The Evaluation of a Decade Barn Owl (Tyto alba) Introduction in Oil Palm Plantations Central Kalimantan

The barn owls (Tyto alba), as a biological control for the rat population, are relatively effective and safe for the oil palm ecosystem. So nowadays, many oil palm plantations are trying to introduce it to reduce the use of rodenticides. This paper’s objective is to evaluate a decade of T. alba introduction in ca 80,000 ha oil palm plantations in Central Kalimantan since the initial releasing three pairs of T. alba in July 2011. We use a quantitative descriptive method to provide the primary data on a monthly monitoring basis, i.e., the T. alba population, including its chick and eggs; the number of barn owl boxes occupied; rat damage percentage and the number of rat baiting applications. The population increment during the year 2011 to 2012 was retarded. However, since the year 2013, the population of barn owl population rapidly increased using a combination of self-released barn owl distribution from field barn owl boxes. It assisted augmentation from barn owl hatchery methods. Based on the monthly census in December 2022, the occupancy rates reached 75% from 4,234 boxes; occupied by 6,390 owls, 1,665 chicks, and 1,531 eggs. The established population of T. alba in an oil palm plantation in Central Kalimantan has reduced the rat damage percentage below the threshold level of 5%. It has no rat-baiting application (zero rat bait).


Introduction
Rats (Rattus sp.) are persistent major pests in oil palm plantations.Rat damage can decline fresh fruit bunches (FFB) production and oil yield in the mature stage and destruction both seeds in the Nursery stage and palms newly planted in the immature stage.These rat attacks damage the oil palm fruit, female/male flowers, apical meristematic tissue, and stem base of young oil palm plants.Rat feed in oil palm plantations consists of oil palm fruit (80%), insects (15%), and other feed (5%) [1].The insect is one of the rat's needs for dietary protein.Rats that consume male flowers can have a significant impact on the population of Elaeidobius kamerunicus, a species crucial for pollination.This it can affect the increase of parthenocarpy fruit bunches (seedless fruit development without fertilization) and lead to a decrease in Fresh Fruit Bunch (FFB) production [2].
Various methods are applied in oil palm plantations for rat control, i.e., trapping techniques, protective guards, hunting, rat poisoning, and using natural enemies such as barn owls (Tyto alba).Currently, anticoagulant rodenticides are the most implemented in controlling the rat population in oil palm plantations [3] 1308 (2024) 012019 IOP Publishing doi:10.1088/1755-1315/1308/1/012019 2 [4,5].However, in line with the sustainable palm oil industry commitment through the implementation of integrated pest management (IPM), anti-coagulant rodenticides have been used in combination with biological control using barn owls [6][7][8].Using barn owls (Tyto alba) as a biological control for rat pests is reported to be ecologically safe, effective, and efficient.It can effectively suppress rats, has no negative impact on the environment, and can reduce costs by up to 45% compared with rat poison application [9,10].
T. alba is one of the raptors that exists as a top predator in the food chain [5].T. alba is a potential predator for rat control as it can prey on 2-5 rats per day [11] and kill more than nine rats per night.T. alba territory can reach a 12 km radius and hear the rat sound from a 500m distance [12].Similarly, with T. alba, rats are nocturnal mammals [13].T. alba's food source has been identified from its regurgitated pellets [14].The highest proportion of food is rats at 58.13% [14], while in a different area, reported at 95.7% -96.2% [15] and 95.8% [16], respectively.This data shows that the significant T. alba prey depends on the availability of its prey, but its dominant prey remains the group of rats.
Currently, the development of T. alba to control rats and minimize rodenticide use in oil plantations are massive.However, not all companies have a good chance of success, as indicated by both the slow increment.T. alba population and the rat attacks level still exceed the threshold level.This study's objective is to evaluate the introduction of T. alba in an oil palm plantation in Central Kalimantan, which has been ongoing for over a decade since 2011.

Materials and methods
The research was conducted from 2011 to 2022 in an 80,000-hectare oil palm plantation in Central Kalimantan.We use a quantitative descriptive method to provide the primary data on a monthly monitoring basis, i.e., the T. alba population, including its chick and eggs; the number of Barn Owl Boxes (BOB) occupied; rat damage percentage and the number of rat baiting applications from January 2011 to December 2022.

Field observation of Tyto alba population
The observation method used is Look See [15], which involves observing the presence of T. alba, such as owls, chicks, eggs, and fresh leftover food.The observation was carried out at night after 6 pm using a flashlight in the oil palm blocks and the housing office complex to determine the presence or activity of T. alba.The initial observation in January 2011 showed that no individual of T. alba was found in the study location.Before the Look See result in 2011, two biodiversity population surveys also have conducted by the company consultants, which stated that no T. alba species were found in the study location [17,18].
It is necessary to observe the existence of the T. alba population in the field to determine the breeding strategy, whether it requires augmenting it from another location or utilizing the existing in-situ population.Generally, T. alba nest in large trees, but in certain areas, they also nest under houses and empty buildings' roofs, especially those far from crowds.The observations exercise in oil palm plantations must be conducted to identify potential nesting sites and active T. alba nests in the field.Its result can provide an overview of the possible presence of T. alba as a basis for implementing the breeding strategy.

Critical success of BOB installation
The installation of BOBs is generally overlooked in T. alba breeding, even though this stage is a crucial part as it is directly related to the level of safety and sustainability of T. alba breeding.Improper placement the BOB can affect T. alba, who is not interested in occupying the BOB.The BOB door should not face the west or east side.It is better to face the north or south side to reduce the intensity of sunlight entering the nest boxes.This is related to the natural behavior of T. alba as a nocturnal animal; it does not prefer sunlight.
To prevent the owls from animal stress, the BOB should also be installed in the center of the oil palm block and far away from crowds or disturbances.
Barn Owl Boxes (BOB) is an artificial nest installed in the field as a breeding and nesting space for T. alba.The BOB installation method is a critical success in the T. alba breeding program.It is directly related to the congeniality and safety aspects for owls, chicks, and eggs from the predators such as python snakes and monitor lizards.The design and material of the BOB are also related to its durability and cost efficiency.Each BOB is given an identification number to facilitate the monthly monitoring exercise.The BOB can be installed on the palm inter-row during the oil palm immature phase.However, it should be installed in a vacant point far away from the palm canopy during the mature stage to avoid predators entering the BOB.Generally, the predators joining the BOB are python snakes (Python reticulatus) and monitor lizards (Varanus salvator).The field observations have shown that python snakes eat the owls and chicks when monitor lizards eat chicks and eggs.The pole should be at least 4.5 m in height, and the wooden pole should be equipped with a protection guard to avoid predators climbing the pole.The hatchery or aviary is the most common method used by oil palm plantations to increase the population of T. alba.However, the evaluation of a pilot project from 2011 to 2012 showed that this solitary hatchery method is less effective as the hatching quantities may be meager.Therefore, establishing the T. alba population in the field may take a long time.The hatchery method is suspected to be unsuitable for T. alba breeding due to environmental factors.Based on this experience, the hatchery is only used to raise the chicks for pairing and augmentation purposes; meanwhile, the primary breeding method is self-released barn owl distribution from the BOBs in the field.Therefore, the primary function of the hatchery is to speed up T. alba population increment in the area.The scheme for assisted T. alba distribution from the hatchery to the field is presented in Figure 3.The combination of augmentation and self-released methods is considered very effective and productive.A significant population increment of T. alba owls, chicks, and eggs from 2014 to 2022 is shown in Figure 6.The augmentation method by collecting some chicks from the field BOBs, raising them in the hatchery, and then releasing them in the area where the T. alba population is still low can allow higher suitability of T. alba pairing in the fields.The hatchery periods take 1 to 1.5 months from chicks to mature owls, to release the pairs in proposed field BOBs.These combined methods, augmentation and self-release also increased the laying period's productivity up to 3 times per year in many occupied nest boxes.
For these reasons, it was decided that the hatchery would only be used temporarily to care for chicks taken from the field BOBs.The size of the hatchery is customized based on the needs of the development program at 12 m length x 6 m width x 6 m height to accommodate 5 to 6 pairs of T. alba.It has built one cage for chicks (below one and a half months old) and two nest boxes for owls inside the hatchery, as shown in figure 4. Once they are 2.5 to 3 months old, T. alba is considered mature and can be distributed to the proposed field BOBs.These rats are obtained by hunting (gropyokan) method in blocks with a high incidence of rat attacks.The gropyokan activity is done by dismantling or lifting the stacking fronds by one worker and another four workers guarding each side.When the rats come out from the stacked fronds, the guard's task worker should catch the rats using leather gloves.The captured rats are then placed in a container and transferred to the rat cage in the hatchery (Figure 5).The distribution of T. alba from the hatchery to the field BOB is carried out according to the planned augmentation program.The locations for releasing a pair of T. alba are selected by the evenness level of the T. alba population in the plantation, indicated by monthly occupancy rate census data.The success rate of T. alba augmentation would be higher if the T. alba distributed is a pair of males and females that showed compatible each other during observation exercise in the hatchery stage.A pair of T. alba will be brought from the hatchery to the field, then put into the BOB, and the doors are closed.After that, four rats are fed every afternoon from 5 pm to 6 pm for 4-7 days for T. alba adaptability purposes.The doors are opened after 4-7 days, which is done in the late afternoon.

Monthly monitoring population of Tyto alba
Monitoring of the T. alba population is conducted monthly to determine the occupancy rate, nest boxes condition, and the evenness level of people.The monitoring census is done by observing the BOBs using a mobile phone camera and a selfie stick.The mobile phone camera can document the owls, chicks, eggs, droppings, feathers, and mice brought to the BOBs.The census results are recorded in the census form, i.e., the BOB number, blocks number, the population number (Owl, chicks, eggs), BOB condition, and other necessary notes (Figure 6).The census results are documented for evaluation and to determine the next T. alba development strategy.The presence of owls can be identified by droppings, feathers, and dead rats around the BOBs.In addition to the T. alba population, the condition of the BOBs is also included in the census report for BOB improvement purposes.The common damages are leaky roofs, damaged walls, floor holes, and collapsed BOB wooden poles.If any fronds are in contact with the BOBs also noted to be immediately pruned to prevent predators from entering the BOBs.

Results and discussion
Evaluation of the development of T. alba is necessary to measure the success of implemented method and as a basis for further management considerations.The evaluation results indicate that the T. alba breeding method at the research site has been successful, as shown in the graph of the increasing population of T. alba and the decreasing number of rat poison applications over the past few years (Figures 7 & 9 The monthly census results as of December 2022 show that the occupancy rate of T. alba has reached 75% from 4,234 installed BOBs in the field, occupied by 6,390 owls, 1,665 juveniles, and 1,531 eggs.This means that one-third of BOBs in the field have been occupied and will continue to escalate based on the potential additions' of 1665 chicks and 1,531 eggs in the occupied BOBs.The increment of the T. alba population also correlates with the increment of installed nesting boxes in the field.The installation ratio of nesting boxes in 2011 was 1 BOB per 40 ha (1:40 ha), and up to the year 2019, the ratio had improved to 1:20 ha.A significant T. alba population increment can improve rat population control in oil palm plantations.Aligns with the data on rodenticide use in the research site, which decreased after the T. alba population was established (Figure 8).The rat population in the oil palm plantation remained relatively below the threshold level before or after the augmentation activities.However, the introduction of owls as biological control agents resulted in a consistently stable alteration in the pattern of rat infestation.The decrease in rodenticide applications proves that increasing the population of T. alba in the field can suppress the level of rat attacks below the threshold level (<5%).The small quantities of first-generation rodenticides (active ingredients Coumatetralyl) were applied in the sporadic area in the year 2016 due to an outbreak of rat attacks in several estates where the T. alba population had not been established yet, thus requiring an assisted first-generation rodenticide application which safe for T. alba.The previous discussion mentioned that the accuracy of BOB placement and the appropriate distribution method predominantly influence the success rate of T. alba breeding.However, T. alba development also can be retarded if we apply second-generation rodenticides (e.g., Brodifacoum, Bromadiolone, Flocoumafen, etc.) in the fields.This can disrupt the population because T. alba is likely to eat rats already consuming second-generation rodenticides, which can cause death in T. alba (secondary poisonous).Only first-generation anti-coagulant rodenticides (Warfarin and Coumatetralyl) with low risk of secondary poisonous can still be applied in limited quantities in the area where the T. alba population is established.Second-generation rodenticide use should be strict to adhere.
The high toxicity of second-generation anti-coagulant rodenticides to barn owls has long been reported [19,20].First-generation anti-coagulants such as warfarin cause sub-lethal effects.However, they are rarely identified as the direct cause of death in owls [13].At the same time, the use of second-generation anticoagulants has been widely reported to cause secondary death in T. alba.The use of rodenticides was secondarily reported as poison for T. alba eggs in their albumen and yolk [21].Owls will experience behavioral abnormalities such as rough breathing, frequent eye-closing, reduced flying activity, and weight loss from day five after consuming poisoned rats [3].Rat control using rodenticide in the long-term harms the T. alba population and the environment [22,23].

Conclusion
Properly installing nesting boxes and combining self-released distribution and augmentation methods can improve the T. alba population development to an established level in the oil palm plantation.Monthly census results in December 2022 showed T. alba occupancy reached 75% of 4,234 nesting boxes occupied by 6,390 owls, 1,665 chicks, and 1,531 eggs.The established T. alba population in the oil palm plantation in Central Kalimantan was able to suppress rat attacks below the threshold level (<5%) and reduce rodenticide use (zero rat bait).

Figure 1 .
Figure 1.a) BOB ready to be installed in the field; b) Calciboard BOB & galvanic pole; c) Wooden BOB & galvanic pole; d) Thermolite BOB & PVC pole (on trial); and e) Wooden BOB & wooden pole.

Figure 2 .
Figure 2. a) Improper BOB installation; b) Python snake in BOB; c) Monitor lizard in BOB

Figure 3 .
Figure 3.The method of assisted T. alba distribution from the hatchery to the fields in the first five years (2011-2015)

Figure 4 .
Figure 4. (a) Barn owl hatchery; (b) nest boxes inside the hatchery; (c) a pair of owls and chicks.While in the hatchery, T. alba chicks are fed 1-2 rats each per day.

Figure 5 .
Figure 5. (a) Rats hunting using the gropyokan system; (b) barn owl distribution to the field BOB.

Figure 6 .
Figure 6.The monthly census activity and documented T. alba population into the nest boxes.