The observant among you will notice that there are only 3 in the picture above, fear not, the others are still alive and well, just a little pre-occupied.
They have decided that they are brooding a clutch of eggs, except they're not, because all the eggs get taken out of the nest box.... This is why people who want lots of eggs don't have pretty, fluffy, posh chickens.
One of them has become so broody that even when you evict her from the nest box she still refuses to accept that she's not sat on a clutch of eggs. She just sits there on the lawn in a trance, eventually she wakes up a little, but it takes her a while!