Also, the eye, or eyes, need to stand out. That can be difficult with some animals, particularly birds, where the eye is the same color as the head - or a band of color on the head. In these cases, a reflection in the eye can create a highlight which draws attention and differentiates the eye from the surrounding body.
Here is an example from shots I took yesterday of a Northern Cardinal male. Both are decently focused and exposed and demonstrate what a difference a highlight in the eye can make. Both were made with a Canon EOS 30D, EF 300mm f/4 IS USM at f/5.6, 1/1000 sec., ISO 100.
Also made yesterday was this shot of a Black-capped Chickadee - again, with a slight highlight in the eye.
|Canon EOS 30D, EF 300mm f/4 IS USM at f/5.6, 1/640 sec., ISO 100|