From time to time–approximately once per day–you’ll see a banner at the pre-battle bird selection screen advertising a 10% health and attack bonus for watching a video. If you click to watch, you’ll see a short advertisement (usually just 15 seconds long) and you’ll get the bonus on your next battle. The bonus only lasts for that single battle, but it can be enormously helpful in the right circumstances. If you’re having trouble with a certain stage of the game, this double bonus can be just the push you need.
To maximize the bonus, be sure to watch the video ahead of a wave stage!
The advertisements can be for just about anything, but usually they’re promoting other games and apps to download or upcoming movies and/or TV shows.