Locomotory behaviour and performance has long been assumed to be the key factor in determining the outcome of competition between animals for resources. However, empirical data to support this hypothesis has proved challenging to collect. In a recent publication in The American Naturalist,Douglas Altshuler sought to experimentally test the hypothesis that flight performance influences both territoriality, and success when competing for niches, by comparing the flight behaviour of several hummingbird species over a range of altitudes.
Although simple measurements of wing disc loading in flying birds had previously been used to approximate the bird's flight performance, Altshuler instead used more complex aerodynamic theory to calculate more realistically the costs of flight. Altshuler also filmed the birds at different altitudes in the wild, and under low-pressure conditions in the lab, to identify their flight kinematics. Finally, Altshuler calculated the power requirements for flight, particularly in relation to maximal or `burst' power, as well as recording features of territorial behaviour during field observations.
Travelling to the Andes and Colorado, Altshuler studied hummingbird flight in three experiments. First, he compared territorial behaviour of hummingbird species in the mountains from 400-3860 m and found that wing disc loading had no effect on territoriality, but burst power did. Territorial hummingbird species had higher burst aerodynamic power outputs than not-territorial species. Second, he studied two species of Selasphorus hummingbirds competing for access to nectar at 1875 and 2900 m. At the lower altitude, S. rufus males appeared to have a competitive advantage over S. platycerus males, but the roles were reversed as altitude increased. S. rufus had relatively lower burst power output and Altshuler found important difference in hovering kinematics between the two species of hummingbirds. Altshuler's measurements showed that S. rufus males used higher stroke angles when hovering and this constrained their ability to modulate aerodynamic force production when compared with S. platycerus. Tempting the hummingbirds with high-quality feeders (sucrose solutions) revealed that not only did altitude affect the way in which the hummingbirds selected feeding sites, but that the density of competitors for feeders also effected the bird's choice. The final component of the study involved laboratory studies of flight performance under high and low pressure conditions. Both species increased power output to compensate for lower air density.
Altshuler's study has provided several intriguing insights into both the mechanical requirements for flight of territorial birds and the influence of environment on competitive interactions. The ability to produce bursts of power appears to be a pre-requisite for successful competitive outcomes at higher altitudes; however, at lower altitudes other flight parameters (such as manoeuvrability) can improve a hummingbird's advantage. Clearly, physiological and aerodynamic limitations become more important as oxygen availability and air density decline with increasing altitude. This study has shown how a bird's flight performance, and a bird's ability to modulate it, can affect its ability to compete with other species for resources.