David M. Larson
Northern Flickers (Colaptes auratus) occur in two distinctively colored subspecies groups—Yellow-shafted (C. a. auratus) and Red-shafted (C. a. cafer). In each case, the subspecies groups contain multiple populations. The names come from the coloration of the shafts and undersides of the flight and tail feathers, a genetically determined distinction caused by the yellow carotenoid pigments in auratus and red 4-keto derivatives of the same carotenoid pigments in cafer. The largely eastern Yellow-shafted and western Red-shafted subspecies groups interbreed in the midwestern states, often giving rise to offspring with intermediate coloration. While the overlap zone is far from the East Coast, it is not uncommon to see Northern Flickers with a few red primaries in New England. So how does that happen? Some researchers have suggested that the multicolored birds are hybrids with periodic shifts in the types of pigments deposited. However, no one has come up with a plausible genetic control mechanism for coloring some feathers and not others, changing coloration patterns in the same individual birds from year to year, and changing the color part way through feather growth.
Hudon et al. (2017) hypothesized instead that red pigments in feathers of auratus are due to exogenous pigments derived from non-native plants. There are examples of this phenomenon in other birds—for instance, the different feather tip colors in Cedar Waxwings that feed on certain berries, though such a mechanism has not been described in flickers. While flicker diets consist largely of ants, they do consume fruit in the fall, including that of the patchy but widely distributed Tatarian (Lonicera tatarica) and Morrow’s (L. morrowii) honeysuckles and their hybrids. If ingestion of exogenous pigments from such plants is the explanation of the anomalous colors in Yellow-shafted Flickers, then the red pigment in the red auratus feathers would be rhodoxanthin from the honeysuckles rather than 4-keto-carotenoids in cafer. Other investigators had shown that carotenoids ingested by birds are rapidly deposited in growing feathers, which could help to explain the shifts from yellow to red along the shafts of some eastern flickers.
Selected feather samples from normal cafer, normal auratus, hybrids, eastern birds with red feathers, and samples of honeysuckle fruit were extracted using standard techniques to concentrate carotenoids and subjected to spectrophotometry and high-performance liquid chromatography to identify and quantify pigments. Without getting into the details of all the spectral differences in samples from the birds, the red feathers in the eastern birds showed clear evidence of rhodoxanthin—and presumed metabolites—but not 4-keto-carotenoids, often starting abruptly along the length of a feather and trailing away during subsequent feather growth. The feather color was not just due to an overlay of exogenous rhodoxanthin on the five normal yellow pigments in auratus flickers, but a change in expression of some of the yellow pigments in the red feathers—for instance, the normally predominant pigment lutein decreased while 3’-dehydro-lutein increased. So it seems clear that the red pigment in some of the feathers of the eastern auratus Northern Flickers had a plant-based source and that regulation of deposition of these pigments is under poorly understood control.
But the question still remained as to whether pigment deposition during feather growth and the availability of fruit were temporally matched. The authors examined molt status records from 134 Northern Flickers processed at the Manomet banding station in Plymouth, Massachusetts, between 1978 and 2014. Of these birds, 105 were actively molting when captured. In fact, 12% of flickers banded at Manomet, and 36% of those captured in two or more seasons—resident birds—had some red flight feathers. Based on molt timing derived from the banding data, a source of rhodoxanthin would have had to be available to the birds in early August in order to effect the color change patterns recorded. While honeysuckle fruits become available in June, they continue to fruit until flickers start ingesting plant materials in August. The only other widespread source of rhodoxanthin is the plump red arils of yew trees (Taxus spp.), though these are not normally available until later in September, so temporally disconnected from the observed color deposition in the flicker feathers. Hence, honeysuckles are the most likely source of this pigment.
Interesting future investigation lines from this research include elucidating the mechanisms by which exogenous carotenoids influence the deposition of endogenous carotenoids, understanding the pathways and functioning of the detected metabolites of rhodoxanthin in the flicker feathers, and determining whether simple spectrographic evidence can suffice to detect rhodoxanthin in feathers. Intriguingly, this work simplifies our understanding of the taxonomic relationship between Red-shafted and Yellow-shafted Northern Flickers by clearing away false suppositions of hybrids beyond the contact zone.
Reference
- Hudon, J., R.J. Driver, N.H. Rice, T.L. Lloyd-Evans, J.A. Craves, and D.P. Shustack. 2017. Diet explains red flight feathers in Yellow-shafted Flickers in eastern North America. The Auk 134: 22-33.
David M. Larson, PhD, is the Science and Education Coordinator at Mass Audubon’s Joppa Flats Education Center in Newburyport, the Director of Mass Audubon’s Birder’s Certificate Program and the Certificate Program in Bird Ecology (a course for naturalist guides in Belize), a domestic and international tour leader, President of the Nuttall Ornithological Club, and a member of the editorial staff of Bird Observer.