"weak" and "frail" are the best option among the words listed in the question.
"Thin" and "gaunt" refer to they physical appearance of a person, who looks "frail" or "weak". The words "weak" or "frail" having more of a figurative meaning, that is still similar to "thin" or "gaunt".
Bluebirds should have a round entrance hole measuring 1½" to 19/16" in diameter, or a 1⅜" x 2¼" vertical oval hole, or a 1⅛" to 13/16" horizontal slot entrance.
