Let’s look at the way that straight lines on a curved surface appear to bend. A little time studying this phenomenon will help you draw and foreshorten anything you see- including the patterns on the breasts and bellies of birds. Study the two diagrams below. Pay particular attention to the way that the latitude and longitude lines curve as they approach the edges of the oval on which they are drawn. Print out and trace these lines until you can produce a reasonable approximation of the shapes and angles freehand. Lines and patterns on bird breasts will follow these same distortions if you are drawing a bird from an angle (as opposed to head on or the side view).
Now let’s look at how these lines guide the patterns on a bird. In the drawing on the left, note how the base of the patterns in the upper breast follow the curve of the latitude line. Observe how the pattern “tucks up” as it wraps around the far side. On the drawing on the right, observe how the lines on the breast pinch out and down on the far side of the body the same way that you see in the longitude lines. At first, it will help to lightly sketch in these orientation lines (perhaps with the trusty non-photo blue pencil). As you get more comfortable with this you can simply imagine them in your head. This same idea also applies to drawing many other objects such as the way a shirt drapes on a human body.