Some birds are difficult to tell from their doppelgangers, such as Cooper's Hawk (above) and a Sharp-shinned Hawk.
With over seven billion people on earth the odds are good that every person probably has someone else in the world that bears an uncanny resemblance to him or her. How many times have you thought you saw someone you know at a distance, only to realize when they came into better view, that it was not the known person at all, just a “friend lookalike”? Certainly more than one unfortunate innocent has gone to jail because of the mistaken identification testimony of some poor victim, who was certain that the “identified” wrong person definitely was the criminal. We marvel, when watching one of those penguin movies or videos, how a parent penguin can find its own chick from among tens of thousands of other tuxedo-wearing nesting penguins, when they all look exactly the same to us. Then too, there are all those very similar species that can only be separated by their call notes or songs: think Willow and Alder Flycatchers.
But among bird species there are several instances where two species are virtually identical except they are different sizes. The first pair of such species that usually comes to mind is the Greater and Lesser Yellowlegs. See my previous article, "Size Matters."
Two other pairs that occur in Florida and throughout most of North America are the Cooper’s and Sharp-shinned Hawks, and the Downy and Hairy Woodpeckers, and while the size difference between the similar species often is sufficient to separate them, there are a few additional clues that can be helpful when one is confronted with trying to positively identify a lone bird of any one of these four species.
Cooper’s and “Sharpies” are particularly difficult to identify, but the first thing to look for is the size difference. The Cooper’s is Crow size, and the Sharpieis Blue Jay size. However, this can be problematic because, as with many raptors, the female in both species is larger than the male, so that a large female Sharpie can appear almost as big as a small male Cooper’s. Their sizes don’t actually overlap, but with a lone bird the size can be close enough to be confusing. Many birders believe the best distinguishing field mark is the Cooper’s rounded tail, compared to the Sharpie’s squared off tail (left). The photos presented here display this difference quite clearly. In flight, the Cooper’s head juts out in front of the leading edge of the wings, while the head of the Sharpie remains aligned with its front wing line. In both species, the juvenile is brown, whereas the adult has a gray back, and peach-colored breast.
As to the woodpeckers, the Hairy (left) is about one-third larger than the Downy; has a much longer, stouter bill than the tiny little bill of the Downy (below, left); and diagnostically, the Hairy has clean white, unmarked, outer tail feathers, while the Downy has three little black bars in its white outer tail feathers. In both species, the male has a red patch in the back of the head, which the female does not have. All of these clues are apparent in the photos displayed here.
The more interesting question posed by these three nearly identical, but different sized species pairs, is: how and why did they get this way? There are a couple of more facts to consider regarding this question: all three pairs are widespread and live side by side, but never interbreed, or create hybrids; and DNA studies disclose that each species in each pair is not really a close relative, but rather, each species is descended from an original ancestor quite removed from the other.
The answer appears to be a concept known as Convergent Evolution. This is an evolutionary process in which distantly related species evolve toward each other in order to take advantage of similar solutions to particular environments or ecological niches. It is the opposite of Divergent Evolution, a process whereby very closely related species evolve different traits to adapt apart to changing ecological circumstances, sometimes changing so significantly that they evolve into new, separate species.
Our three species pairs, under discussion here, are excellent examples of Convergent Evolution; while Darwin’s Finches on the Galapagos are the most familiar example of Divergent Evolution, as they evolved into four separate species from a common ancestor in response to changing environmental circumstances on the islands. The amazing aspect of our Yellowlegs, Cooper’s/Sharpie and Downy/Hairy pairs, in their convergent evolution, is the level of similarity that the species have achieved, considering they apparently began evolving millennia ago from quite divergent ancestors.
So, if and when I ever stumble upon the poor soul among the seven billion on earth who is unfortunate enough to look like me, I will know that the similarity is not a result of either Convergent or Divergent Evolution, but just a matter of pure happenstance, for assuming he is human, we will definitely both be members of the same species. I certainly hope it won’t be under any circumstance where I might be misidentified, unless the viewer mistakes me for Robert Redford. Then, of course, I will know he or she is blind.
For identification tips on Cooper’s and Sharp-shinned Hawks, see:
For identification tips on Downy and Hairy Woodpeckers, see:
For more information on Convergent Evolution, see:
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