Note 1: This post is about orange throats in normally red-throated male Ruby-throated Hummingbirds. If you’re trying to identify a hummingbird with an orange throat, start with Rufous Hummingbird. For additional hummingbird ID help, please refer to A Field Guide to Hummingbirds of North America in the Peterson Field Guide Series.
Note 2: This is a blog post, not a peer-reviewed article, and I’m personally acquainted with the people mentioned. Therefore, I’m dispensing with the artificial formality of referring to them by their last names.
The late-season color shift in hummingbird gorgets, a phenomenon familiar to hummingbird banders, has caught the attention of David Sibley. Unfortunately, a red herring had David barking up the wrong tree (it was an arboreal herring).
The source of the misdirection is an article in the September 2009 issue of Birding, “The Alternate Plumage of the Ruby-throated Hummingbird,” in which Donna Dittman and Steve Cardiff documented late summer/early fall molt (another phenomenon well known among hummingbird banders, though apparently none were consulted for the article). Extrapolating from Donna and Steve’s contention that Ruby-throated Hummingbirds undergo a more-or-less complete fall molt into “alternate” plumage (only to molt again in late winter—a dubious scenario), David hypothesized that the orange gorget color observed in some male Ruby-throateds in fall and winter is acquired by molt and constitutes a dull winter plumage. Comments from hummingbird banders Cathie Hutcheson and Scott Weidensaul encouraged him to reconsider, but I’d like to take this opportunity to review what we do and do not know about seasonal color changes in hummingbirds.
Though they don’t fade in the way pigment-produced colors do, the iridescent colors of hummingbirds do change over time. The exact mechanism by which this happens has yet to be documented (at least in published form), but the short answer is that it involves wear and/or bleaching rather than an additional complete molt.
To get to the long answer, it helps to know a bit of the science behind the colors. Iridescence is produced by thin layers of substances of different refractive indices, such as a film of oil on water. The refractive index is the speed at which light passes through a substance; it’s responsible for the bent appearance of a pencil in a glass of water. Refractive index values are based on the speed of light through a vacuum, which is assigned a value of 1. The higher the number, the slower the speed. The refractive index of air is 1.000293, water’s is 1.3330, and that of ordinary glass ranges from 1.523 to 1.925.
In the feathers of hummingbirds, layers of microscopic bubble-filled discs of melanin, known as platelets, are the primary source of the refractive and interference effects that create the birds’ brilliant colors. According to Crawford Greenewalt (1960), the refractive indices of the melanin and the bubbles are 2.2 and 1.0, respectively. Different colors are produced by variations in the relative thicknesses of the melanin matrix and the bubbles (the average refractive index). Thicker melanin (higher average refractive index) pushes the iridescent color toward the red end of the spectrum; larger bubbles (lower average refractive index) push it toward the violet end. Using a spectrophotometer, Greenewalt found an average refractive index of 1.85 for hummingbird feathers that iridesce red and 1.5 for those that appear blue. Following the order of colors in the spectrum, a green feather’s average refractive index would fall between 1.5 and 1.85, while the value for a violet feather would fall below 1.5.
In his follow-up post, David points out that a change in wavelength from red to orange would require a change in the thickness of the platelets. He imagines this as a collapse, but physical abrasion and/or degradation by exposure to sunlight seem like far more plausible explanations. This is supported by detailed examination of individual feathers, which show a color shift on more exposed parts and the original color on more protected parts (illustrated in the photo at right).
The change in refractive index may result from thinning of the feather’s outer layer of transparent keratin (refractive index = 1.56; Osorio and Ham 2002), complete removal of the keratin layer and abrasion of the melanin matrix of the top layer of platelets, or changes in porosity that alter the refractive index of the keratin and/or melanin. Any of these would lower the average refractive index of the iridescent structures and push the color toward the violet end of the spectrum. Over time, a feather that started out bright red would be expected to shift to orange, yellow, and perhaps even green as more of the higher refractive index material (melanin and/or keratin) is removed or degraded, and that’s what we see in nature (even in the less intense green iridescence of the back feathers, which tend to be more golden green in spring and more emerald in fall).
There’s little doubt among hummingbird banders that the shift from longer to shorter wavelengths is the result of wear and aging rather than molt, but only electron microscopy of fresh and worn feathers can reveal the mechanism responsible. I don’t personally have the resources to pay for specimen preparation and EM imaging, but if someone with deeper pockets and/or university connections can provide the microscopy services I’m sure I can round up some feathers.
Addendum: Another photo of a male Anna’s showing the color contrast between extremely worn and new crown feathers.
Greenewalt, Crawford. 1960. Hummingbirds. (Dover reprint, 1990.)
Osorio, D. and A. D. Ham. 2002. Spectral reflectance and directional properties of structural coloration in bird plumage. Journal of Experimental Biology 205, 2017–2027. link