Comparing a field guide depiction with a photo is a great way to build an image of a bird in your mind. A White-headed Barbet depicted in a field guide is shown next to a photo from the internet.
I dislike bird field guides with photos but use them all the time.
That probably sounds strange. Field guides, such as Sibley, that use paintings of birds are virtually always superior. The way a bird looks in a photograph can be affected by the lighting conditions, the film or exposure used, the angle it is standing at, as well as; its age, plumage, and molt. An artist can compensate for all that, showing the bird at the angle best used to portray field marks and depicting it as one normally appears in the field.
Or almost - some artists come close, but not even Sibley’s paintings truly look like a bird you see in the field. The only way to get a true sense of what a bird looks like is to look at the real bird. That failing, looking at photos in combination with a field guide with artist’s plates is the best way to study birds. I’ve found there is no substitute for photos in establishing a quick-recognition image in the mind.
The old NAS Master Guide to Birding is a great source of photos. As shown here, comparing the relative darkness of the head and neck on Cassin’s vs. Couch’s Kingbirds, an unlikely field problem. . . until a Cassin’s shows up in south Texas, as they sometimes do.
Long ago, in the dark ages before the internet, I would use the three volume National Audubon Society Master Guide to Birding as my main source of photographic images of North American birds. Nowadays, images of virtually any North American bird, and many foreign ones, can be found by simply doing a Google image search.
Photographs are particularly useful to establish a sense of an unfamiliar bird’s structure, like, how massive is the bill or how much of a crest does it show? However, photographs of birds are best studied in a series, rather than a single photo, all the while remembering that one individual of a particular species may look slightly, or substantially, different from others of that species.