Golden Egg #31 is hidden on level 26-15 (Short Fuse). To obtain it, aim the Yellow Bird down as far left as possible in the mini passage. Aim the shockwave bird so it forces the yellow bird left, so that the yellow bird gives you a golden egg. Or you can just simply fling a Boomerang Bird down the passage.
To complete the upside-down level, aim the yellow bird so it knocks as much potions down as possible. These potions will effect some of the pigs. Aim the boomerang bird so it kills the two left pigs. Then use the other birds if you need to.