In the photo above, a Northern Flicker is showing off her red nuchal (neck) collar. She is a female, since she lacks a black mustache (malar) stripe. Compare her with the male below. His red nuchal collar is hidden (but present), and you can clearly see his black malar stripes. Look closely at the female's right under-wing and the male's tail--the feathers are edged yellow. These eastern birds used to be called Yellow-shafted Flickers.
The flickers in the western United States are quite different (see below). They lack the red neck collar and the male's mustache is red, not black. Similar to the eastern female, the western female lacks a mustache. If you look closely at the tail and the wings of the western bird, you will note that they are edged reddish rather than yellow, hence these birds used to be called Red-shafted Flickers.
Until relatively recently the two types of flickers were considered to be distinct species. However, they interbreed across the mid-west, from Texas to southern Canada and northwest to Alaska. Hybrids have combinations of field marks and usually salmon-colored wing shafts. I have seen hybrids in South Dakota with each wing containing both red and yellow-shafted feathers. Recent studies show little significant genetic difference between these populations, despite the differences in field marks. Yellow-shafted and red-shafted birds are all now considered to be one species named Northern Flicker.
The scientific hypothesis is that these flickers were isolated about the time of the last glaciation, about 10,000 years ago. When the populations were separate, they differentiated. As the populations came back into contact after the glacier receded, they had not changed enough to prohibit them from massively interbreeding.
The top photo is of a bird recently banded near Northfield, Minnesota; the middle on is at a feeder in Aberdeen, South Dakota; the red-shafted flicker is on a cottonwood in Missoula, Montana.