From Badger Run Wildlife Rehab

If you have Barn Owl nesting boxes the time to clean them is when they are unoccupied. For most parts of the country that is going to be between Halloween and EARLY December.

These birds don’t remove the regurgitated pellets they cast after their meals so these boxes can get pretty gross by the end of a nesting season.

Barn Owls bring no nesting material into their nest boxes to lay their eggs and will use regurgitated pellets as a nest substrate to lay them on, HOWEVER, it’s best not to rely on these in man-made boxes.

An owl laying their eggs in a natural spot like a tree cavity will have a nice uneven, soft floor of softened wood to lay eggs on. Hay bales likewise make a soft, uneven surface. In both of these cases, eggs won’t be rolling around on a hard surface like a nest box floor of wood or plastic. So, in man-made boxes it’s best to add 3 or 4 inches of bedding over the entire floor.

Avoid fine material like laboratory bedding, sawdust, ground corn cob, loose straw, or shredded wood. As the parent tears prey apart to feed the youngest owlets, pieces of fine bedding stick to the food and are eaten by the youngsters. Large-pieced mulch such as garden mulch is perfect. This can be hardwood, pine, fir, or many other types of wood. Avoid cedar because it emits irritating fumes.