How do chicken eggs hatch at the same time?

Chickens are creatures of habit. They can change nests, and often do, but for a stretch, they like one nest and one nest only. And there are usually fewer nest boxes than hens. When a broody hen claims one box to sit in, and other hens really like that particular box, there is a bit of a problem as to everyone getting time enough to sit. They will fuss and fume, but not actually fight for the nest.

It's more interesting to watch ducks. They will chose a nesting area. If one duck builds a good nest, then several ducks will share it. I've seen as many as 6 ducks at one time sharing a nest. It's almost like they like each other's company. The last duck to lay a egg is the one responsible for covering the eggs with down or dried grass. I've never seen them fuss about being the primary duck in the nest, like the chickens do.

In the wild, this may be a protective feature of the birds. If all the eggs are in one "basket", then all the hens/ducks will share in protecting the eggs from danger.