I wish I ____ a bird. 

The sentence "I wish I _____ a bird" is talking about something that is not true in the present, so we need to use the past tense to indicate that we are wishing for something that is not currently the case. In this case, we should use "were" because it is the past subjunctive form of "to be." This form is used after the word "wish" to indicate that we are expressing a desire for something that is not true in the present. Therefore, the correct answer is "were."

I wish I were a bird.