The nocturnal barn owl, Tyto alba, has been studied and used as a biological agent for the control of rats in oil palm plantations, but diurnal birds have never been investigated for their impact on oil palm pests. Only birds which damage the fruits have been recorded. However, many species are present from soon after planting up to mature oil palms.At the Indonesian Oil Palm Research Institute, Sumatra, we have carried out an inventory of these birds in two plantations and on blocks of different age. Their population and composition vary according to the time since planting and availability of food supplies. Whilst some are granivores feeding on seeds of grasses, or fruit-eaters feeding on weed berries and oil palm fruits, most of them are insectivores with their population fluctuating with caterpillar outbreaks. Their biology has been studied, especially the feeding and nesting behaviour. Their gut contents have been analysed and the amount of insects found in some species is astonishing and may have an important impact on oil palm pests, especially caterpillars. Among the 29 species observed, their dominance varies according to plantations: Pycnonotus goiavierbetween 24.4% to 28.7% of all individual birds, Prinia spp. between 27.59% to 34.4%, Parus major 9.54% to 10.56%, Copsychus saularis 4.47% to 8.62%, and Halcyon smyrnensis 5.07% to 6.90%. Parus is found on higher palms, Copsychus and Prinia more common on younger planting, Pycnonotus on both older and younger palms, as well as others such as Centropus bengalensisonly at the level of the lower strata among the cover crops.
These dominant insectivorous species feed on caterpillars, small beetles, grasshoppers and ants. For example, a pair of Parus major providing food for their young (2 or 3) collects up to 139 insects, mostly nettle caterpillars, per day from 6.55 am to 18.15 pm, with the male and female bird catching 60 to 79 of these limacodid larvae respectively.The Centropus feeding at the level of the soil breaks the nettle caterpillar cocoons and up to 12 pre-pupae and pupae of Setothosea asigna can be found in the gut of a single bird. Pycnonotusand Cosyphus are less specific, catching a wider range of insects including small beetles, caterpillars and ants. Halcyon catches bigger insects in flight such as grasshoppers and beetles. The Orthotomus, one of the species which is increasing in population, has modified its feeding behaviour by preying only on Elaeidobius kamerunicus. This may result in a reduction of the population of this pollinator, with the consequence of decreasing the oil palm fruit-setting during the first years of production.