What's Different About These Common Birds? Part 3

This male, photographed near Grants Pass, Oregon on 13 January 2007, is typical of the Acorn Woodpeckers (subspecies bairdi) found in California, Oregon, and Washington, and matches depictions of this species found in the popular North American field guides. The underside of this bird exhibits the following features: a broad and nearly solid black breast band that frames the throat and connects to a broad black “mask” surrounding the eye, and a mostly clean white lower breast, belly, undertail, and flanks. In the U.S., Acorn Woodpeckers show dense black streaking immediately below the black breast band, but virtually no streaking on the white belly and sparse narrow black streaks on the otherwise white flanks. (Photograph taken by Phil Hicks using an Olympus C750UZ camera)

This female Acorn Woodpecker, part of a small isolated population (subspecies angustifrons) in Baja California Sur (Koenig et al. 1995), was photographed near San Antonio de la Sierra, B.C.S., Mexico on 12 January 2009. We encountered 75+ Acorn Woodpeckers in this area, and all those seen well were similar in appearance to this bird. They had dark eyes and were heavily streaked along their flanks from their lower breast to their undertail coverts. Only the very middle of the belly lacked streaking. Their black flank streaks were much broader than those of birds in the U.S. I’ve seen hundreds of Acorn Woodpeckers in Oregon and California and none have shown the amount of heavy black flank streaking exhibited by this individual. The portion of the breast band that is solid black is not as broad in the B.C.S. birds and, as evidenced by this bird, they show lots of white-tipped feathers in the black breast band. Northern birds show far fewer white-tipped feathers in the breast band, giving their breast bands the appearance of being broader and more solid black. The primary difference between males and females is the head pattern. In males, the red crown patch extends farther forward and the black does not extend across the forehead, which it does in females. (Photo by David Irons using a Canon EOS XSI 450D camera and an EF 100-400mm f/4.5-5.6 lens)

Variations in plumage and eye color between the B.C.S. population (on left) and birds in the U.S. (on right) are evident in this side-by-side comparison. These close-ups were cropped from the two photos above. The most obvious difference between these birds is eye color. The dark eye (red iris) seen on the bird on the left is unique to the B.C.S. population. All other populations of Acorn Woodpeckers in the U.S., mainland Mexico, Central America, and northern South America show white irises as adults. Note also that the somewhat circular, light-colored area wrapping around the face, referred to as the “facial circle” in this discussion, is entirely yellow on the bird on the left and mostly white with just a bit of yellow on the throat of the bird on the right. Again, only the B.C.S. birds show an all yellow facial circle. There are also subtle differences in the width of the facial circle. It is a little wider between the eye and the bill and a little narrower across the forehead on the B.C.S. birds. In all other Acorn Woodpecker populations, the light-colored facial circle is predominantly white with varying amounts of yellow on the throat.

This female Acorn Woodpecker, likely the subspecies striatipectus (Koenig et al. 1995), was photographed at Chinchona, Costa Rica in February 2007. Its features are somewhat intermediate between the B.C.S. birds and the typical birds found in North America. Like all Acorns (other than those in B.C.S.), it has white irises. The underparts are more streaked than birds in the U.S., but less heavily streaked on the flanks than those of the B.C.S. population. The black breast band is similar to the B.C.S. birds, in that it is not as broad and has a lot of white spotting. However, the facial circle of this bird is mostly white, with yellow restricted to the throat. This bird shows a broader white band across the forehead compared to the rather narrow band of yellow across the forehead of the B.C.S. birds. Note that this individual shows a few red-tipped feathers at the base of the throat, where the yellow on the throat meets the top of the black breast band. This smattering of red-tipped feathers is always shown by Acorn Woodpeckers from Oaxaca, Mexico to Panama (Koenig et al. 1995), but appears on only about 5% of the U.S. specimens (subspecies bairdi and aculeata) in the San Diego Natural History Museum (SDNHM). All four of that museum’s angustifrons specimens have red-tipped breast feathers (Phil Unitt pers. comm.). I took several images of the San Antonio de la Sierra bird shown in the images above, and found only one angle where the bird appears to show a single red-tipped breast feather. (Photograph was by Bill Tice using an Olympus D-550 camera through a Zeiss Diascope 65T)