Shulchan Arukh, Yoreh De'ah 42/שולחן ערוך·יורה דעה מ״ב

8There are types of birds that do not have bile, such as doves and pigeons, and they are not prohibited because the entire species is like this. The deer does not have bile in its liver, but it has it below, near its tail." Gloss: If a person comes before him with a bird whose species he does not recognize, and no bile is found in it, it is not said with certainty that this species does not have bile. Rather, it is assumed that it has bile until it is known that this species does not have bile (Responsa of the Rashba, section 108).