The Slingshot is what fires your birds and spells into the enemy lines! There are many different types of Slingshots, and they are unlocked by collecting hat sets.

Wanna level up your Slingshot? That's easy! You just need to make sure you've completed any Hat Set, and that ads a permanent +1 Power boost to your Bird Power and your Flock Power. 

You can find the Hats in the Tower of Fortune, the Mighty Eagle Shop and Daily deals, or you can buy them in your Hat Shop. You can also see every Hat and every Hat Set that you have in this shop, read more about it here.

Remember that all hat sets give the same amount of boost to your Slingshot level, so it's definitely worth to collect all of them, despite the rarity.