Grizzly bears and Kodiak/Alaskan big brown bears used to be considered different species, and everyone knew it was B.S. They were pretty much the same animal except that the Kodiaks were somewhat larger, though not always, and there was overlap in size anyway. The species range was a relatively arbitrary line along the Alaskan coast, which would be odd since these bears can wander widely. It was like that for a long time, and even my young self could tell it was an artificial distinction when I got my first Peterson Mammal Field Guide back in middle school, yet it persisted because people wanted to believe that they were different.
Hell, when I was in middle school (my obsession was overwhelming...I was an odd kid) I got an older book from the Community College about North American mammals (it was a two volume set of huge books), and they had grizzly bears broken up into dozens of species. If you've seen the range map of the bears and know how much territory they need then you would have to assume that some species only had maybe 20 bears total, which would be quite the genetic bottleneck. But, obviously, there weren't that many species of grizzlies, just like there weren't two species of North American brown bear. In fact, when people went out and tested the genetics of the populations they found that, not only were they the same species (obviously), but that all brown bears in North America, Asia, Europe, and formerly North Africa were the same species.
Looking back, it seems obvious, right? But it persisted for years (along with things like the Kaibab/Abert's squirrel distinction) despite what now seems to be obvious simply because there was a set precedent. I blame typological species thinking and use of type specimens, but these things lasted well within the time of the Modern Synthesis.
Today I am going to look at birds that seem to be like that, you know, birds that we all know are probably the same species but don't want to say anything because it will winnow our life list down. But instead of using science and all to figure this out, I am doing this simply by impression and feel when I see them. So let's start out with some rules and notes!