The Shining Sunbeam (Aglaeactis cupripennis) is a widespread and relatively common high-elevation hummingbird in the tropical Andes. Despite this, there is no comprehensive record of its natural history. In this study we present our findings on the diet and territorial behavior of Shining Sunbeam at sites in Peru and Ecuador. Using radio telemetry to track and observe individuals, we examined territory size and vegetation characteristics, activity budgets, diet composition, and territorial aggression. We found that average territory size was 0.13 ha (SD 0.05) with 100% minimum convex polygon estimation and 0.19 ha (SD 0.06) with 95% kernel density estimation. We found high variation in territory sizes, which was not explained by locality, year of data collection, or flower density. The diet of the Shining Sunbeam within our study sites was composed primarily of nectar from flowers of the tree Oreocallis grandiflora (Proteaceae), which comprised 93% (SD 9) of all nectar feeding events in Peru and 99% (SD 1) in Ecuador. Other flowering plant resources included Brachyotum, Centropogon, Fuchsia, Gaultheria, and Macleania. Insects made up 7% (SD 4) and 3% (SD 1) of observed foraging events of A. cupripennis in Ecuador and Peru, respectively. Mean activity budgets across individual birds were 68% (SD 16) perching, 28% (SD 15) foraging, 3% (SD 3) in aggressive behavior, and 2% (SD 1) in nonaggressive flight. Of the observed aggressive interactions, the majority were directed toward other hummingbirds (77% in Ecuador and 84% in Peru). When only considering interactions with hummingbirds, most aggression was intraspecific in Ecuador (71%) but interspecific in Peru (95%). We observed aggressive behavior directed toward other common, non-hummingbird nectarivores, such as Diglossa flowerpiercers, while aggression directed toward non-nectarivores was rare. Our results highlight the need for comparative studies to better understand hummingbird foraging and territorial behavior in the Andes, and the utility of radio telemetry for studying larger hummingbird species like the Shining Sunbeam.