이/가 are used to mark subjects of clauses and the subject complements of the irregular verbs 아니다, 있다, and 없다.
- 남자는 닭이 아닙니다 = "The man is not a chicken."
- 남자는 닭이 없습니다 = "The man does not have a chicken."
- 요리사는 닭이 있습니다 = "The chef has a chicken."
- 요리사는 남자가 먹은 닭을 요리합니다 = "The chef cooks the chicken that the man eats."
Duolingo, You should consider 'a/the/none' and 'singular/plural' distinction/flexiblity in translation more carefully.
"A light chicken eats more." and "Light chickens eat more." are both appropriate here, and "The light chicken eats more" and "The light chickens eat more." can be fine in some contexts.
This is an instance where the dynamic translation ability of the organic mind out shines AI. Humans can easily make the following connections light = light weight = not heavy = skinny = scrawny = small = slim = this chicken eats a lot of food but doesn't put on much weight = is this chicken bulemic or does it have a high rate of metabolism = either way, this chicken is strictly pet material.