The tropical Andes rank first among the world's 25 “hotspots” of biodiversity and endemism yet are threatened and little studied. We contrast population trends in avian diversity in montane cloud forest (bosque altoandino) and similar forest degraded by the planting of introduced tree species (bosque introducido) in the Mazán Reserve, Ecuador. We describe changes in bird diversity and abundance in these habitats over 12 years and evaluate the nature of change within these avian communities. On the basis of 2976 count detections and 419 net captures of 76 species of landbirds, indices of similarity between the habitats were low, with only 47.6% of species occurring in both forest types. From 1994–95 to 2006–07, species richness decreased from 54 to 31 in bosque introducido and from 67 to 30 in bosque altoandino. Capture rates also declined from 56.0 to 28.5 birds per 100 mist-net hr in bosque introducido and from 38.0 to 22.4 birds per 100 mist-net hr in bosque altoandino. We explore various potentially interacting factors that might have caused the observed changes in bird communities, including changes in vegetation within the Mazán Reserve and environmental changes resulting from global warming. But our results also suggest that local and regional changes in habitat outside of the Mazán Reserve were likely responsible for some community changes within the reserve. We argue for increased population monitoring to verify trends and to strengthen the effectiveness of conservation efforts in the Andes.