MC is a detective in his school .He's trying to catch a strange creature ,that the student in his school are gossiping about.The school Public Moral Department is chasing this creature too like him , but they're like rivals competing with each other.
He tried to catch it (?) on the stair but failed and felt down. (it seems that he received infos about this creature from the student ,some say it's a dog ,some say it's a cat but he did not so sure about those theories- he also overheard a member of Public Moral Department said "I heard that it's a rabbit" -He thought that "It seems their infos about that creature are no different like mines" )
MC : "どういうことだ？ここに来て、まさかの鳥説……！？" (>> this is the line which i don't understand)
Strange creature (cat/bird ??)「んなな～っ！」
I can only guess he meant "Could it be the theory people (the student in his school) said that you're a bird was true ... !?" but i'm not sure .Much thanks if anyone can help me understand the correct meaning of what he said (there is a "....!?" missing part so I don't know what was he trying to say here too)