Genetic research is revolutionizing our understanding of how birds are related and of what constitutes a species. The Yellow-shafted and Red-shafted Flickers I recently wrote about are a good example. They look different but they are genetically similar and they interbreed. Therefore they are considered to be the same species, now called the Northern Flicker.
At first glance, Indigo Buntings (the first photo taken in Northfield, Minnesota) and Lazuli Buntings (the second photo taken in Aberdeen, South Dakota) appear to be a similar example. Although they look quite different, where the eastern Indigo comes in contact with the western Lazuli, these two buntings often hybridize. Thus, Alan Phillips, in his book Birds of Arizona, combined the two into a single species, which he named the Common Bunting.
What is a "species?" Most ornithologists adhere to the Biological Species Concept. A species is a group of individuals that are at least potentially capable of interbreeding. In other words, a species consists of individuals that share a common gene pool. Other scientists lobby for the Phylogenetic Species Concept. Here a species is a group of individuals that share a relatively recent common ancestry. One result of this second concept is the recognition of many more species than are currently illustrated in our field guides.
My source for much of the genetic information is Payne, Robert B. 2006. Indigo Bunting (Passerina cyanea), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online: http://bna.birds.cornell.edu.bnaproxy.birds.cornell.edu/bna/species/004