Long-term dynamism in forest vegetation composition has significant implications for biodiversity and conservation. In the mid-hills of Kumaon in the Western Himalaya (1500 m -2400 m asl), the expansion of pine into oak-dominated hardwood forest has been of concern, due to its possible impact on biodiversity and livelihoods. Yet this vegetational shift is poorly studied in terms of spatial extent, the underlying drivers or the ecological consequences. We used field-based and remote-sensing methods to quantify oak-to-pine transition over three decades and studied the effect of this shift on forest bird communities in a 1285 sq.km multiple-use forest landscape in Kumaon. Our analysis indicates a gradual replacement of hardwoods with pine over the period of study: specifically, 22% decline in dense (protected) oak, a 29% decline in degraded oak and 74% increase in pine- over their original extents respectively- from 1991 to 2017. We found significant roles for micro-scale spatial variation in habitat, topography and climate in driving the oak-to-pine transition. Spatial analysis of forest bird communities further suggests that the transition of hardwood forest to pine may result in significant defaunation over time. For instance, pine forests support 33-46% lower forest bird species richness than hardwood oaks. Hardwood forest specialists could be especially vulnerable to forest change, showing 93-97% lower abundance in pine forest sites in comparison to dense oak sites. Additional effects of warming and degradation may further intensify species losses at the regional scale, and require further study.