I am El Capitano of the HAP as well as the founder. I am pretty sure that I coined the phrase as it it used commonly now in bird photography.

First off I offer the two excerpts below from The Art of Bird Photography II (916 pages on CD only).

The first is an introductory paragraph from the Advanced Composition and Image Design Chapter.

Understanding Light-Angle, Subject-to-Film Plane Orientation, and Head-Angle

The failure to understand the importance of how light-angle and head-angle relate to both subject-to-imaging sensor or film plane orientation and to the quality of the final image ruins more bird photographs than all other artistic and design factors combined. The major flaws in at least three-quarters of the images of birds presented during BIRDS AS ART Instructional Photo-Tour critiquing sessions involve either improper light angle or improper head-angle (or both). As you approach a bird or a flock of birds, your primary consideration in nearly all cases will be light-angle, so that is where we shall begin.

Skipping ahead:

Head-Angle

Understanding the importance of the angle of the bird’s head relative to both the light-angle and the imaging sensor or film plane are critical considerations if one wishes to create powerful, appealing images on a consistent basis. Images of birds facing slightly or well away from the camera are rarely successful. Ideally, when the subject is roughly parallel to the film plane, the bird’s head should be parallel to the film plane, or—better yet in most cases—just a bit inside of parallel, that is, turned two or three degrees towards the imaging sensor or the film plane. Assuming a perfect light-angle, I actually prefer the latter for several reasons. With the subject’s head turned slightly towards the film plane, the tip of the bill will generally be on the same plane as the eye, so even when you are working at wide open apertures the image will appear sharper overall than an image in which the bird’s head is precisely parallel to the film plane. Furthermore, when the bird’s head is cocked slightly towards you and the sun is directly behind you, the face will be illuminated immaculately and the image will almost always feature a catch-light in the eye.

With birds facing to some degree towards the camera, it is usually best to make the photograph either when the bird is looking directly at the camera or when its head is aligned naturally with its body. When a bird’s head is parallel to the film plane and its body is angled towards the film plane the results usually look at least a bit unnatural. When the body is angled partially or totally away from the film plane, it is—as noted above—almost imperative that the bird’s head be parallel to the film plane. At times, it is possible to get a bird to turn its head towards you by making a sharp raspy “queek” sound (though this may scare the subject away). Often it’s best to simply wait (and pray) for the bird to turn its head, as birds as a rule are constantly looking around to check for predators.

As far as moderators disagreeing with one another that is both the nature of the beast and--as I mentioned in my introductory letter to the group--to be expected. And yes, a very few degrees can make a huge difference. Take a superb image of a a great bird, sharp with wonderful light and a soft, oof background. If the bird's head is turned as little as one degree away from me, the image will be disappointing; most times I will consider it as a complete failure. I can remember dozens of times when I have begged a bird to give up a head angle only to have it fly away without ever looking back at me... If the bird's head is parallel to the imaging sensor, or turned a degree or three towards me, I will usually be thrilled. And if the bird in a posted image has it's head turned away as little as one degree, I will point that out. Some might consider commenting on a single degree nit-picking; I view it as an attempt to help folks improve their bird photography.

As far as the degree of head turn, that is of course personal and subjective. As above, if the bird is parallel to the imaging sensor, my great preference is for the bird's head to be angled two or three degrees towards me. At times, the best head angle is when the bird is looking right down the lens barrel. In various situations I might prefer just about any head angle...

The next time that I am out I shall save a few examples to post.

later and love, artie