Saccharum sp. take several years to develop and therefore need protection from conversion to cropland but likely can sustain partial harvesting and grazing. Furthermore, without such harvesting and/or grazing, ecological succession might over time lead to Saccharum being replaced by woody shrubs. We therefore argue that wet grasslands on these river islands would be best managed for the long term if a regime of moderate grazing and grass harvesting could be established and maintained. This, with care, would help to retain both grassbird species and maintain some livelihoods for local people. Achieving total protection of such wetland grasslands would be a major challenge in river islands that are remote from any conservation authorities, have long established patterns of human use, and are subject to frequent change due to floods and erosion nearly every year. Strategies to limit the amount of agriculture, diversify crops (for Striated Grassbird) and ensure that other bird-friendly practices such as minimizing pesticide use are adopted would be appropriate to conserve both species. In addition, leaving some farm fields fallow each year would be a good additional measure and possible compromise for local farmers. That would help to provide links for grassland dependent birds to use larger areas and to move between patches of tall grasses, by allowing medium height grasses and forbs to grow, in this way providing usable habitat for a number of grassland bird species of management concern. This management approach will require cooperation between conservationists, local governments and local people to develop local plans to zone river islands and regulate uses in different zones to achieve a reasonable balance between bird conservation and human livelihoods. This will also necessitate negotiation and an adaptive approach to monitor and adjust practices based on longer-term monitoring of target wet grassland dependent species.

Acknowledgements: The authors express sincere gratitude to the field team (Shimul Nath, Sultan Ahmed, Hossain Ahmed Tanvir, Naim Rashid, Sajib Biswas), local bird guides (Nuru, Nazirul, Kalam), British bird ringers (W M Jones, Mike Jackson). Thanks to Hem Sagar Baral, Mike Crosby (BirdLife International) for recommendations for field work funds. We thank Bangladesh Bird club, Rajshahi Bird Club, Bangladesh Forest Department, Prof. Sayed Abu Touab Shakir for their technical assistance throughout the research. Lastly, thanks to Dr. Larkin Powell, Dr. Dusit Ngoprasert, Dr. Naruemon Tantipisanuh for their help with data analysis.

Funding statement: This research was supported by Petchra Prajom Klao Doctoral Scholarship Program (Scholarship ID-36/2560) of King Mongkut’s University of Technology Thonburi, Thailand and National Geographic Early Career Grant (Grant # EC-325R-18).

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Differential responses to habitat structure and degradation by two Grassbirds (Fam. Locustellidae) syntopic in floodplain grassland

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Abstract

Introduction

Methods

Study sites and study species

Survey methods

Habitat measurements

Analysis

Vegetation associations

Grassbird density

Results

Detections

Habitat associations

Effects of grazing and harvesting

Density

Discussion

Bird-vegetation structure associations

Impacts of human use

Density

Conclusions

Declarations

References

Additional Declarations

Supplementary Files

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