This is one of a series of sketches for piece I want to do for my portfolio. Like many I remember watching Heavy Metal or reading various fantasy novels and comics where people fly around on giant birds. Now just like dragons without magic birds run into some hard physic problems. Aerodynamics just doesn't let birdies get big enough to let them carry human aloft, but it is possible for a mouse! Dangerous for the mouse yes. Besides being eaten by your mount you could freeze to death or fall to your demise. Would a mouse have to be crazy... it probably wouldn't hurt.
The other thing I needed to consider was how would the mouse would control and guide the bird. Most people who have done bird flying mounts have just stuck a horse saddle and reins on it and called it done. Unfortunately a bird operates in three dimensions unlike a horse so you must work out a method of guiding the animal through this space. Next the birds wings seriously restrict the locations where the rider can sit since they move through a wide range of movements. Finally the seat arrangement must keep the rider in place in the slipstream and if the bird rolls over in a dive.
So I realized before I could design a saddle and control arrangement I first needed to know how an Eagle was put together, so I started with skeletal studies to understand what was going on under all those fluffy feathers.