Golden Egg #21 is located in level 1-14 of Summer Pignic. Zoom out and you’ll see the Golden Egg is hidden behind the precipice on the far right hand side of the screen. You’ll need to fling a yellow bird almost straight up — tapping quickly — so that it creates a high arc and drops down right on top of the Golden Egg. If you hit the top part of the pile of boulders the yellow bird might roll down on top of the Golden Egg, so there is a little wiggle room with this shot.
Note, this is a two part video. The first part shows how to find the Golden Egg and the second is a walkthrough on how to pass the actual Golden Egg level.