Breeding jackdaws have a habit of putting lumps of earth into their nest. Why they do this, we are not sure. One function may be to provide a foundation to their nest, given that natural cavities in trees or among rocks may sometimes run deep, and filling them up may avoid accidents. A second reason may be that it may make life of creatures living in the nest material uncomfortable, and making their life hard may well benefit the nestlings.
Below is a picture illustrating that over the years a lot of earth can accumulate in a nest box – this is what was in a single box that had fallen from a tree. The width of the box standing directly behind it is ±25 cm.