Birdo Eggs are a type of item that hatches into a Birdo. The white versions first appeared in Doki Doki Panic, which were shot from Birdo's mouth, while the spotted pink ones first appeared in Mario Kart: Double Dash!!. A Birdo Egg with bow-shaped spots appeared in Mario Super Sluggers.
In New Super Mario Bros. D.I.Y., it was revealed that pink Birdo Eggs hatch into Birdos, and the white ones are shot from Birdo's mouth. Birdos can be ridden in the game for the first time.
A Birdo Egg, shot from Birdo's Mouth. Note that it looks like an ordinary chicken egg, without spots or anything