This is normal when they are sitting on eggs. They don’t want to soil the nest, so they will hold their poop sometimes all day or all night, depending on which shift they are taking, and when they emerge, they expel a large, and often smelly poop. This is one of the many reasons why you have to rest your pairs between clutches, because nesting, incubating eggs, and feeding chicks take a toll on both the male and female’s health. When the chicks have left the box, the nest box should always be removed immediately. This prevents the pair from nesting again too soon, and keeps the parents focused on caring for and weaning the chicks. Even if the eggs do not hatch, the nest box needs to be removed once they abandon the eggs. The pair should be rested for 6 months before you return the nest box. A pair should be limited to two clutches per year, which is one more than they would have in the wild.
Thank you for asking Lafeber,