A bird's egg can be obtained by a bird nest that falls out of trees while Woodcutting. They can come in three different colours: green, blue and red which represent the colours of Guthix, Saradomin and Zamorak, respectively.
These eggs can be offered to the shrine in the Woodcutting Guild to receive 100 Prayer experience and a Bird nest with seeds as a reward. There is a 1/300 chance of obtaining a piece of the evil chicken outfit when offering eggs. Before this update, the eggs had no use.
Considering the value of pieces of evil chicken outfit and the chances of obtaining one, the average value of one egg is 3,518.