Black pearls are very valuable. You can collect those from the various rewards throughout the game. You can use them to purchase hats for your flock. Hats give an additional multiplier to your birds and the more expensive the hat, the higher the multiplier.
When you are able to collect all of the hats from the same set, your slingshot will be upgraded and it will also give a higher multiplier to your overall score. Even if you have a very valuable hat set, it is worth going back to purchase those cheaper hat sets to boost your slingshot.