I think it is okay to say "C'est un oiseau?", as you said before, just as it is okay to say "Est-ce que c'est un oiseau?" Remember, French is a language that has many ways to say the same thing, especially in terms of interrogation.
"C'est un oiseau?", lit. "That's a bird?"
"Est-ce que c'est un oiseau?" lit. "Is it that this/that is a bird?" (Yes, that's grammatically valid), loosely meaning "Is that a bird?"
Either way, both are perfectly valid.
I been studing for some months and been worderful, but lately the course is starting to get more "difficult" due to the big variety of words that look like the ones in my own language, portuguese, but doesnt mean the same thing and after some time i notice a thing: What blocks my mind from learning isn't the difficulty but yes "my mental resistance to accept the french language reality as it is". I bet that is others studants difficulty if you get to think better.
Yes KarikJorge1, It is now becoming very difficult to continue the initial pace of learning. I am facing a mental blockage now due to multiple factors: 1. Vocabolary (there are more than 350 words in my word list now) 2. Variations (there so many different ways to say same thing. Words like because, since and questions can be used in multiple ways in French).
Keeping myself motivated to learn French; very tough task.