Birds
Class: Aves
So where to begin?
One way is start narrowing identification down by asking yourself, what family of bird does the bird I'm looking at fall into? Is it a large hawk circling over my runway or is it a small song bird on the perimeter fence? Also what are it's prominent colors? How does if fly? What's its behavior? A lot of these details along with good binoculars and good guide can help you. It also takes practice, not unlike learning types of aircraft!
One thing I did when I first started out (and still do to this day) is use my Nikon not only as binoculars, but the photos allow me to more closely examine the bird on my own terms, long after it has flown past.
You'll notice that you field guide divides the species up by families. So kind of knowing the general characteristics of some of the common families you see in your day-to-day at your airport can greatly cut down the search.
Guides
Books
By far my favorite is Sibley Guides and no, this isn't a product placement. Sibley does the best job of giving you different morphs of the species, sexual dimorphism, etc. They're just detailed, which is what you want in a guide! Having just one or two pictures of a bird often isn't enough and Sibley brings the diversity.
Other big names are: Kaufman, Peterson, National Geographic Field Guide to the Birds of North America, Smithsonian Field Guide to the Birds of North America
And of course if you visit your local book store or even an outdoors store like REI, you're bound to find others and even more specific guides to birds in a particular area (i.e. the American Birding Association's Area Field Guides).
And if you're looking for just humorous gag of guide there's, The Field Guide to Dumb Birds of North America.
Online
For online, it's got to be Cornell's online Bird Guide and if you want and are into it they even have an app called Merlin -- look for it where you would normally get your apps. (And Cornell offers various online courses, in case you find yourself wanting to know more.)
A close second is the National Audubon Society's online "Guide to North American Birds" (which ironically uses "All illustrations © David Allen Sibley") and their Audubon Bird Guide App.
Alright, so this one's a little more advanced. This is honestly more helpful for strikes when you have a physical specimen in front of you, but only as an assist to someone who is already pretty knowledgeable. If you're not a birder (even if you are) if there's any doubt whatsoever, send it to the Smithsonian Feather ID Lab.
The Slater Museum Wing & Tail Image Collection