Mystery Eggs were introduced on November 12, 2009 with the addition of the Chicken Coop. You can obtain a mystery egg by collecting eggs from your Chicken Coop. There is a 1.5% chance of obtaining an egg for each chicken in the coop per harvest. A full coop of 20 chickens will give a 30% chance of obtaining an egg for each harvest. Once you find an egg, you can post it to your feed.
When collecting from chicken coops does not produce any Mystery Eggs, a random coin bonus is given to the player.
Mystery Eggs can also be found in neighbors' Chicken Coops. After fertilizing, the player is prompted to feed the neighbor's chickens, increasing the chance to find Mystery Eggs in this coop. He can also find a Mystery Egg (max. 3 per day).
Beginning on April 30, 2010, Mystery Eggs that are found on a wall feed will then gift a farmer a Mystery Egg of that color, which must in turn be opened on the farm.
Previously, if you wanted to increase the chances of finding a Mystery Egg, a practice known as "coop stuffing" could be utilized. This was later disabled from the game, and can no longer be used. More information about coop stuffing can be found here.
Mystery Egg contents
Items that were previously available in Mystery Eggs are: