Easter Eggs were added in the Easter update on April 15th, 2017 into Ruins. The eggs spawned in random locations and there is a chance that they will drop from crates or dead players. The Eggs, once picked up, release a baby chick, also called a duckling. The ducklings follow the player around until the player takes damage. They then die and give the player 1 HP.
The eggs colors are varied, from blue and purple to green and orange, however, they always released 1 chick, identical to the rest. People used to grab as many of these eggs as they could without taking damage, so that they would have an army of ducklings following them around.
This mode has been removed since Easter passed already.