Gravitation-enabled Forward Acceleration during Flap-bounding Flight in Birds

Flap-bounding, a form of intermittent flight, is often exhibited by small birds over their entire range of flight speeds. Its purpose is unclear during low to medium speed (2 m·s⁻¹–8 m·s⁻¹) flight: aerodynamic models suggest continuous flapping would require less power output and lower cost of transport. To explore its functional significance at low speeds, we measured body trajectory and kinematics of wings and tail of two zebra finches (Taeniopygia guttata) during flights between two perches in a laboratory. The flights consisted of three phases: initial, descending and ascending. Zebra finch first accelerated using continuous flapping, then descended, featuring intermittent bounds. The flight was completed by ascending using nearly-continuous flapping. When exiting bounds in descending phase, they achieved higher velocity than that of pre-bound forward by swinging their body forward similar to pendular motion with conserved mechanical energy. We recorded takeoffs of three black-capped chickadees (Poecile atricapillus) in the wild and also found similar kinematics. Our modeling of power output indicated finch achieved higher velocity (13%) with lower cost of transport (9%) when descending, compared with continuous flapping in previously studied pigeons. Flap-bounding could be useful for unmanned aerial vehicle design by mimicking descending flight to achieve rapid take-off and transition to forward flight.