The Mariana islands have a species-poor but functionally diverse and largely endemic bird assemblage that varies due to biogeographic legacy and anthropogenic impacts. The largest island in the chain, Guam, is the setting for one of the most extreme examples of recent avian population declines, indicating the capacity for avifaunal collapse and loss of function in neighboring islands. We performed a systematic survey of resident land birds in remnant karst forest on the Mariana Islands' 3 largest islands following Guam to assess the status of the avifauna in this habitat, characterize inter-island heterogeneity in bird communities, and consider the resulting differences in the functional roles of birds across the archipelago's native forests. We identified significantly greater functional diversity on Rota than either Saipan or Tinian, but lower bird population densities, species richness, and Shannon diversity. We recommend continued monitoring of avian population trends across the archipelago and assessments of ecosystem functions like pollination, seed dispersal, and food web dynamics.