The head angle for an optimum bird portrait is one thing, but what about what the birds see? When individuals interact socially either to fight or prepare to mate or whatever, they generally look directly at each other. When I capture an image of a bird's head, face-on, I am often impressed with the patterning- this is the species and sometimes sex-specific signal that one bird is giving to another. To illustrate this I've chosen a recent face-on image I made of a Bohemian Waxwing (Bombycilla garrulus) at Sheffield Mills, Nova Scotia.
Take a look at some of your images, in particular the ones face-on. Study them, and don't throw them away!
Other examples welcome!