Biomechanical and Behavioral Determinants of Flapping Frequency in Cliff Swallows

One of the central predictions of applying aerodynamic theory to the flapping flight of birds and other animals is the quadratic relationship between wingbeat frequency and airspeed. This relationship has been observed in theoretical models of flapping flight and experimental studies of birds and bats flying in wind tunnels. The U-shaped curve arises from the varying power requirements for producing lift and overcoming drag at different air speeds. Because changes in wingbeat frequency have the most scope for varying muscle power output, the U-shaped cost of flight relationship is expected to lead to a U-shaped wingbeat frequency curve in all flying birds. However, the degree to which these U-shaped relationships are characteristic of natural flight behavior and the extent to which birds might modify this basic aerodynamic response to accommodate other behaviors are less well-known. We analyzed videos of wild Cliff Swallows (Petrochelidon pyrrhonota) engaged in tandem (i.e., following or being followed by a conspecific) and non-tandem flight behavior to collect birds' wingbeat phases and 3-D position, velocity, and acceleration through time. These data were used to determine the relationship between wingbeat frequency, air speed, and other kinematic properties such as elevation gain and centripetal acceleration. After accounting for the Cliff Swallow's intermittent flapping flight style, we found that non-tandem birds had a wingbeat frequency-speed relationship consistent with theoretical models and wind tunnel experiments, though they typically did not fly at speeds faster than the minimum-frequency speed. Cliff Swallows engaged in tandem flight behavior flew with higher wingbeat frequencies that did not vary systematically with speed, contrary to aerodynamic expectations. They also reached faster flight speeds than were seen in non-tandem behavior. These results support the applicability of the predicted wingbeat frequency-speed model to natural flight, but also show that birds can substantially modify this underlying pattern to support different flight behaviors.Bachelor of Scienc