Guest Birds visit the game from time to time and during their visits you're able to play the levels with the Guest Bird instead of the permanent characters. You're also able to collect golden feathers on the levels during the Guest Bird visit.
When a Guest Bird is visiting, you'll see an event button on the map screen. The button displays which character is visiting as well as how long the visitor will be available. To be able to play with the guest, tap the picture of the Guest Bird on the pre-level screen before you enter the level.
Golden feathers can be collected during the Guest Bird visit when you play with the visiting bird. The score bar at the top displays a tiny feather in addition to the three stars to mark the target score you need to reach in order to get the golden feather. When you reach the score and pass the level, you'll earn a gold feather for the level. The feather will be displayed on the map screen, attached to the level button.