Hummingbirds and insects have evolved for sustained hovering ?ight from vastly different ancestral directions, and their distinct phylogenies underlie the differences in their aerodynamic styles. In all other birds?and, presumably, hummingbird ancestors?the downstroke provides 100% of weight support during slow ?ight and hovering. Given that many birds possess the mass-speci?c power (using anaerobic metabolism) to hover for short periods, the selective pressure on hummingbird ancestors was probably for increased ef?ciency (resulting in stiff wings with greatly simpli?ed kinematics), and an upstroke muscle (the supracoracoideus) that makes the recovery stroke rapid, while contributing enough to the hovering power requirements to allow the downstroke muscle (the pectoralis) to operate within its aerobic limits.
In other words, this pseudosymmetrical wingbeat cycle is good enough, and although hummingbirds do not exhibit the elegant aerodynamic symmetry of insects, natural selection rewards ?good enough? as richly as it does our aesthetic ideals