Here is the full list of all 22 BirdGut achievements. It takes around 2-3 hours to unlock all of the achievements on Windows.
Recieve the Blessing of the Nine
Discover the Skull of the Eighth
Discover the Skull of the Seventh
Discover the Skull of the Fifth
Discover the Skull of the Fourth
Discover the Skull of the Third
Discover the Skull of the Second
Discover the Skull of the First
Obtain the launch ending
Obtain the escape ending
Obtain the launch and escape ending
Discover the Skull of the Ninth
Reach the Missile Chamber
Reach the colon
Reach the large intestines
Discover the Skull of the Sixth
Reach the small intestines
Reach the Acid Ocean
Reach the stomach city's upper zone
Reach the stomach city
Reach the stomach of the bird
Get eaten by the bird.