Cypsiurus is a genus of the swift family of birds in the family Apodidae, of the order Apodiformes.

1/ Asian palm swift (Yến cọ, Cypsiurus balasiensis) 

As the name apodiforme—meaning "footless" in Greek—suggests, the legs of hummingbirds, swifts and treeswifts (or 'crested' swifts) are rather small and offer limited functional uses.
The family name, Apodidae, is derived from the Greek ἄπους (ápous), meaning "footless", a reference to the small, weak legs of these most aerial of birds.
The genus name Cypsiurus comes from the Greek words kupselos (swallow) and ouros (tailed).