Scientific detective work reveals that yellow woodpeckers with unusual red wing feathers are not the result of genes borrowed from their red-pigmented cousins, but instead, they are the product of eating the berries of introduced invasive plants.
Birders and ornithologists have long been baffled by the unexpected presence of red feathers on a yellow woodpecker that lives in North America: where did these red feathers come from? Might they result from red pigment genes borrowed from a red cousin?
The enigmatic bird is the yellow-shafted form of the northern flicker, Colaptes auratus, a medium-sized woodpecker that lives throughout North America. The northern flicker comprises nine visually distinct subspecies including the yellow-shafted flicker, C. a. auratus, which lives east of Rocky Mountains, whilst the red-shafted subspecies, C. a. cafer, occurs west of the Rockies.
Where these two subspecies’ ranges overlap, they produce hybrids with a mix-and-match combination of markings and colors from both parents. Based on this information, it would appear that all yellow-shafted flickers with reddish primary wing feathers might be hybrids — except these birds are spotted far to the east of any hybrid zones. Which raises the question: is the yellow-shafted auratus‘s anomalous red coloring due to genes or to some other cause? Is it “nature” or “nurture?”
This is where Jocelyn Hudon, Curator of Ornithology at the Royal Alberta Museum enters the picture. Dr. Hudon is fascinated by birds’ bright colors, especially by how they are produced and change over time, and he uses a variety of techniques to investigate these processes. For this reason, those yellow-shafted flickers with mysteriously aberrant red flight feathers intrigued him and sent him on a quest to understand what he was seeing.
The red color for auratus and cafer feathers is not from the same pigment
The first clue came when Dr Hudon and his team made side-by-side comparisons between the red flight feathers of the red-shafted cafer and the yellow-shafted auratus: the red color of the eastern auratus is more “coppery” in tone than that of the western cafer and hybrids (ref) (although I think auratus looks more orange; Figure 1):
FIGURE 1. Coloration of the shafts of the outer primaries of MCLA 16 compared to those of a Red-shafted Flicker from Alberta (RAM Z66.64.13; at the top). doi:10.1642/AUK-16-63.1).
Since these colors didn’t look the same, they likely aren’t the result of the same pigment. So Dr. Hudon and his colleagues got to work. They identified two auratus museum specimens (one male and one female) with red feathers in the wings and tail, that showed no indication of hybridization.
Using spectrophotometry, they measured the light absorption and transmission wavelengths of the feather pigments in these birds and compared them to those of the red-shafted cafer subspecies and to hybrid yellow-shafted X red-shafted flickers. These spectral characters are distinctive for each compound. The team found that auratus colors are indeed, different from those found in feathers from both cafer and the hybrids.
Dr. Hudon and his colleagues then extracted the pigments from the feathers. They separated the carotenoid pigments from each other using chromatography, and found that the mysterious auratus pigment is rhodoxanthin. This purple pigment is chemically similar to yellow pigments in egg yolks and to orange pigments found in carrots, but rhodoxanthin is found in only a few plants. In North America, it is found almost exclusively in the berries of two species of nonnative bush honeysuckles, Lonicera spp., which are now naturalized in eastern North America and the American Midwest.
Could honeysuckle berries be the source of auratus‘s rhodoxanthin-red feathers? Previous studies established that the berries of two nonnative bush honeysuckles, L. morrowii and L. tatarica, and their hybrid, L. X bella, contain rhodoxanthin, and thus, they could be sources of this pigment. Further, these berries are available in early August and persist even late into August and September in parts of Canada.
Several more clues were provided by a 30-year-old study reporting that the moult pattern for flight feathers coincided nicely with the presence of these anomalous “coppery” feathers (i.e.; ref). The authors of that paper also documented that affected feathers can differ from one year to the next in the same individual (ref).
To finally nail this down, when Dr Hudon and his colleagues examined flight feather replacement in northern flickers caught by the bird-banding station at Manomet, in Massachusetts, they were able to infer that these aberrantly colored yellow-shafted flickers on average acquired the unusual red pigment in early August. But this effect is not limited to yellow-shafted flickers: honeysuckle berries have also been implicated as the source of unusual orange feathers in cedar waxwings.
Does it matter if yellow-shafted flickers have strange colors?
“This is the pinnacle of a lengthy series of papers on the pigments deposited in primary feathers,” said Alan Brush, an expert on feather color who is retired from the University of Connecticut, and who was not involved with the study.
“Hudon et al. make use of the most up-to-date spectrometric and biochemical analyses to identify and quantify the pigments,” said Professor Brush in a press release. “In addition to demonstrating that the red pigments in the molting yellow-shafted feathers are derived from their diets, not the result of interbreeding with the red-shafted form, they illuminate the dynamic nature of pigment deposition during molt, an accomplishment in itself.”
How did these alien plants become so widespread? In a word: people.
“At one point considered valuable wildlife habitat and widely disseminated, the naturalized Asian bush honeysuckles are now considered invasive and undesirable in many states,” said Dr. Hudon in a press release.
Could aberrant plumage colors affect the behavior and ecology of these birds?
“The ready availability of a pigment that can alter the coloration of birds with carotenoids in their plumages could have major implications for mate selection if plumage coloration no longer signaled a bird’s body condition,” Dr. Hudon said.
However, Dr. Hudon and his colleagues noted, it is unlikely that northern flickers rely on color to choose high-quality mates.
“Such selection for red color would have been surprising given that flickers do not appear to discriminate on the basis of the color differences when pairing in the hybrid zone,” they write.
“Pockets of aberrantly colored flickers can be explained by the patchy distribution of the rhodoxanthin-bearing berries in the east.”
Of course, this study has made bird watching a little more challenging, since birders must now rely solely upon other field marks to distinguish hybrids from “honeysuckle berry-dyed” individuals.
“This is clearly not the last we have heard of aberrantly colored birds,” Dr Hudon said.
Jocelyn Hudon, Robert J. Driver, Nathan H. Rice, Trevor L. Lloyd-Evans, Julie A. Craves, and Daniel P. Shustack (2017). Diet explains red flight feathers in Yellow-shafted Flickers in eastern North America, The Auk, 134:22–33 (published online on 12 October 2016 before print) doi:10.1642/AUK-16-63.1
Frederick H. Test (1969). Relation of wing and tail color of the woodpeckers Colaptes auratus and C. cafer to their food, The Condor, 71:206–211. [PDF]
James L. Ingold and Charles M. Weise (1985). Observations on feather color variation in a presumed Common Flicker intergrade, Journal of Field Ornithology, 56:403–405. [PDF]