Flight and size constraints: hovering performance of large hummingbirds under maximal loading.

As the smallest birds, hummingbirds are the only birds capable of prolonged hovering. This suggests that hovering locomotion scales unfavourably with size. Is the hovering performance of larger hummingbird species more constrained by size than that of smaller ones? Maximal load-lifting capacities of the two largest species of hummingbirds found in the United States, the blue-throated (Lampornis clemenciae, 8.4 g) and magnificent (Eugenes fulgens, 7.4 g) hummingbird, as well as the two other local small species, the black-chinned (Archilochus alexandri, 3.0 g) and rufus (Selasphorus rufus, 3.3 g) hummingbird, were determined under conditions of short-burst performance. The power reserves of hummingbirds are substantial relative to normal hovering performance. The two large species lifted maximal loads close to twice their body mass for a very brief duration of over 0.4 s. The small species lifted maximal loads approximately equal to their own mass with a longer duration of over 0.6 s. For the two large species under maximal loading, estimates of burst muscle mass-specific mechanical power output assuming perfect elastic energy storage averaged 309 W kg-1, compared with 75 W kg-1 during free hovering without loading. For the two small species, these values were 228 W kg-1 and 88 W kg-1, respectively. The differences in aerodynamic force production and power output between the large and small size classes occur despite their similar wing stroke velocity. This indicates that, during burst performance in these hummingbirds, the larger ones had a higher load-lifting capacity and generated more muscle power. In spite of the twofold difference in body mass, both large and small hummingbirds have evolved to become potent aerial competitors in order to exploit their common food resource, nectar. Both size classes have evolved to cope with the multi-dimensional effects of size constraining their aerodynamics, muscle mechanics, metabolism and ecology.