The egg was introduced in most vanilla maps in update 0.152 for Easter 2011.
Mice can collect eggs by touching them and getting back to the hole. After touching an egg, the counter at the lower right corner will show a +1. If you got the egg and then back to the hole, the counter will increase.
Eggs are very similar to Presents, as it can unlock items, can be collected and both are on the same locations on vanilla maps. However, mice don't need to wait for a map to unlock items, and they can't get cheese with it.
Eggs are located in most vanilla maps, except the Cheese Thief Maps and map 777 (Fishing event). On some maps, mice will need to co-operate to get the egg to avoid death (Balance Maps), or build things in more difficult to reach places, just to reach the egg (Building Maps). Players often give up their chance of getting first place in favor of getting the egg (Racing Maps).
There must be at least 4 mice for the egg counter to go up.
Unlockable items can be found at Easter 2011.