Here is the full list of all 13 The King's Bird achievements.
Destroy the Cage and usher in a new era of freedom
Escape the Fallen Kingdom in under 10 minutes
Restore the Barrier and usher in a new era of peace
Win the Battle for the Last Kingdom
Complete the Fallen Kingdom
Found all the spirit birds in the Sky Kingdom
Found all of the spirit birds across the world
Complete all levels in the Sky Kingdom
Found all the spirit birds in the Lake Kingdom
Complete all levels in the Lake Kingdom
Complete all levels in the Forest Kingdom
Found all the spirit birds in the Forest Kingdom
Found the elusive purple tree