flightlessness vs unfitness

flightlessness and unfitness both are nouns.

Word	Noun	Adjective	Verb	Adverb
flightlessness	Yes	No	No	No
unfitness	Yes	No	No	No

As nouns, unfitness is a hypernym of flightlessness; that is, unfitness is a word with a broader meaning than flightlessness:

flightlessness: The condition of being unable to fly, usually used with birds such as the penguin, ostrich, and emu.
unfitness: lacking the power to perform

Other hypernyms of flightlessness include inability.

flightlessness (noun)	unfitness (noun)
The condition of being unable to fly, usually used with birds such as the penguin, ostrich, and emu.	the quality of not being suitable
	lacking the power to perform
	poor physical condition; being out of shape or out of condition (as from a life of ease and luxury)