Hairy and Downy Woodpeckers look so similar to each other that even experienced birders sometimes have trouble telling them apart. Both have the same black-and-white pattern on their wings and heads, white bellies and backs, and red spots on the back of the crown in males. Their calls sound different and they are slightly different sizes, but if one can neither hear them nor see the two side-by-side identification can be very difficult. With such similar features, laymen and experts could easily assume that they are each other’s closest relatives. However, with the advent of genetic analysis, much of our knowledge of bird taxonomy has changed. According to a study published in January 2016, the DNA evidence suggests the Hairy and Downy Woodpeckers are not closely related enough to be placed in the same genus. (Weibel et. al.).

The striking resemblance of these species is probably the result of convergent evolution, a force that causes different species to resemble each other. Some examples include the similar body shape of sharks and dolphins, flying squirrels and sugar gliders, and hawks and falcons. Many forces can drive convergent evolution, and one of them is territoriality. One theory for why Hairy and Downy Woodpeckers look so alike is that because they share a niche, looking the same makes it easier to identify each other as competitors.

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Weibel, A.C., Moore W.S. Plumage Convergence in Picoides Woodpeckers Based on a Molecular Phylogeny, with Emphasis on Convergence in Downy and Hairy Woodpeckers. The Condor 107(4):797-809. 2005

A more recent article not cited in this posting:

Dufort, M.J. An augmented supermatrix phylogeny of the avian family Picidae reveals uncertainty deep in the family tree. Molecular Phylogenetics and Evolution 94: 313-326. 2016.