ALTSHULER, Douglas L.: Burst flight performance and competitive ability of hummingbirds along elevational gradients.

Flight at high elevation presents two challenges: reduced air density demands high mechanical power output whereas reduced oxygen partial pressure can constrain metabolic input. Despite these challenges, the highest species diversity of hummingbirds occurs along the slopes of the Andes. I examined the power requirements of hovering hummingbirds across a broad elevation gradient (0 – 4300 m) in southeastern Peru. Power requirements of free flight were compared to maximum power production during load-lifting. Both power requirements and maximum power production among species were invariant with increasing elevation, indicating considerable morphological and kinematic compensation for increasing flight costs. Nonetheless, the power margin for burst performance during load-lifting compared to normal hovering decreased with increasing elevations. Accordingly, hummingbirds at high elevations have reduced capacity for burst flight activity.

The relationship between burst capacity and competitive ability was then studied using two hummingbird species in the Colorado Rockies. Selasphorus rufus is a short-winged hummingbird with a small power margin whereas long-winged S. platycercus has considerably greater power reserves. The hummingbirds differed in burst capacity and altered flight modes across elevations, resulting in a shift in competitive ability. At low elevations S. rufus were dominant whereas at high elevations, S. platycercus were dominant. Thus, burst capacity of hovering hummingbirds affects species interactions and dominance relationships across elevations.