This paper describes changes in bird communities following the conversion of lowland forest to
commercial oil palm and rubber plantations. Conversion of forest to plantations resulted in a
reduction in species richness of at least 60%, with insectivores and frugivores suffering greater
losses than more omnivorous species. Of the 128 species recorded across all habitats, 84% were
recorded in forest, and 60% were recorded only in that habitat. Of the 16 Globally Threatened
or Near-Threatened species recorded in the study, 15 were recorded only in forest. Species
occurring in plantations were significantly more widespread in Thailand than species recorded
only in forests and had a tendency towards smaller body size. Species richness in plantations was
unaffected by plantation age or distance from nearest forest edge, but was significantly greater
where undergrowth was allowed to regenerate beneath the crop trees. Bird communities in oil
palm and rubber plantations were extremely similar, and there was a strong positive correlation
across species in their relative abundance in each plantation type. The results indicate that a high
proportion of species formerly present in the region are unable to adapt to conversion of forest
to oil palm and rubber plantations, resulting in large losses of bird species and family richness
and the replacement of species with restricted ranges and high conservation status by those
with extensive ranges and low conservation status. Initiatives that reduce pressure to clear
new land for plantations, for example by increasing productivity in existing plantations and
improving protected area networks, are likely to be more effective in conserving threatened
forest birds than initiatives to improve conditions within plantations, though both should be
encouraged.