"The falcon has a perfect vision" not accepted. While there isn't the word "ένα" in the original sentence, it sounds more natural in English
Eh? To me, that completely changes the sense of the sentence.
"a vision" (countable) to me would be meaning 3 at https://www.dictionary.com/browse/vision ("an experience in which a personage, thing, or event appears vividly or credibly to the mind, although not actually present, often under the influence of a divine or other agency")
While "has perfect vision" (uncountable) would be appropriate for meaning 1 ("the act or power of sensing with the eyes; sight"). He has perfect vision = his power to see is perfect.
the hawk has a perfect sight why is this wrong??
"sight" in the meaning of "ability to see" is not countable:
https://dictionary.cambridge.org/dictionary/learner-english/sight_1 meaning "ability"
Thus "a sight" (in this sense) is not correct English.