Ventral-clap modes of hovering passerines.

Some small birds typically clap their wings ventrally, particularly during hovering. To investigate this phenomenon, we analyzed the kinematic motion and wake flow field of two passerine species that hover with the same flapping frequency. For these two birds, the ventral clap is classified as direct and cupping. Japanese White-eyes undertake a direct clap via their hand wings, whereas Gouldian Finches undertake a cupping clap with one wing overlaying the other. As a result of their morphological limitation, birds of both greater size and wing span cup their wings to increase the wing speed during a ventral clap because of the larger wing loading. This morphological limitation leads also to a structural discrepancy of the wake flow fields between these two passerine species. At the instant of clapping, the direct clap induces a downward air velocity 1.68 times and generates a weight-normalized lift force 1.14 times that for the cupping clap. The direct clap produces a small upward jet and a pair of counter-rotating vortices, both of which abate the transient lift at the instant of clapping, but they are not engendered by the cupping clap. The aerodynamic mechanisms generated with a ventral clap help the small birds to avoid abrupt body swinging at the instant of clapping so as to maintain their visual stability during hovering.