Birds fly. At least, most can. In fact, “flight is the defining characteristic of birds” (as Jonathan Alderfer and Jon Dunn and Kenn Kaufman write). Then why don't field guides teach us to identify birds in flight?
Depending upon the type of birding you are doing, your first detection of a bird may be of a bird in flight. For instance, you are walking along a hedgerow and previously unseen sparrows fly up from the grass and disappear into the brambles. You turn a corner and a bird takes off from a tree and flies away. Movement catches your eye and you see a flock of birds flying in formation overhead. You scope out the ocean from a promontory and view numerous birds winging by offshore.
These are the common ways that we see birds. It only makes sense that your field guide should tell you how to identify these birds in flight. But they don’t. Oh, some occasionally mention the unique flight styles of a certain few birds. There are a few specialized guides for hawks and ducks in flight. But these books are often based only on shape and pattern, not actually the behavior of flight that might help you to identify distant or quickly-viewed birds.
This manual teaches you the basic mechanics of flight and gives you the vocabulary necessary to describe a bird in flight in such a way that you may identify it to family or even species by flight alone. Combine flight style with flight call notes or key observable plumage field marks, and most birds in flight are readily identifiable.
The above text is the introduction to my 38-page manual on how to identify birds in flight. As far as I know there is no other book or manual that explains how to go about describing flight characters in order to identify birds. Flight style mechanics, silhouette and shape, flapping and flight path progression, and other ID clues are all brought together in a "Bird flight style identification worksheet." This is followed by 11 examples describing the flight style differences between such groups and species as West Coast Shearwaters, European Starling versus Cedar Waxwing, Crows and Ravens in flight, Late autumn sparrows in the weedy marsh, Flight style comparison: Red-winged Blackbird and Brown-headed Cowbird, Flight of American Robin, and several more.
Using what you learn here, and the examples given, you will be able to describe flight characters and learn the ID of flying birds you observe.
The entire manual is now loaded into a stand-alone page on my Greg in San Diego blog. It is here: How to Identify Birds in Flight.