I had the same problem. It sounded more like a statement to me as opposed to a question. I then checked with a translation app and it indicated that the correct way to ask the question would be "isso é um passaro?"
There are exceptions to this rule, but most masculine words end with an "o" whereas most feminine ones end with an "a." For example, gato = male cat ,gata = female cat. You can also tell by the word that comes before the subject. If "um" or "o" are used, it is masculine. If "uma" or "a" are used, it is feminine. Take for example this sentence, "É UM pássarO?“