FLIGHT AND SIZE CONSTRAINTS - HOVERING PERFORMANCE OF LARGE HUMMINGBIRDS UNDER MAXIMAL LOADING

As the smallest birds, hummingbirds are the only birds capable of prol onged hovering, This suggests that hovering locomotion scales unfavour ably with size, Is the hovering performance of larger hummingbird spec ies more constrained by size than that of smaller ones? Maximal load-l ifting capacities of the two largest species of hummingbirds found in the United States, the blue-throated (Lampornis clemenciae, 8.4g) and magnificent (Eugenes fulgens, 7.4g) hummingbird, as well as the two ot her local small species, the black-chimed (Archilochus alexandri, 3.0g ) and rufous (Selasphorus rufus, 3.3g) hummingbird, were determined un der conditions of short-burst performance, The power reserves of hummi ngbirds are substantial relative to normal hovering performance, The t wo large species lifted maximal loads close to twice their body mass f or a very brief duration of over 0.4 s. The small species lifted maxim al loads approximately equal to their own mass with a longer duration of over 0.6 s. For the two large species under maximal loading, estima tes of burst muscle mass-specific mechanical power output assuming per fect 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 di fferences 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 hummi ngbirds, the larger ones had a higher load-lifting capacity and genera ted more muscle power, In spite of the twofold difference in body mass , both large and small hummingbirds have evolved to become potent aeri al competitors in order to exploit their common food resource, nectar, Both size classes have evolved to cope with the multi-dimensional eff ects of size constraining their aerodynamics, muscle mechanics, metabo lism and ecology.