Bird diversity in the enrichment experiment plots of an oil palm plantation

Agricultural habitat heterogeneity could promote the bird biodiversity. Up to six native tree species had been planted in different plot sizes by the EFForTS (Ecological and Socioeconomic Functions of Tropical lowland rain forest transformation Systems) Project in Jambi Province, i.e., Petai (Parkia speciosa), Jengkol (Archidendron pauciflorum), Durian (Durio zibethinus), Sungkai (Peronema canescens), Meranti (Shorea leprosula) and Jelutung (Dyera polyphylla). This study is aimed to investigate the effect of native tree enrichment to bird diversity in oil palm plantation. Data was collected from 27 July to 10 August 2022 in enrichment experiment sites. The enrichment plots surveyed were those of 20 m x 20 m (13 plots) and 40 mx 40 m (12 plots) sizes. Point counts with a radius of 50 m were used to survey from 6.00 to 10.00, and, when necessary, from 15.30 to 17.30. A total of 33 bird species of 20 families were observed during the study, but only 24 species of 13 families were observed within the point count radius. Bird diversity was slightly higher in the larger enrichment plots (H’= 2.649; E= 0.845) than in the smaller plots (H’= 2.291; E= 0.826). Seven feedingguilds based on major dietwere identified, with the majority (48.5%) are insectivorous species. Two carnivorous species were recorded outside the point count radius, i.e.,Changeable Hawk-Eagle (Nisaetus cirrhatus) and Barn Owl (Tyto alba).


Introduction
Massive rainforest transformation intocash crop plantation has resulted in the loss of critical habitat formany plants and animals of the forests.The loss of biodiversity has been showed to reduce the capacityof ecosystem to give multiple services that need by humans [1].Many studies proved that forest bird diversity is affected by land conversion into agricultural land especially oil palm plantation.
Habitat heterogeneity or complexity is one factor that might increase the bird diversity.A study by [2] showed habitat heterogeneity increase bird diversity through providing temporary refuge as well as migratory stop over for birds.Agroforestry that combines crop production with trees is one way to obtain yield while maintaining the biodiversity [3].The creation of tree islands in oil palm plantation will increase habitat heterogeneity that is important for biodiversity while fulfilling the livelihood [4].An experiment in oil palm showed that maintaining understory vegetation in oil palm increased the bird species richness by 3.6 times [5].High bird diversity in oil palm plantation might be advantageous for oil palm plantation through the service provided by the birds.Information on the diet of birds is necessaryto support pest control in agroecosystem.A study on the diet of insectivorous birds in agricultural areasshowed that most birds tend to feed on insect pests from the order of Coleoptera, Lepidoptera and Orthoptera [6].This finding showed that birds are potential to be used as pest control in oil palm plantation whose major insect pest are from the order Lepidoptera.
Identifying functional diversity is another way to examine the effect of habitat changes, and to examine the role of bird community in agro-ecosystem.Because species are lumped into guild or their function in the ecosystem, the functional diversity will provide a more detailed information [7].There are feeding guild differences between forest habitat and oil palm plantation [8].
In 2013 the EFForTS Project, a collaboration between several Indonesian universities and University of Goettingen, Germany, has established a large-scale and long-term biodiversity enrichment experiment (EFForTS-BEE) on oil palm plantation in Jambi, Indonesia using six native tree species that were enriched in the EFForTS-BEE sites.Theplantedtree species includes tree-producingfruits (Parkiaspeciosa -Petai, Archidendronpauciflorum -Jengkol and Duriozibethinus -Durian), timber (Peronema canescens -Sungkai and Shorea leprosula -Meranti) and natural latex-producing trees (Dyera polyphylla -Jelutung) [4].The age of oil palms ranging from 15 to 21 years [4].Survey conducted a year after the establishment of enrichment plots showed that the number of bird species remained relatively the same (20-21 species) but there are changes in bird species composition, and that the tree enrichmenthad positive effect on bird and invertebrate communities [4].The question is whether there is an increase in the bird diversity after nine years of planting.Therefore, the objectives of this study areto count bird species richness and feeding guild of birds in the experimental enrichment plots of an oil palm plantation.

Method 2.1 Study site
The study was conducted in oil palm plantation of PT Humusindo, Jambi Province.In 2013 the EFForTs (Ecological and socio-economic functions of tropical lowland rainforest transformation systems) project established biodiversity enrichment experimental (BEE) plots to examine the effect of increasing habitat heterogeneity in oil palm plantation on biodiversity.
A total of 56 plots with various sizes (5 m x 5 m, 10 m x 10 m, 20 m x 20 m, and 40 m x 40 m) and plantedthem with 0, 1, 2, 3 and 6 species of native tree species [4].In this study, observation was conducted onplot sizes of 20 m x 20 m and 40 m x 40 m with a total of 25 plots (Figure 1; Table 1).Consideration in selecting the observation plots include the condition of planted native tree vegetation and easy access to the location.

Data collection
Data was collected from 27 July to 10 August 2022 in enrichment experiment sites.Point counts with a radius of 50m were used to survey the birds in the area, using the enrichment site as the center.The observation tookplace from 6.00 to 10.00, and, when necessary, from 15.30 to 17.30.Counts were only conducted during fine weather.All birds detected within the radius by visual or audio were recorded.Birds detected outside the radius of point count was also recorded but are not included in the calculation of bird diversity.

Data analysis
Bird diversity is expressed as species richness (S) and indices of diversity Shannon (H') and Margalef (DMg) [9], using the formula (1) where pi is the proportion of number of individuals of species-i to the total number of individuals.
Evenness index was calculated using the formula (2) and the richness index was calculated using Margalef's formula (3) where N is the total number of individuals.Birds are classified according to the major feeding guilds.

Species richness and composition
A total of 33 bird species of 20 families were observed during the study, but only 24 species of 13 families were observed within the point count radius (Table 2).The bird families are represented by 1-4 species, with Columbidae and Sylviidae families have the highest number of species.Members of Columbidae are either frugivorous or granivorous, while Sylviidae are insectivorous.There was one protected species detected outside the radius of point count, i.e., Changeable Hawk-eagle.However, allspecies are fall within the category of least concern (LC) based on the threat status (IUCN 2022).Another carnivorous species detected during this studywas Barn Owl (Tyto alba) that is known as pestcontrol inoil palm plantation.All bird species observed during this study are residents [10].
Compared to the result reported by Teuscher [4] who found 20 bird species in the BEE plots, there is an increase in the number of the species.We did not measure the vegetation; however, based on the visual observations there are scattered tree vegetation that improved the complexity of the habitat (Figure 2).

Abundance, dominance, and diversity indices
The total records from all plots are 345 birds, which consisted of 144 records in plots of 20m x 20m and 201 records in plots of 40m x 40m.The most abundant species was Yellow-vented Bulbul (Pycnonotus goiavier) with a total of 66 records followed by Ashy Tailorbird (Orthotomus ruficeps) with a total of 50 records.Ashy Tailorbird is a member of the family Sylviidae whose major diet is insect and is classifiedas primary andsecondary insectivores [6], therefore is highlypotential in controlling the insect pest.
Bird diversity was slightly higher in the larger enrichment plots (H'= 2.649; E= 0.845; DMg= 4.148) than in the smaller plots (H'= 2.291; E= 0.826; DMg=3.018).Bird species richness and diversity varied among plots of different number of enrichment plant species (Table 3).Shannon diversity indices (H') ranges from 1.252 in plot of 20m x 20m with 2 species of enrichment plants to 2.665 in plot of 40m x 40m with 2 species of enrichment plants.The highest values of diversity indices are found in plots with one species of enrichment plant; however, this might be due to the higher number of samples (Table 1).

Feeding guilds
Seven feeding guilds were identified, with the most dominant feeding guild is insectivores (48.5% of total species, 44.6% of total records), followed by frugivores (18.2% of total species, 28.5% of total records) (Figure 3).The higher number of insectivorous bird species in oil palm habitat compared to other feeding guilds had been reported by several previous studies [5,11,12].Tseucher [4] also found that insectivores dominated bird species in the experimental plots.A bird-exclosure experiment by [13] suggested that insectivorous birds in oil palm plantation served as pest control by consuming herbivorous insects harmful to oil palm.

. Conclusion
The study showedthat larger plots of enrichment planting harbour higher species richness and diversity.As in other studies in oil palm plantation feeding guilds are dominated by insectivores.The bird species richness has increased in Biodiversity Enrichment Experiment (BEE) plots by 57 % after 9 years of plantingcompared to the baselinedata and one year after planting, suggestingthat the enrichment plants had a positive impact on bird diversity in oil palm plantation.The high number of insectivore species and abundance provide hope for the biological control of pest in oil palm plantation, although a more detailed study is necessary.

Figure 2 .
Figure 2. Habitat condition in enrichment plots of oil palm plantation, showinga more complex habitat than ordinary oil palm plantation

Figure 3 .
Figure 3. Feeding guild composition4.ConclusionThe study showedthat larger plots of enrichment planting harbour higher species richness and diversity.As in other studies in oil palm plantation feeding guilds are dominated by insectivores.The bird species richness has increased in Biodiversity Enrichment Experiment (BEE) plots by 57 % after 9 years of plantingcompared to the baselinedata and one year after planting, suggestingthat the enrichment plants had a positive impact on bird diversity in oil palm plantation.The high number of insectivore species and abundance provide hope for the biological control of pest in oil palm plantation, although a more detailed study is necessary.

Table 1 .
Number of bird observation plots based on enrichment plot sizes and number of plant species

Table 2 .
List of bird species recorded during the study

Table 3 .
Variation of speciesrichness, number of records anddiversity indices in different plot sizes and number of enrichment plant species