a new layer of down feathers that begin to emerge by the second week after an eaglet hatches, from follicles interspersed among the natal down follicles; they are thicker than the natal down feathers with greater insulating capability; also called thermal down; in eagles these feathers cover most of the body and hide but do not replace the natal down feathers, which are pushed out by the emerging contour feathers; a few months after an eagle fledges the thermal down molts and is replaced by a new layer called definitive or adult down