When four or more chicks are hatched in one clutch, the nest becomes to small to adequately house them all. Commonly, one or more is prematurely found on the floor as they get squeezed out by the other chicks. Generally the bigger clutch have this problem as they approach 14 days old. (Typically, I expect the first chick to leave the nest on the 18th day.)
At that point, I like to add a second nest as close to the other one as possible and divide the chicks between the two nests. I am always surprised and delighted to see them switch nest and at times three will be in one nest and only one in the other.
The white nest is the original nest and the clay nest is the additional one added around the 14th day.