Mighty Eagle’s Dojo is unlocked after defeating Level 4 of Old Nesting Barrows. Once unlocked, you can upgrade the classes (headgear) of your birds. Three training sessions are available at a time, and refresh every 3 hours (or immediately for a fee of 5 Lucky Coins, but this is not advised).
Upgrading headgear to Veteran Class costs 500 Snoutlings and increases health and attack by 20%. Elite Class headgear costs 2000 Snoutlings and increases health and attack by 40%. Finally, Master Class headgear costs a whopping 300 Lucky Coins, but increases health and attack by a staggering 60%!
It is highly advised that you upgrade your favorite headgear to at least Elite Class before attempting the hardest levels in Angry Birds Epic.