[Kung Fu Chuck]

You’re not the only one. I took the liberty of e-mailing Rovio about this issue and here’s their response:

To get the Birdday Song sticker you need to enter the Mighty League and listen through the track that plays – while originally only obtainable while the Birdday Song played, it works too if you listen through the usual Mighty League theme. I hope this helps!

Is that even true? I played the Mighty League theme thruout and still no sticker. Has anyone even tried this and gotten the sticker that way?
 

[Kung Fu Chuck]

Thanx! I never realized that till now! :D