flightlessness vs inability

flightlessness and inability both are nouns.

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

As nouns, inability is a hypernym of flightlessness; that is, inability 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.
inability: lacking the power to perform

Other hypernyms of flightlessness include unfitness.

flightlessness (noun)	inability (noun)
The condition of being unable to fly, usually used with birds such as the penguin, ostrich, and emu.	lacking the power to perform
	lack of ability (especially mental ability) to do something