They are multiplying!

Flying, Hexproof
On your upkeep create two 1/1 Birb tokens with
flying. you may stack Birb tokens on top of
They are multiplying!.
They are multiplying! gains +1/+1 for token stacked
on They are multiplying!.
{t} pay 2 life, create a 1/1 Birb token with flying.