Birdchaser has a great analysis of how bird field guides featuring photographs rather than illustrations have changed over the last 30 years. Great stuff, especially for field guide aficianados (you know who you are!)

Written by Mike
Mike is a leading authority in the field of standardized test preparation, but he's also a traveler who fully expects to see every bird in the world. Besides founding 10,000 Birds in 2003, Mike has also created a number of other entertaining but now extirpated nature blog resources, particularly the Nature Blog Network and I and the Bird.