We analyzed seasonal variation in avian species richness and relative abundance at the community and guild level during a 13-month period in central Bolivia in an 11-ha patch of Polylepis (Rosaceae) forest, a high-Andean ecosystem that has suffered from extreme anthropogenic fragmentation. Birds were surveyed audio-visually (supplemented with mist net data) and assigned to five relative abundance categories for 2-month survey periods. We recorded 35 core species, including 16 insectivores (46%), 11 frugi-granivores (31%), seven nectarivores (20%), and one carnivore (3%). Core species richness varied from 25 (June–July) to 33 (October–November). Insectivores had a significantly higher proportion of year-round residents (81%) than frugi-granivores (45%) or nectarivores (14%); the same trend was evident with respect to seasonal variation in species richness of each guild. Although most species varied considerably in their relative abundance, no guild showed significant variation in relative abundance scores among survey periods. However, frugi-granivores and nectarivores combined, both of which depend upon plants as food resources, reached a significant minimum in mid-winter (June–July), and the same result was found at the community level. The insectivore guild thus was the most temporally stable both in terms of species richness and abundance. Qualitative observations indicated that the fluctuations in frugi-granivores and nectarivores were related to the availability of food resources.