Young of the year: flight feathers all in roughly same stage of growth. Saw some buff-tipped body feathers as well. First two pictures of the tail feather fan, then the rest presented in order of finding.
Feathers distributed across a somewhat large area, less like a pile in one place and more like a scattered collection plucked from a point higher up (i.e. a tree).
Diameter of sphere about 13 cm, diameter of opening about 3.5 cm. In a suburban yard, adjacent to an open area of grassland. Possibly Northern Pocket Gopher?