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.
No, that would be 가벼운 닭을 더 먹어요. Particles are extremely important for discerning the grammatical relations and cases of words: 을/를 denotes the direct object, and 이/가 denotes the subject.