Two aspects of watching birds are that they move, and that their appearances can be highly variable. Even the best field guides—whether using paintings or photographs—are limited by having to represent everything through a small number of two-dimensional, static images. Given these inherent restrictions, books can still do a very good job—especially if one takes the time to read the text!
Nonetheless, the potential of video footage to show moving birds and to portray more plumage variations than even a specialized field guide suggests that the medium will become increasingly common in the field of bird identification.