Many raptors have strongly contrasting feathers. You can identify them by the bold dark and light patterns from a great distance. This makes them ideal subjects to explore with drawings on toned paper. With paper that is a mid-value gray or brown, you are not limited to simply pushing the darks by adding graphite with your pencil but can also pull the lights, adding highlights or white feathers with a white pencil or white gouache. Click on the illustrations below to enlarge them.
As shown in the illustration below, the key to a successful drawing on toned paper is to use the color of the paper as one of your values. Do not fill up the page with graphite or white pencil but use the mid-value of the paper as a value in your drawing. This is hard to do. Once you find yourself adding the white pencil it is very easy to fill up the whole drawing.
Make sure your colored paper selection is a mid value. If it is too dark, the pencil lines will not show up. If it is too light, the white pencil will not read clearly. Non-photo blue pencils are too light to effectively block in a preliminary shape. Instead use a graphite pencil and press lightly. Block in the posture, proportions and angles. For more details on getting started with how to draw raptors, see How to draw a hawk- first lines.
Now draw over this framework with details. You do not need to draw in every feather. It is better to suggest detail and the locations of major feather groups such as the scapular feathers and wing coverts.
Add dark and mid-values with a graphite pencil. Note that I have not added any graphite on the back. Be sure to leave some areas of the subject  untouched paper. You expand your value range by intentioanaly using the color of the paper as a value in your subject. Here the sunlight shines from the left side of the drawing, making the middle of the bird’s back lighter.
Since you are using graphite, use can use a tortilion or paper blender to smooth and darken the values on the back. The blender fills in many of the little spaces on the surface of the paper, making the overall value darker and a stronger contrast against the gray paper.
Now the really fun part. Create highlights and white feathers using a white Prismacolor pencil. This is what makes the drawing “pop” by increasing the value range. Note that the bird’s white breast is not all filled in with white. I use the value of the paper to create the shadow on the lower belly. By adding a cast of white pencil into the background, the back stands out against the pale sky. Observe how the same value of the paper can be used to represent the gray back of the bird, caught in sunlight and the white belly, cast in shadow.
You do not need a toned paper sketchbook. You can cut a few pieces of toned paper to fit inside your regular sketchbook, and slip them in the back of the book to be used at your convenience. When you make a sketch on the toned paper, trace the outline of the paper into your sketchbook as a place holder so that you will have room to glue the sheet into the book, maintaining the chronological order of your sketchbook entries.
Give it a try and let me know how toned paper works for you.