The problem is that in mid continent, in a small area where the distributions of the birds formerly known as the Yellow-shafted and Red-shafted Flickers overlap, there is a zone where it's not uncommon to spot flickers which, when they fly away, flash a color between yellow and red -- a sort of salmon-color.
The story is that the yellow- and red-feathered birds can mate to produce offspring with intermediate characteristics. The sticking point is that part of the definition of "species" is that individuals belonging to one species shouldn't be able to easily mate and produce vigorous, reproducing offspring with individuals belonging to another species. In other words, no matter how unlike two individuals look, if they tend to mate with one another and produce healthy, fertile offspring, they should be considered members of the same species.
Therefore, nowadays experts have "lumped" Yellow-shafted and Red-shafted Flickers into one species, and that one species is known as the Northern Flicker. The two former "species" are now thought of as "races," or "subspecies." They are considered races in the same sense that we think of the various human races. Black people and white people can mate to produce brown babies exactly the way Yellow-shafted and Red-shafted Flickers mate to produce flickers with salmon-colored shafts. This means that black and white people are members of the same human species, and it means that Yellow-shafted Flickers and Red-shafted Flickers also are the same species, despite once having different names.
Of course, the process also works in the opposite direction. For example, botanists have always had a hard time with blackberry species. When I was learning my plants, one manual (Britton & Brown) reported 18 blackberry species in northeastern North America. At the same time another famous expert (Bailey) distinguished in the same area over 400! In this case, Britton & Brown tended toward lumping, but Bailey definitely was into splitting, for he split every new-looking population he found away from the commonly known designation and gave it a new name.
Nowdays it is recognized that some groups of organisms do tricky things at the genetic level so traditional classification techniques often can't be applied equally to all groups of organisms. Once you become a little expert in a certain field, you just have to know what the tricks are for working in that group. The main thing we can do to deal with the problem is to use up-to-date field guides,a nd to just keep in mind that this lumping and splitting situation exists, and that there's not much we can do about it.