Step 10: Use the arc on the left as a guide to draw the top part of the harpy eagle's beak. Follow the basic path of the arc and darken the top and left side of it. At the bottom, extend the line downward a bit to create a pointy tip. Curve the line to the left to create the bird's mouth. The line should end close to the bottom of the eye.
Step 11: Use the initial arc as a guide to draw the bottom part of the beak too. Follow the path of the arc but curve the line more as you darken it to make the beak more organic. Add a series of short strokes on the right side for the feathery base of the harpy eagle's beak. Inside the beak, near the middle, draw a couple of short lines for the nostril.
Step 12: Use the initial circle as a guide to draw the rest of the harpy eagle's head. Follow the top edge of the circle as you draw the head but make the shape flatter. Draw a series of short, curved lines along the right side for the base of the feathers that stand up. Continue to draw these short, curved lines along the bottom edge of the initial circle. Add a few lines under the bird's beak for the neck feathers. Add another row of short, curved lines on the right side for more feathers.
Step 13: Draw a series of curved lines along the outer edge of the initial circle for more head feathers. Follow the path of the row of feathers from the previous step and stagger the curved lines for new feathers. Add flatter lines along the bottom for more neck feathers.
Now add longer, more arc-like curved lines along the right side of the harpy eagle's head for the feathers that are standing up. The feathers should be smaller near the base and longer toward the right side. The bottoms of the feathers should overlap a bit. Don't draw too many feathers, otherwise they'll look crowded. Make the feathers smaller and flatter the closer they are to the neck at the bottom.
Step 14: Inside the body, lightly sketch a long, curved line for the harpy eagle's folded wing. This line should start near the neck at the top and then curve down and to the left near the tail. When you get the shape of the wing right, darken the lines. Make the line kind of choppy along the bottom for a more feather-like texture. The tip of the bird's folded wing should be pointy on the lower, left side. Add a series of curved lines across the middle of the wing for a row of feathers.