Collect the Eggs is a limited time event that gives boosters as rewards for collecting eggs that are hidden in bubbles of levels.
How to play the event?
During the event, you'll see bubbles with eggs on them in the levels. Popping those egg bubbles will drop eggs that then get added to the egg counter.
During gameplay in level, you see an egg counter placed beside the bird character in the bottom left corner of the game screen. The counter shows you how many eggs you have collected in the level. When you pass the level, the eggs you have collected will be added to your total, and you'll be rewarded for any egg collection milestones you reach. In case you fail or quit the level, then the eggs you have collected on the level will be lost.
You can view the available rewards from the event button that is visible on the map screen while the event is on-going.