Exotic Honeysuckles Change Feather Color

The finding underscores the unpredictability of invasive plant impacts.

Tatarian Honeysuckle (Lonicera tatarica) in early July. The fruits remain on the plant into August.

Most introduced plants – species imported from their natural ranges – are not harmful. The introductions that damage the environment, the economy or human health are called invasive, and their stories are much alike.

Arriving by accident or on purpose and without predators or pathogens to check their spread, invasive plants escape and eventually dominate areas outside cultivation, outcompeting native plants for water, nutrients and light. Some of them change soil chemistry or host other invasive species, further altering the communities they invade.

To a large extent, that’s the story of exotic bush honeysuckles. Four species are present in Minnesota and neighboring states: Tatarian Honeysuckle (Lonicera tatarica), Morrow’s Honeysuckle (L. morrowii), Bell’s Honeysuckle (L. x bella) and Amur Honeysuckle (L. maackii). All were introduced in the 1700s or 1800s as ornamentals, for erosion control or for wildlife habitat, and all are now recognized as invasive.  Once they are established, they displace native plants with their extended growing season, large, fast growth and prolific fruit production, which is their primary means of natural spread (1, 2). Together, their effect on native plant communities is considered second only to common buckthorn (2).

That’s sobering enough, but there’s another concern. Decades of observations have found that some birds are affected both indirectly by alteration of their habitats and directly by alteration of their appearance. Specifically, the berries of some exotic honeysuckles can change the color of their feathers, turning yellow parts orange and orange parts red.

Looking Back

The first reports of reddened feathers, called plumage erythrism, are from the early 1960s, when bird banders at Powdermill Nature Reserve in southwest Pennsylvania found Cedar Waxwings with orange tail bands instead of the normal yellow (3, 7). More observations followed, involving more species. White-throated Sparrows turned up with orange, not yellow, lores, the area between the base of the bill and the eye (4). Yellow-shafted Flickers, an eastern subspecies of Northern Flicker, were found with orange to red, not yellow, coloration on the undersides of their flight feathers (5). And since the early 1990s, male Baltimore orioles have been observed with red instead of orange feathers, particularly in the northeast U.S. and southeast Canada (6, 7).   

Left: An adult Cedar Waxwing, Bombycilla cedrorum. Photo by Ken Thomas via Wikimedia Commons. Notice the yellow tail band. Right: Tail band of a Cedar Waxwing showing normal yellow and abnormal orange color caused by deposition of rhodoxanthin. Photo courtesy Powdermill Nature Reserve, Carnegie Museum of Natural History.  

It took decades to find the cause. In the late 1980s, Jocelyn Hudon and Alan Brush, then with the University of Connecticut, analyzed the pigments in the yellow and orange tail bands of cedar waxwings. Along with several other pigments, they found rhodoxanthin, a red pigment then known primarily from the berries of yew (Taxus) and the leaves of some gymnosperms (Arborvitae, for example; (8).

Rhodoxanthin is different from the pigment that adds a splash of red to the birds’ wing tips. Because cedar waxwings can’t produce rhodoxanthin from other pigments they ingest, Hudon and Brush suspected it was introduced directly from the birds’ diet. In addition, because most of the birds with orange tail bands were juveniles, they suggested that the pigment came from something the nestlings were fed by their parents in July and August, when they were growing tail feathers. Whatever the food source was, they thought it was seasonally available, and because erythrism was recent – it was not documented in birds before the 1950s and 60s – the dietary source also had to be recent. They speculated:

If a dietary change is involved, the sudden appearance [of aberrant feather colors] might reflect the appearance of a new food source, a change in the abundance of a native or established source, or a change in adult food choice. (8)

Suspicions Confirmed

Speculation that change in feather color was caused by a change in food source was correct. Around the same time color aberrations were more widely observed, exotic bush honeysuckles were increasing in abundance. They and their cultivars were widely promoted and planted, and they found available habitat not just in landscapes and wildlife plantings but also in grasslands, brushlands and open woods, where they escaped. Feeding experiments and observations of wild birds confirmed that the source of rhodoxanthin was honeysuckle berries, especially those of Tatarian and Morrow’s Honeysuckle. Furthermore, plumage erythrism was found to affect not only nestlings but also adults that ate the berries during their summer molt. Rhodoxanthin consumed as they replaced their feathers was deposited like yellow or orange pigments, reddening the color of the tail bands (9, 10).

As exotic honeysuckles have spread, so have instances of plumage erythrism. Birders in the Midwest, including Minnesota and Wisconsin, have reported erythrism in cedar waxwings, white-throated sparrows and yellow-shafted flickers. Reports also come from Idaho, Montana, Utah and Alberta, Canada (10).

White-throated Sparrows (Zonotrichia albicollis) with normal yellow lores (left)

and abnormal orange lores (right). Photo courtesy Powdermill Nature Reserve,

Carnegie Museum of Natural History.

The consequences of changed feather color aren’t yet fully understood, but some ornithologists wonder if plumage erythrism could affect reproductive success (7, 10). Intensity of feather color is a measure of fitness. If males retain their reddened feathers into the breeding season, females may choose them over males that acquire their color “honestly,” that is, from activity and genetic advantage that signal better fitness. The result may be selection of breeding partners that are less able to produce and raise healthy offspring.

How to Help

The effect of exotic honeysuckle berries on feather color also points to the unexpected outcome of some plant introductions. Although not all introduced plants become invasive, those that do may have surprising – and still unknown – impacts on ecosystems, economies and health.

Fortunately, many landscape alternatives are available to anyone wanting to avoid the negative effects of invasive plants. Here are several sources of information.

• To learn which plants are invasive, start with this Minnesota DNR web page or see the Invasive Species tab above.
• To learn how to identify exotic honeysuckles, see this guide from MnDOT. More resources specific to introduced honeysuckles are listed on this Minnesota DNR web page.
• If you decide to replace non-native plants with native ones, you may be encouraged to know that an effective planting can be any size. To learn how one ecologist and gardener converted her backyard into a diverse, pleasing habitat, read Home Is Where the Habitat Is from the Minnesota Conservation Volunteer.
• If you’re looking for alternatives to invasive plants, there are many helpful resources. For trees, shrubs and woody vines, try the Landscapes Alternatives web page from the Woody Invasives of the Great Lakes Collaborative (WIGL). A free, downloadable brochure and a mobile app are available under the Landscape Alternatives pull-down menu.
• Another resource is Landscape Alternatives for Invasive Plants of the Midwest, a brochure that can be downloaded at no cost from the Midwest Invasive Plant Network.
• For lists of native plant suppliers and services, see the links under More to Explore, above right.

Finally, if you’re a birder, report any observations of birds with abnormally reddened feathers. This will help researchers learn more about the extent of plumage erythrism and its possible impacts. State ornithological societies and eBird may be appropriate places to submit reports. 

References

1. Woody Invasives of the Great Lakes Collaborative. Species accounts for Amur Honeysuckle, Morrow’s Honeysuckle and Tatarian Honeysuckle accessed July 27, 2021. https://woodyinvasives.org/
2. Smith, W R. (2008). Trees and Shrubs of Minnesota. University of Minnesota Press, Minneapolis.
3. All About Birds: Cedar Waxwing, accessed July 23, 2021. https://www.allaboutbirds.org/guide/Cedar_Waxwing/overview.
4. Powdermill Nature Reserve Avian Research Center, Carnegie Museum of Natural History. 2014 Late Fall bird banding report, https://powdermillarc.org/pictorial-highlights/late-fall-2014/. Accessed July 23, 2021.
5. Hudon, J., Driver, R.J., Rice, N.H., Lloyd-Evans, T.L., Craves, J.A., and Shustack, D.P. (2016). Diet explains red flight feathers in Yellow-shafted Flickers in eastern North America. The Auk 134(1): 22-33. 
6. Hudon, J., Derbyshire, D. Leckie, S., and Flinn, T. (2013). Diet-induced plumage erythrism in Baltimore Orioles as a result of the spread of introduced shrubs. The Wilson Journal of Ornithology 125(1): 88-96.  https://www.jstor.org/stable/41932838
7. Flinn, T., Hudon, J., and Derbyshire, D. (2007). The Tricks Exotic Shrubs Do: When Baltimore Orioles Stop Being Orange. Birding magazine, September/October 2007.  https://www.aba.org/birding_archive_files/v39n5p62.pdf
8. Hudon, J., and Brush, A. (1989). Probable dietary basis of a color variant of the Cedar Waxwing. J. Field Ornithol. 60(3): 561-568.
9. Witmer, M.C. (1996). Consequences of an alien shrub on the plumage coloration and ecology of Cedar Waxwings. The Auk 113(4): 735-743.
10. Hudon, J. and Mulvihill, R. (2018). Diet-induced plumage erythrism as a result of the spread of alien shrubs in North America. North American Bird Bander 42:95-103.