Sorry in advance to anybody who has suggested this already ^^.   My idea is that nichlings with Double digger paw can make dens on certain types of dirt called soft dirt, which could be either  light brown or light green. These dens would be see-through when you zoom in on them, like the trees are, and a small one could fit 1-2 nichlings inside, as well as a nest. However, if you found a patch of soft dirt about 3-5 tiles large, you can dig them all up and create a large den that could