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