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BIRD SPECIES RICHNESS, ABUNDANCE AND THEIR FEEDING GUILD ACROSS OIL PALMS DEVELOPMENT THROUGH MIST-NETTING METHOD IN BETONG, SARAWAK

There are a lot of studies comparing birds in oil palm plantation with forests and other crop ecosystems but less on bird across drained peat swamp forest (DPSF) converted to oil palm plantation. This study assessed the bird species richness, abundance and their feeding guild change throughout the oil palm development phases including DPSF, cleared land (CL), one-year-old palm (1YOP), two-year-old palm (2YOP), three-year-old palm (3YOP), four-year-old palm (4YOP) and five-year-old palm (5YOP). Sixty-seven species of bird were recorded across the development phases through mist-netting method, of which 35.8% were only recorded in DPSF, 19.4% of the DPSF bird species continued to be recorded after the forest conversion to oil palm plantation, and 44.8% of bird species were additional species from DPSF species, appearing following the planting of oil palm. Species richness of bird was significantly higher in DPSF than CL but similar level to those in oil palm plantation. Even though level of species richness in oil palm plantation was similar level to those in DPSF, Non-metric Multidimensional Scaling revealed that bird compositions were different according to the three grouping habitats; DPSF, CL and oil palm plantation. Species richness of insectivorous guild was ranked higher along the oil palm development phases. Species abundance of insectivorous was ranked higher at the early stage of development then omnivorous guild recorded abundant once the Yellow-vented Bulbul started to dominate oil palm of more than 2YOP. Good understanding on bird’s distribution change across the conversion of forest to oil palm development gives a better idea on how to minimise land disturbance during plantation operations.

Being one of the mega biodiversity countries, Malaysia is committed to the pledge at the Earth Summit in 1992, to keep at least 50% of its land as forest cover and the protection of Malaysia's forestland cannot be compromised (Parveez et al., 2019). Oil palm is an important commodity crop to Malaysia and with approximately 5.9 million hectares of cultivation, the industry is committed in addressing issues related to sustainability and its safety put forward by global concern (Parveez et al., 2020).
Across a range of agricultural systems, there is a general pattern of biodiversity change when natural habitats are converted to agricultural areas, and further change of biodiversity as such systems are intensified (Donald, 2004). The conversion of

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forest into oil palm plantation systems has resulted in reduced biodiversity as compared to the native forest (Gibson et al., 2011;Hawa et al., 2016;Pimm et al., 2014;Savilaakso et al., 2014). To minimise the disruption on the pattern of biodiversity, various studies have been conducted to enhance biodiversity level within an oil palm plantation, including retaining forest remnants (Laurance et al., 2018), promoting greater landscape heterogeneity (Azhar et al., 2011), and increasing riparian reserves width to 40 m for each river (Mitchell et al., 2018).
Birds are good indicators to examine the degree of habitat disturbance (Barlow and Peres 2004), fragmentation effects of the conservation value of tropical rainforest (Campbell and Reece, 2002;Mohd-Azlan et al., 2019), landscape-scale processes (Pearman, 2002), florististic composition and availability of food resources (Barlow and Peres, 2004). This is mainly because they respond quickly to changes in vegetation structures and compositions. According to Stouffer and Bierregaad (1995), birds are specialists in their foraging techniques and habitat preference. For example, they use specific habitats and microhabitats and require large territories for their survival. Monitoring birds can show early warning of environmental changes (Gregory et al., 2003) and as well as important agents for seed dispersal and pollination (Padoa-Schioppa et al., 2006), pest control in agricultural environment (Koh, 2008a), and habitat quality and changes (Mohd-Azlan et al., 2019). The species composition (richness and abundance) of birds at each habitat is likely to be influenced by various processes associated with habitat structure, food availability, microclimate changes and spatial variations (Sodhi and Smith, 2007;Yap et al., 2007).
There have been several studies of bird in the literature reporting peat swamp forest is more diverse than that in oil palm plantation, and these results are likely attributed to peat swamp forest having complex habitat structure, better habitat, greater plant diversity and food resources (Azhar et al., 2011;Posa, 2011;Yule, 2010). Habitat structure change along the oil palm development phases is expected to be primary determinant of bird diversity over the development, yet few researchers have documented it. In the recent decades, there has been increasing interest in suggesting best management practices to promote bird diversity in oil palm landscape. There are several best practices in oil palm production as suggested by Azhar et al. (2013) which imply the occurrence of bird diversity according to feeding guild. For example, maintaining ground layer vegetation cover, pruning of oil palm canopy to permit light penetration to ground layer, re-vegetation of parts of oil palm landscapes with native trees and retention of natural and/or secondary forest patches within the boundaries of the plantation.
Most of the literatures on the bird species richness were focused on the comparison between oil palm plantation with forests and other agriculture activities (Jambari et al., 2012;Koh, 2008b;2008c) which were located at different locations. There were research studies focusing on bird species richness that compared logged peat swamp forest with oil palm plantation with different ages and management (Azhar et al., 2011;Azman et al., 2011;Hawa et al., 2016). In addition to these primary data, investigation on bird starting from drained peat swamp forest to oil palm plantation are still needed for a better understanding of their species richness, abundance and feed guild change overtime. Drainage system is common in oil palm plantation on peatland for water management (Hasnol et al., 2010) and flood control (Azhar et al., 2013). Establishment of road and drainage system were carried out in the early operating procedure before clearing logged peat swamp forest for oil palm development (Harun et al., 2011). Hence, the aim of this article is to study the bird changes in species richness and abundance, and their feeding guild following the conversion of DPSF to planting with oil palm and until the palm reached five years old.

Study Site
The study was conducted in an oil palm plantation located on peatlands and covered an area of 3100 ha in Betong Division, Sarawak, Malaysia ( Figure 1). Bird surveys were conducted between July 2013 and November 2019. During this period, the study site was developed as oil palm plantation over several phases from the DPSF to cleared land (CL) and planted with oil palm from one-year-old (1YOP) to five-year-old palms (5YOP). DPSF is defined as logged peat swamp forest that has been drained for establishment of main road and drainage system according to plantation blocking before clearing phase for construction of field drains, planting rows and harvesting paths. CL phase is defined as the area that has been cleared for planting oil palm. Four sampling sessions (4320 net/hr) were carried out for each of the oil palm development phases except for CL phase (3240 net/hr). For each sampling session, two replicate sites that have undergone similar development were selected (Site A and Site B), and sampling was conducted repeatedly from both sites to monitor changes of bird population over time. We only completed three sampling sessions in CL phase due to the site being planted immediately after clearing. The surrounding area consisted of peat swamp forest and oil palm plantation belonging to other company. The distance between both sites was 1 km apart and the nearest forest

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close to Site A and Site B was 1 km apart. After the CL was planted with oil palms, the natural vegetation such as Nephrolepis biserrata, Melastoma sp., and Stenochlaena palustris grow naturally on the ground until the palm reached two years and nine months. Spraying activities started before the palms reached three-years-old and this operation only focused at palms circle and harvesting path. Harvesting activities started once the palms reached three years.

Bird Sampling
In this study, a total of 15 mist-nets [12 m (long) x 2.6 m (height) x 30 mm (mesh-size)] were set up at each site (two sampling sites) to capture birds (Hawa et al., 2016). This method restricted our samples to bird flying within the height range between 0.5-3.0 m above the ground. In the newly planted oil palm sites, the nets were set up at the harvesting path in the plantation. The nets were laid out for three days from 0600-1800 hr with regular inspections every 2 hr interval. Captured birds were gently disentangled from the net and were placed separately in the cloth bags. The captured birds were identified to the species level at the camp site and ringed before being released immediately at the site of capture to reduce stress. Ring was used to avoid double counting for similar individual at the same sampling period (Sutherland et al., 2004). Species identification was based on Myers (2009) and Phillipps and Phillipps (2014). The feeding guild of the bird species were grouped into five classes according to the bird species and food resources at the study sites (Phillipps and Phillipps, 2014). Classes of feeding guild involved in this study were namely; insectivorous (feeds on insect and small invertebrate), carnivorous (feeds on vertebrates), omnivorous (feeds on insect, vertebrates and plant parts), granivorous (feeds on grains or seeds) and frugivorous (feeds on fruits).

Data Analyses
R statistical software version 4.00 (R Core Team, 2020) was used to estimate analyses as mentioned below using package 'vegan' by Oksanen et al. (2007). Shannon index, H' was used in this study to calculate the species diversity of birds at different oil palm development phases. Species diversity can be measured if we have the number of species and the number of individuals in a given area or in a given sample (Spellerberg and Fedor, 2003). Pielou's evenness (J) was used to measure how homogeneous or even a community or ecosystem is in terms of the abundances of its species. A community in which all species are equally common is considered even and has a high degree of evenness. J compares the actual diversity value (such as the Shannon Index, H′) to the maximum possible diversity value (when all species are equally common, Hmax=lnS where S is the total number of species). J is constrained between 0 and 1.0, and the more variation in abundances between different taxa within the community, the lower the J. One-way analysis of variance (ANOVA) was used to compare species richness and abundance among development phases. Once the p-value was less than 0.05, Tukey honestly significant difference (HSD) tests were used to test for significant differences in terms of species richness and abundance between phases. To determine the similarities of the bird community in seven development phases we

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performed an ordination of sites, based on the number of individuals of bird of each species at each site, using Non-metric Multidimensional Scaling (NMDS). To determine whether the bird composition was significantly different among sites we implemented Analysis of Similarity (ANOSIM).

Species Richness and Abundance
Across the seven development phases, we captured 715 individual birds representing 67 species, of 32 families (Table 1), including 10 migratory species and two Bornean endemics (Bornean Brown Barbet, Calorhamphus fuliginosus and Dusky Munia, Lonchura fuscans). We recorded 37, 3, 14, 21, 19, 20 and 19 bird species in DPSF, CL, 1YOP, 2YOP, 3YOP, 4YOP and 5YOP, respectively. The calculation in this data suggests that species richness of bird was reduced 43.34%-62.16% from the total number of species recorded in DPSF. Only one species was recorded in all development phases, Oriental Magpie Robin (Copsychus saularis). Our species accumulation curves approached an asymptote for all sites and confirming that we had sampled the avifauna well enough to assess differences in richness between sites except for 4YOP which was still rising and has not shown any sign of levelling off to reach asymptote suggesting more species to be recorded through longer and more intensive sampling (Figure 2). Species of conservation concern comprised 15% of all species recorded across the development phase. Interestingly, oil palm plantation recorded one globally threatened and two near-threatened species, Hook-billed Bulbul (Setornis criniger) and Black-throated Babbler (Stachyris nigricollis), with Grey-chested Jungle Flycatcher (Cyornis umbratilis). Unexpectedly, we captured peat swamp forest specialist species, Hook-billed Bulbul at 3YOP.
Of the 67 species of birds recorded in this study, 35.82% were recorded only in the DPSF, 46.27% were recorded only in the plantations and 17.91% were recorded both in the plantations and forest. Out of 37 species of birds recorded in DPSF, 3%-22% bird species continued to be recorded after forest conversion to oil palm plantation (< 5YOP) and 9, 15, 13, 12 and 13 additional species from the species recorded in DPSF species were recorded in 1YOP, 2YOP, 3YOP, 4YOP and 5YOP, respectively ( Figure 3). In this study, Yellow-vented Bulbul, (Pycnonotus goiavier), C. saularis and Plaintive Cuckoo (Cacomantis merulinus) were birds recorded relatively abundance in oil palm phases as compared to DPSF (Table 1).
Our results indicated a significant difference in species richness (p<0.05) between DPSF (mean=13.25 species per sampling) and the CL (mean=1.67 species per sampling) but similar to those in palms in the ages of less than 5YOP (Figure 4). There was no significant difference in bird abundance (p>0.05) caught between different phases. Bird Shannon diversity index was greater in DPSF than CL and young palm. Species abundances were more evenly partitioned at the 1YOP and DPSF than CL and 2YOP until 5YOP. NMDS showed that the bird community (ANOSIM: the number of permutations = 999; Global R: 0.5378; p=0.001) showed three clear habitat groupings; DPSF, CL and oil palm plantations ( Figure 5).

BIRD SPECIES COMPOSITION WITH RELATIVE ABUNDANCE CAPTURED IN DRAINED PEAT SWAMP FOREST (DPSF) THAT UNDERWENT OIL PALM DEVELOPMENT PROCESSES, SUCH AS CLEARING (CLEARED LAND) AND PLANTED WITH OIL PALM ONE-YEAR-OLD PALM (1YOP), TWO-YEAR-OLD PALM (2YOP), THREE-YEAR-OLD PALM (3YOP), FOUR-YEAR-OLD PALM (4YOP) AND FIVE-YEAR-OLD PALM (5YOP) (continued)
Family

P R E S S Feeding Guild
Based on the results, out of 67 species recorded in this study, 49.25%, 20.90%, 19.40%, 7.46% and 2.99% were insectivorous, carnivorous, omnivorous, frugivorous and granivorous birds, respectively. Our results show that, the insectivorous guild recorded highest in species richness in all development phases but recorded only second in terms of species abundance starting 2YOP until 5YOP after omnivorous guild. There was no significant difference in insectivorous species richness and abundance between sites (p>0.05). During CL phase, there was no other guild recorded except for insectivorous. It was found that frugivorous guild was only recorded in the DPSF, 3YOP and 4YOP phases while granivorous guild was only recorded at the early stage of oil palm development (1YOP, 2YOP and 3YOP).
Based on the results from Table 2, insectivorous (mean=6.8 species and 15.5 individuals per sampling) in DPSF was the most diverse and abundance guild followed by omnivorous (mean=5.0 species and 13.5 individuals per sampling), carnivorous (mean=1.0 species and 2.8 individuals per sampling) and frugivorous (mean=0.5 species and 0.75 individuals per sampling). None of the granivorous guild has been recorded in DPSF. Diverse insectivorous guild in DPSF was contributed by a good representation of bird species from woodpeckers (Family Picidea) and bulbuls (Family Pycnonotidae) with five species each. The Olive-winged Bulbul, Pycnonotus plumosus from omnivorous guild was the most dominant species recorded in DPSF with 25 individuals, followed by Bold-striped Tit Babbler, Macronus bornensis with 12 individuals, and Buff-necked Woodpecker, Meiglyptes tukki with 11 individuals. Macronus bornensis and M. tukki were species categorised as insectivorous guild. Carnivorous guild in DPSF was well represented from family Alcedinidae (kingfisher). Frugivorous birds recorded in the DPSF were pigeons, barbets and parrots which mostly fed on fruits.
Meanwhile, in our survey of CL, 100% (three species) were insectivorous birds. During land clearance, one species each from Locustellidae, Apodidae, and Muscicapidae family were recorded. A total of 88.89% of the individual birds recorded in the CL were Rusty-rumped Warbler, Locustella certhiola. In this study, this species was recorded until the palm reached 5YOP.
After planting with oil palm, our results indicated that young palm (<5 years) could potentially comprise of 40%-70% more diverse insectivorous guild than other feeding guilds (omnivorous: 10%-25%, carnivorous: 14%-29%, frugivorous: 0%-15% and granivorous: 0%-14%) with most of the insectivorous guild coming from Cisticolidae family (tailorbirds, prinias). Insectivorous bird, L. certhiola was still dominant in the first year of planting oil palm, however, omnivorous bird namely Yellow-vented Bulbul drastically increased in number and became dominant starting from 2YOP until 5YOP with more than 45% from the total individuals recorded in each palm age. Omnivorous guild recorded in the plantation was contributed by the presence of Pycnonotidae family (bulbuls). The carnivorous guild in oil palm plantation were represented by wetland bird and small bird predators. In turn, these birds were preyed upon by small predators, such as Long-tailed Shrike (Lanius schach) and Greater Coucal (Centropus sinensis) which occurred abundantly in the early stage of oil palm plantation. Frugivorous guild recorded in the plantation block were mostly seed and fruit eaters, such as spotted dove. This species was not recorded in the DPSF but was captured at 2YOP and 5YOP. Furthermore, this species was spotted feeding on loose fruits on the roads. Granivorous birds were only recorded at the early stage of oil palm development (1YOP, 2YOP and 3YOP) and these species, L. fuscans and Chestnut Munia (Lonchura atricapilla) were grass seeds eaters on the plantation blocks.

DISCUSSION
The findings highlight that the logged peat swamp forest that has been drained for oil palm plantation supported higher bird species richness than CL. DPSF supported comparable levels of bird species richness to young palms at the age of less than five years old, and eight times higher than CL. Higher species richness in DPSF than CL has been attributed to the availability of complex habitat structure, better food resources and better habitat shelter in the forests (Azhar et al., 2011;Hawa et al., 2016;Posa, 2011). There was no significant difference in species richness among different ages of young palms and this result is similar to the findings by Azhar et al. (2011) and Aratrakorn et al. (2006). This study reveals that converting DPSF to oil palm plantation may potentially reduce 43%-62% of the bird species. This is consistent with the findings by Azhar et al. (2013) whereby converting forests to oil palm plantation possibly remove 48%-60% of bird species. The research study by Aratrakorn et al. (2006) also found that conversion of forest to plantations results in a reduction of species richness of at least 60%. While there were many additional species appearing after converting DPSF to oil palm plantation such as tailors, prinias, bitterns, swiflets, munias, swallows, waterhen, warblers, shrike and sparrowhawk. This result indicates that new species compositions were formed after transition from DPSF to oil palm plantation and this finding was supported though our NMDS ordination plot showing three clear grouping habitat along the development phases. Aynalem and Bakele (2008) suggested that distribution and abundance of bird are determined by the composition of the vegetation and surrounding landscape that forms a major component of their habitat. We had a surprising results, we managed to capture large-sized bird species such as Chestnut-winged Cuckoo and Japanese Sparrowhawk in oil palm plantation and this finding is similar to the study conducted by Hawa et al. (2016). Of the 10 Globally Threatened and Near-Threatened species (IUCN, 2020) recorded in this study, three species were recorded in oil palm plantation and the others were restricted to DPSF. Out of these three species, one species was known as specialist species to peat swamp forest by Sheldon (1987) namely, S. criniger and two individuals from this species were recorded at 3YOP. The abundance of this species in peat swamp forest is due to ecological release from competitor (Sheldon et al., 2014). The presence of this species, possibly for foraging activities in the oil palm plantation, was due to the adjacent peat swamp forest located 1 km from the sampling site.
Insectivorous birds from the families Picidae (woodpeckers) and Pycnonotidae (bulbuls) were recorded to be diverse in the DPSF. The finding is consistent with previous studies at peat swamp forest (Azhar et al., 2013;Hashim and Ramli, 2013;Peh et al., 2006;Sheldon et al., 2010). Good representation of woodpeckers in peat swamp forest is due to the existing remnants of dead standing trees found in the peat swamp forest which would attract bark-gleaning and hole-nesting birds for foraging and nesting activities (Styring and Zakaria, 2004). Interestingly, in CL abundance of insectivorous migratory bird, L. certhiola showed that this species was attracted to the newly opened peat swamp land which was consistent with the findings of Phillipps and Phillipps (2014) who stated that the presence of this species is due to their preference for open wetland area for their foraging activity. Our findings suggest that insectivorous guild recorded highest in species richness in oil palm plantation than other guilds and this guild was well-represented by the family Cisticolidae (tailorbirds, prinias). The primary contributor was the presence of ground layer of natural vegetation which attracted a high number of arthropods (Azhar et al., 2013;Hood et al., 2020;Jambari et al., 2012;Turner and Foster, 2009) hence providing food resources for this guild.
Diverse species of bulbuls in DPSF indicated that these coloniser species are common species recorded in disturbed areas such as regenerating forest from previous logging activities (Hashim and Ramli, 2013;Sodhi and Smith, 2007;Zakaria et al., 2002). Abundance of omnivorous bird, P. plumosus in DPSF indicates that this habitat was common in disturbed forest areas. Uneven species abundance in oil palm plantation was contributed by the presence of omnivorous bird P. goivaier as a dominant species starting from 2YOP, 3YOP, 4YOP and 5YOP. This result is in line with earlier literature by Hawa et al. (2016) and Amit et al. (2015) who found that P. goivaier has successfully dominated oil palm plantation due to their preference for diet with diverse range of food from plant materials to arthropods. Plantation practice by maintaining ground-layer vegetation growing naturally provide food resources for this omnivorous bird (Azhar et al., 2013;Hood et al., 2020;Turner and Foster 2009).
Carnivorous guild in DPSF was represented by the family Alcedinidae (kingfisher) while in early oil palm plantation revealed that carnivorous guild was well represented by wetland bird such as Amaurornis phoenicurus, Ixobrychus cinnamomeus and Ixobrychus sinensis and small bird predators such as L. schach and C. sinensis. Construction of drainage systems to control the ground water level in oil palm plantation on peat create habitat for aquatic life hence was more attractive for wetland carnivorous birds due to foraging opportunities (Azhar et al., 2013;Jambari et al., 2012). Diverse birds from family Cisticolidae (tailorbirds, prinias) in oil palm plantation provide food resources for small predators that feed on small birds. This finding was observed during sampling period where C. sinensis was seen feeding on tailorbirds and prinias captured on the nets.

CONCLUSION
It is important to understand the changes of bird species richness, abundance and their feeding guild throughout the oil palm development processes so that the effects of land conversion especially on peatland can be reduced and managed. Our study demonstrates that DPSF supported higher bird species richness than CL but similar level to those in early stage of oil palm plantation. This study has shown that species abundance is similar among sites. Good representation of insectivorous guild in terms of species richness in all development phases. Oil palm plantation recorded abundance of omnivorous guild starting at 2YOP until 5YOP. Through letting ground layer vegetation grow naturally and maintaining water quality of the drainage system in the early stage of oil palm development by the plantation management attract birds that prefer to this habitat hence provides food resources for their survival.