The sentence 雞吃了 (jī chī le) has a topic-comment structure and allows a different reading depending on context. If the noun 雞 (jī/ chicken) is viewed as the agent, the sentence means "The chicken has eaten." On the other hand, when 雞 (jī/ chicken) takes up the role of patient in the sentence, the sentence should be translated as "The chicken was eaten."
Reference: Chinese: a linguistic introduction by Chao Fen Sun
