Uncultivated Field Margins Support Greater Farmland Bird Diversity than Cultivated Margins in Oil Palm Smallholder Plantations

Agricultural expansion into tropical forests has led to extensive biodiversity loss, with oil palm plantations being a major driver. Within these landscapes, field margins are among the few semi-natural habitats that may support wildlife, yet their ecological role in smallholder systems remains poorly understood. Here, we examined how fine-scale habitat features and broader landscape heterogeneity influence bird assemblages in oil palm smallholdings. Using data from 45 line transects and Generalised Linear Mixed Models, we assessed bird species richness and abundance in relation to field margin length and type, number of crop types and crop abundance within field margins, height of vegetation cover, distance from peat swamp and mangrove forests, and survey time. Overall, we recorded 29 species, dominated by resident birds, with additional migrants and introduced species. Both bird richness and abundance were significantly shaped by field margin type, number of crop types, distance from peat swamp forest, and survey time. Specifically, both metrics were higher in uncultivated field margins than in cultivated margins, increased with the number of crop types, and were higher at sites farther from the peat swamp forest. Both responses were also higher during morning surveys compared to afternoon surveys. Field margin length, crop abundance, and height of vegetation cover showed no significant effects. These findings suggest that biodiversity-friendly management of smallholder oil palm landscapes should prioritise the retention of uncultivated field margins with natural vegetation, while recognising that crop-type heterogeneity within cultivated margins may also contribute to farmland bird richness and abundance. Our results indicate that these margins may function mainly as supplementary habitats for generalist and disturbance-tolerant bird species, reinforcing the value of local, fine-scale vegetation structure for supporting biodiversity within smallholdings.