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Birds are one of the few vertebrate groups in which viviparity does not occur; every bird begins life by hatching from an egg. Bird eggs vary widely in size, color, and shape, though their internal structure is consistent. For example, cavity-nesting birds typically lay spherical eggs, while rock-nesting birds, like guillemots, lay conical eggs that are less likely to roll off cliffs. Cavity nesters often produce pure white eggs, as color camouflage is unnecessary in the darkness of their nests and white requires less energy to produce. In contrast, birds that leave their eggs exposed or unattended typically lay camouflaged eggs. The ostrich lays the largest egg of any bird, yet the bird with the largest egg-to-body mass ratio is the kiwi, which typically lays only two eggs spaced weeks apart. Although an ostrich egg accounts for only about 2% of its body mass, the eggs of tiny hummingbirds can make up as much as 25% of the mother’s body mass.
