Definition of "on the wing"

Next comes a supposition, that the bird sleeps on the wing, and that the great distance from land at which it is seen at close of day is thought to favour the supposition […] .

1835 February, Thomas Hutton, “Observations on an Article in Loudon's Magazine of Natural History, on the subject of the Albatross”, in Journal of the Asiatic Society of Bengal‎, volume 4, number 38, page 108

By surviving the rigors of a life on the wing, birds earn another chance to pass on their genes.

1999, Scott Weidensaul, Living on the Wind: Across the Hemisphere with Migratory Birds‎, North Point Press, published 2000, page 346