The very first meal a monarch butterfly caterpillar eats is its own eggshell. In order to hatch, the caterpillar eats its way out of the egg, and then polishes off the remainder of the eggshell. It then wanders around the leaf, and if it finds another monarch egg, it will start to eat that, too.
Female monarch butterflies lay 300 to 500 eggs over the course of two to five weeks. Normally, a monarch only lays one egg at a time (on the underside of a tender, young milkweed leaf). It is fairly rare to find more than one egg on a leaf, or even on the same plant. After a female lays an egg, several seconds up to a minute goes by before she lays another egg. During this time, she usually moves on and finds another milkweed plant on which to lay the next egg. This lapse of time between the laying of each egg – referred to as a refractory period – probably evolved to encourage the dispersal of a female’s eggs on different milkweed plants, to decrease the chances of cannibalism occurring.
According to the renowned monarch entomologist Lincoln Brower, a cluster of monarch eggs on any given milkweed leaf indicates either that milkweed is in short supply, or that the female that laid the eggs is sick, very old, or has been flying for a very long time.