Behavior and biomechanics: flapping frequency during tandem and solo flights of cliff swallows

Aerodynamic models of bird flight, assuming power minimization, predict a quadratic relationship (i.e. U-shaped curve) between flapping frequency and airspeed. This relationship is supported by experimental bird flight data from wind tunnels, but the degree to which it characterizes natural flight, and the extent to which birds might modify it to accommodate other behaviors, is less known. We hypothesized that the U-shaped relationship would vary or vanish when minimizing power is not a primary consideration. We analyzed videos of wild cliff swallows (Petrochelidon pyrrhonota) engaged in solo and tandem (i.e. following or being followed by a conspecific) flights to collect bird flapping frequencies and airspeeds. Solo birds had a U-shaped flapping frequency to speed relationship. Birds engaged in tandem flights had the opposite pattern; their flapping frequencies varied with speed as an inverse U-shaped curve and were up to 2.1 times higher than solo birds at the same speed.