Painters paint what they see; sculptors sculpt what they know.
The bird sculptor must know and understand bird anatomy and the major feather groups.
Break your subject down into simple shapes. When looking at your subject, instead of seeing feathers, you'll see the ovals that makes up the large body shape and scapular feather groups. Notice the circle shape of the head and the triangular shape that defines the nape and the rump. The feather groups of the wing should be simplified and understood. Keep your drawing simple. Shapes make up the whole.