OOps! I guess that would be the idiom referencing "being in motion" that is the most commonly used, wouldn't it? My bad.
on the wing
Fig. while flying; while in flight. (Usually refers to birds, fowl, etc., not people or planes.)
There is nothing as pretty as a bird on the wing. The hawk caught the sparrow on the wing.
See also:
wing
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.
on the wing
in motion
He started to run, but a rifle shot caught him on the wing. Usage notes: based on the literal meaning of
on the wing (flying):
The sight of so many enormous birds on the wing filled me with awe.
(thank you)