DNA metabarcoding confirms Copsychus saularis, the common insectivorous species in oil palm plantations as biological control of ground-dwelling insect pests.

Pests pose a major challenge to the oil palm industry, causing significant yield losses and economic damage. While entomopathogenic pesticides have been used for pest control, repeated applications have led to insect immunity. Chemical pesticides, on the other hand, risk secondary poisoning of natural predators. As a sustainable alternative, biological pest control using insectivorous birds has gained attention. However, previous studies often lack direct evidence linking specific bird species to pest suppression. This study examines insect composition in the diet of two insectivorous birds namely, Copsychus saularis and Copsychus malabaricus across three landscape types: monoculture plantation (MCP), forest patch plantation (FPP), and next to forest plantation (NFP). Findings show that C. saularis' diet is influenced by landscape type, with a limited diet in MCP compared to NFP. Notably, C. saularis prefers pest insects over beneficial insects, indicating its strong potential as a biological control agent in the oil palm landscape. Conversely, C. malabaricus mainly preys on neutral insects, contributing little to pest suppression. This study also reveals the functional spillover of the sympatric species in utilizing the modified landscape, in which C. malabaricus exhibits minimal functional spillover in terms of pest control, whereas C. saularis demonstrates significant functional spillover by actively consuming insect pests across wider oil palm landscapes. This distinction is crucial for understanding the true ecological service provided by different bird species in human-modified environments.