What is the X in X-mas?
any member of one of two groups of birds, the swifts and the hummingbirds, that are very different from one another in general appearance and way of life. The two groups, considered suborders, are the Apodi, which contains the families Hemiprocnidae for the tree swifts (also called crested swifts) and Apodidae for the true swifts; and the Trochili, which contains the single large family Trochilidae for the hummingbirds. The most obvious common characteristic of the two groups is a superb mastery of the air; both swifts and hummingbirds are agile fliers and share, in this connection, some peculiarities of wing structure. Some ornithologists believe that these anatomical resemblances are convergent (the result of the same type of natural selection acting on unrelated organisms) and that the true relationships of hummingbirds and swifts lie with other groups, not with each other