Here is the full list of all 39 Leave The Nest achievements.
Fly with the train for 10 seconds
Unlock Touky the tough toucan
Unlock Cardy the charismatic cardinal
Fly through 10 zones
Get a 50X multiplier
Unlock Henry the hyper hummingbird
Unlock Finny the finch who doesn't flinch
Unlock Elly the energetic eagle
Unlock Polly the pirate parrot
Fly with the train for 5 seconds
Collect 100 coins
Collect 50 coins
Unlock Olly the observant owl
Fly through 5 zones
Unlock Philly the fastest falcon
Unlock Billy the bold bluejay
Get a 15X multiplier
Unlock Sally swan the swift skimmer
Unlock Spoopy the spooky bat
Unlock Manny the mallard with valor
Get a 5X multiplier
Unlock Firdy the fast bird
Fly above the clouds for 8 seconds
Fly above the clouds for 5 seconds
Touch the sky
Fly above the clouds for 2 seconds
Collect 10 coins
Fly for a minute
Dive for 2 seconds
Fly through 50 windows
Fly through 10 windows
Fly through 25 windows
Fly for 5 minutes
Fly for 10 minutes
Fly through 15 zones
Visit every zone in one flight
Unlock Corey the caw caw crazy crow
Unlock Gully the seagull with no equal