In most of the avian species, lower eyelids are more mobile than the upper counterparts (see my previous post – click)
However, woodpeckers seem to mostly use upper eyelids when they close their eyes, like owls. My theory is that most of the diurnal birds are visually vigilant to potential predators who are frequently above their heads or at their eye level; therefore, closing their eyes with the lower eyelid raised from below gives more chance of detecting potential danger till the last moment.
Whereas for woodpeckers, their feeding activity is more important than the surveillance for predators. If they use lower eyelids to close their eyes, their eyes should be completely closed before they start pecking on the tree. Otherwise, all the dust and particles generated from their head-banging on the tree will fall onto the eyes, which will be difficult to remove even over to the nictitating membranes. Hence, they use their upper eyelids to efficiently protect their eyes and they don’t need to completely close their eyes.
Additional difference from most of other birds I noticed in woodpeckers is that canthal tilt is quite steep (lateral canthus is much higher than medial canthus), thus upper eyelid faces the forehead rather than the crown. Moreover, the movement directions of the nictitating membrane and of the upper eyelid seem to be similar, which is partically due to the eyelid position, I guess. Whereas in most of other birds, upper eyelid moves vertically while the nictitating membrane starts unfolding from the medial canthus and runs across the globe, so the two eyelids make more or less 90 degrees. These are never mentioned in any of the references and I am excited with my findings.