The Win Streak rewards you with powerful Bird Boosters for starting the next level. How it works: You must pass consecutive levels without failing to keep the streak going. 

Level 1 Win: Start the next level with a Red booster.
Level 2 Win: Start the next level with a Red and a Chuck booster.
Level 3+ Win: Start the next level with a Red, Chuck, and Bomb booster.

Failing any level will reset your Win Streak, and you will lose the accumulated starting boosters.