Abstract

An exploratory study on the presence of diurnal predatory birds and their relationship with rat damage was carried out in a young oil palm plantation, situated on the Caribbean coast of Honduras. An area of approximately 2.5 Km² was visited bi-weekly over the period of a year to note the presence of predators. The rat population was estimated by observing the amount of fresh damage caused to the petioles of the lower leaves of young oil palms. Rat activity was quantified according to the level of damage caused to the palms and by measuring the growth of the affected palms alongside some control palms. The abundance of two types of hawk (Buteo pos. platypterus and Elanus caeruleus) was associated with the increase in the rat population, (increase in "fresh" damage to the palms) within the area. The incidence of other rat-eating birds (Polyborus planctus, Tigrisoma mexicanum) did not appear to be affected by the fluctuation in the rat population. Rat damage increased during the rainy season, particularly in those areas with poor superficial drainage, and where tall grass was prevalent. Repeated rat attacks on the same palms adversely affected growth and early yield.

Introduction

Rats cause the most severe rodent damage of oil palm in Central America. Plantations in some areas also occasionally suffer attacks from taltuzas (Orthogeomys spp). (Chinchilla 1993, Hilje 1992b). In Malaysia, the predominant rat species is Rattus tiomanicus (Liau et al .1991), which principally damage the bunches (Mohd Mat Min 1985). Rats can not survive on an exclusive diet of oil palm fruits, but they can easily complement the proteins they lack by consuming the abundant number of insects living inside the plantations (Liau 1984)

In Central America the species of rat which is prevalent concentrate on attacking young plants, where they can feed from the leaf bases closest to the ground. Frequently the damage is located at the base of the stem. As a consequence of this, the palm can lose varios leaves which are still functional and the exposed tissue may attract adults of Rhynchorus palmarum . The combined effect of the adult activity along with the insect larvae which develop in these tissues, can lead to the plant's death. There is no study that documents the composition of the rat species in oil palm plantations in Honduras. According to casual observations, the predominant species is Sigmodon hispidus , although other species such as Zigodontomy brevicaudata and Oryzomys spp. could also be present (Hilje 1992a).

Rats are hunted by various species of vertebrates, of which, birds of prey are the most prominent (Duckett 1982). The real role achieved by these predators in reducing the rat population within the level of economic damage, is not easy to determine and their importance is frequently brought into question (Wood 1986). However, predatory birds are very important elements to take into account when designing strategies to tackle rat plagues (Wood 1986, Duckett and Karuppuah 1989, Lim et al . 1991).

In Malaysia, the proliferation and use of birds of prey inside plantations have been recommended in varying degrees. The Barn Owl Tyto alba , has received particular attention owing to he fact that it has adapted itself very well to oil palm plantations, where it survives on a diet comprising of 98% rats (Duckett 1982, Duckett and Karuppiah 1989, Lim et al . 1991). One advantage of this bird is that it does not show much territorial behavior when there is abundant quarry. It is estimated that a pair of these animals together with their chicks consume around 1.300 rats a year (Duckett and Karuppia 1989).

Plantations in Malaysia have started an initiative to supply these owls with places to breed. Some model nesting structures are accepted by the birds (Duckett 1982). The bird's observation perch can also be reconstructed, particularly in young plantations. The use of such perches has the following advantages:

1. An increase in the prey's vulnerability.
2. They represent an energy saving for the bird in the search for prey.
3. They offer a place to rest for the birds of prey (Hall et al. 1981).

Programs which attempt to manage the rat population through the help of birds of prey have suffered from various problems. Second generation anticoagulants (in particular brodifacoum) can adversely affect a bird that consumes a poisoned animal. As a solution to this problem, the use of first generation anticoagulants which have been proven to be safe for birds of pray have been suggested (Duckett and Karuppuah 1989).

There is little documentation about the use of predatory bird species in combating rodent pests (Wood 1986), and in Central America there is even less such information. The present study had these objectives:

1. To recognize and to fallow the fluctuation of different predatory diurnal bird populations present in a young oil palm plantation.
2. To document the damage to the vegetative growth of plants caused by rats, and to associate this damage to the predatory population.
3. To evaluate, in a preliminary form, some strategies to stimulate predatory bird activity inside the plantation.

Materials and Methods

Observations were taken in a commercial oil palm plantation located in the Department of Atlántida in the Littoral Atlantic of Honduras. The observations were carried out on a young planting.

Estimation of the predatory bird population

Every two weeks over the period of a year (January - December 1992) an area of 2.5 Km² was covered. Each visit lasted approximately four days. Predatory bird species were noted and observed on perches, or similar ledge above ground (such as oil palm trunks from the previous planting). Such perches were built with bamboo stems mately one meter from the highest extreme, a transversal rod, smaller in diameter, and one meter long was placed. To facilitate observation, Jason "permafocus" 2000, 4x21 binoculars were used.

The density of birds was estimated through the "Line transect" method, Gates (1896):

D = (n-1) / 2LY

where:

D= estimated density (birds/Km²)

n= number of individuals observed during one visit

L= length of the observation line

Y= average perpendicular distance of the observation line.

The neighboring areas of the plantation were visited in search of the birds' nesting sites. The presence of other posible predatory species, such as snakes, were noted during the visit. It was not determined whether the same bird was observed more than once during the same visit or on different visits.

Quantification of damage caused by rats

In the same area of study of the population of birds of prey, a section of approximately 25 ha. was chosen to assess quarterly data of vegetative growth (Corley and Breure 1981) in healthy palms and with different levels of damage caused by rats. During the last two evaluations this included the number of bunches per plant. Measurements started after 2 months from field planting of commercial Deli x AVROS palms.

The following categories of damage were used:

• Healthy palm: without any apparent damage.
• Light damage: up to two leaf bases damaged.
• Medium damage: more than two leaf bases damaged, but without damage to the stem.
• Severe damage: the leaf bases and the stem damaged.

The number of plants evaluated in each category was 30. Owing to the increase in the attack on the period of observations, which meant that those palms were not include in the final analysis of the data.

The active population of rats was estimated in an indirect form by counting the number of palms showing recent damage (Wood 1982). Data about the number of plants attacked by rats in 1990, 1991 and 1992 was obtained from the Phytosanitary Department registers.

Results and Discussions

Description of the more common birds of prey

Buteo pos platypterus (Broad-Winged Hawk). Grayish head and crest, body is reddish-brown, yellow abdomen, with a wingspan of around 43 cm; bodyweight is up to 450 g. Its habitat is south of the United States, towards Peru and northern Brazil.

It prefers undisturbed areas and nest in high trees. However, it has also been found nesting inside oil palm plantations in the taller palms. It prefers to hunt in open or semi-open areas. Averagely territorial, it hunts alone or in small groups of up to seven individuals.

The Broad-Winged Hawk hunts small mammals, particularly rodents, insects and small birds. It commonly uses observation perches located within young oil palm plantations. The hunting groups change their area fairly frequently, probably owing to the shortage of food.

Elanus caeruleus (Black-Shouldered Kite). Reaches a wingspan of 41 cm and 350 g in bodyweight. It looks distinctive in flight and his ability to hover is very characteristic.

Its plumage is white and pale grey with black shoulders. Its habitat is south of the United States, Panama and parts of South America. It prefers open areas such as pastures and farm land for hunting. It feeds on small rodents, insects, snakes etc. It is probably more territorial than B. platypterus .

Tigrisoma mexicanum (Bare-Throated Tiger-Heron) Reaches a height of 32-80 cm. The predominant colors of its feathers are grey, greenish-yellow and black.

Its habitat is from Mexico down to Colombia. It stands motionless for periods at the water's edge. It feeds on fish, crustaceans and small rodents.

Relationship between some predatory birds and rat damage

The Crested Caracara ( Polyborus plancus ) and the Bare-Throated Tiger-Heron ( Tigrisoma mexicanum ) are generalist predatory birds and relatively insensitive to fluctuations in the rat population ( Fig. 1 ). However, a few Crested Caracaras have been found dead inside the plantation. They could have been poisoned after eating moribund rats which had eaten baits with brodifacoum. The consumption of animals poisoned with anticoagulants was related to the death of the Common Barn Owl in Malaysia Tyto alba (Duckett 1989). The Crested Caracara feeds on carrion, which meant that it was particularly prone to finding animals who had died from anticoagulants poisoning.

Some hawks are migratory birds, who take flight between March and September. Their effect on the rat population is possibly of little importance, because they are absent during most part of the rainy season when the population of rats increases ( Fig. 2 ).

The Broad-Winged Hawk ( Buteo platypterus ) and the Black-Shouldered Kite, ( Elanus caeruleus ) apparently do respond to the fluctuation in the rat population assessed as "fresh" damage on oil palm leaf bases. ( Fig.1 and Fig. 2 ). The level of damage caused by the rats increased in May 1992, which coincided with the suspension of the use of brodifacoum baits. The original intention was to give an opportunity to the biological control in keeping down the rat population. However, an increase in the percentage of palms recently damaged by rats ( Fig. 2 ) motivated the administration to resume the baitting programme.

The presence of a large number of B.platypterus and E. caeruleus in the area. ( Fig. 1 ), was related to the increase in the population of rats during this period (May 1992). The activity of these two bird species does not seem to respond to large migratory movements, moreover to a shift in search of food within the area.

Other predatory birds observed in the plantation (Fig. 1) were not associated to rat damage at any point during the year ( Fig. 2 ).

The use of brodifacoum was suspended for the first few weeks of September 1992 and was substituted by Warfarin and "Racumín". The objective was to avoid causing damage to the predatory bird population hunting the rats. The birds which appeared to be most affected by the use of brodifacoum were the Broad Winged Hawk and the Crested Caracara. The change in the type of anticoagulants used was associated with a slight increase in the number of B. platypterus but not in the population of E. caeruleus. The level of damage caused by the rats showed a diminishing tendency.

Seasonal variation in the percentage of palms damaged by rats

The highest percentage of palms which presented recent damage attributed to rats, was observed during the rainiest months of the year ( Fig. 2 ). The increase of damage observed in May 1992 was associated to the suspension of the use of brodifacoum baits. The increase observed in July 1990 was linked to the suspension of pest control activities owing to a labor problem. Differences in magnitude of the damage were probably due to the period of the year in which pest control was interrupted: in April, rainfall was low (Fig. 2), and the rat population was also lower, in June, rainfall was heavier and the proportion of palms damaged by rats increased.

The increase in the level of damage in the rainiest months is associated with more favorable conditions for rat activity and reproduction. High grass growth, particularly Paspalum fasciculatum, in areas of poor superficial drainage, especially favored the proliferation of these rodents.

As oil palm grows, the damage caused by rats diminishes to such a point that it practically disappears towards the end of the unproductive period. Rat species in Central America concentrate their attacks on the base of the petioles and the stem at the soil surface. They do not usually climb up the plant. In addition, the start of harvesting brings more frequent visits to the area. Also, "sanitary pruning" of the palms removing the oldest leaves (plus the ripe or rotten bunches), create less favorable conditions for rats.

Quantification of Rat Damage

Rat damage tends to be repeated to the same plants, particularly in those which grow in favorable conditions for an attack (superficial drainage poor for example). For this reason, some palms were already showing some damage by the time of the first field visit. All vegetative parameters were adversely affected in plants damaged by rats ( Fig.3 , Fig.4 , Fig.5 ) A high level and damage was related to a reduction in vegetative growth. The production of bunches was also less in those palms that had been attacked ( Fig. 5 )

CONCLUSIONS

The initial enthusiasm for the possibility of achieving good biological control of vertebrates has diminished as more aspects about the relationship between the predatory animals and their quarry are known. For example, predatory birds appear simply to act as a regulation of prey wich are above the "carrying capacity" of the environment. This means that their role is reduced to the elimination of an over-population of animals, particularly the weakest and most infirm. The final effect of this is more likely to favour the population of hunted animals by the selection of stronger individuals (Wood 1986).

In Malaysia, Tyto alba has been noted as preferring rats with a heavier bodyweight, which are usually males. This could be due to a greater mobility of males. To this effect, the hunting behavior of owls could change the sex of the rat population towards and abundance of females, thus increasing the reproduction rate. The initial rat population would therefore be maintained or would tend to rise (Lim et al . 1991).

The population of vertebrate predators appears to be affected by the amount of prey, but not the other way around. However, if the environment is manipulated in such a way that it can only support a smaller population of rodents, then the predators would effectively be able to help to maintain the population at the limit of the new lower carrying capacity. Equally, when a method of pest control such as anticoagulants is used, a higher population of predators can help to lengthen the period in which the population of prey recuperates to its initial level (Wood 1986).

The introduction of vertebrate predators within a new environment, has created additional problems to those that already existed. Some predatory birds have preferred to hunt other native species because they are easier to find and kill than rats. This situation has been further complicated by predators' attacks on domestic animals (Wood 1986).

Rat control must follow an integrated approach. The management of the palm's environment is of outmost importance, and should be done in such a way that it be as inhospitable as possible to the rat population. Emphasis has been given to the destruction of places which serve as refuges or reproduction sites for rats. Such locations include piles of stones, branches or mounds of rubbish etc. In particular, holes and other depressions should be filled in, as these could serve as a rat's breeding ground. Experience also shows that weed control in the interlines and the "circles" surrounding each plant is important.

Rats proliferate in areas with tall grass, such as "gamalote", Paspalum fasciculatum . This type of grass, along with other gramineae, prevail in areas with poor superficial drainage, and provide a very adequate breeding-ground for rats (PCARRD 1985). Given that rats avoid moving between areas with are stripped of vegetation, it has been recommended to maintain a border free of weeds in areas adjoining other infected by rats.

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