to groom the feathers with the beak by grasping one near their base and sliding the beak out to its tip; removes grit and parasites and helps interlock the barbs and barbules on the vanes if they have become misaligned; often the eagle gathers oil from the uropygial (preen) gland at the base of the tail and spreads the oil on the feathers as it preens; the oil helps condition the feathers but does not waterproof them; eaglets begin preening in their second week when juvenal contour feathers emerge, helping break up the feather sheath at the mouth of the follicle and align the barbs and barbules as the vanes unfurl