Posted by: Sheri | August 15, 2011

Orange-throated hummingbirds: Not so mysterious after all

The gorget of a male Ruby-throated Hummingbird in mid-September.

The gorget of a male Ruby-throated Hummingbird in mid-September consists mainly of older orange feathers with a few fresh red ones.

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.

Anna's Hummingbird gorget showing wear

Anna’s Hummingbird gorget showing shift to coppery orange on exposed distal portions of the feathers and retention of fresh hot pink color on basal portions protected by overlying feathers.

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.

Anna’s Hummingbird crown in early fall, during replacement of gorget and crown feathers.

References:

Dittmann, D. L. and S. W. Cardiff. 2009. The Alternate Plumage of the Ruby-throated Hummingbird. Birding 41: 32–35. Part 1 Part 2

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

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Responses

  1. Hi Sheri, Thanks for posting the explanation and photos. I’m glad to know all this and I’ve added an update at my website. You say above that two molts each year is “dubious”, and I assume you are questioning the existence of the late winter molt? But that molt is the only possible explanation for the very brilliant red throats of the returning birds in the spring. Were you suggesting something else?

    • Thanks for commenting, David. Extrapolating mainly from molt patterns in Black-chinned and Anna’s with the addition of our tiny Ruby-throated data set, the late summer/fall molt that Donna and Steve wrote about is consistent with early onset of the single annual molt that completes in late winter/early spring. It’s possible that some of the feathers replaced in late summer are replaced again in late winter, but the evidence I’m looking at does not support the hypothesis of two complete body molts per year.

      Anna’s are easiest to study since some are with us all year. Adults take up to six months to complete molt, from the first loss of contour feathers in late spring to the replacement of the chin and forecrown in fall. I posted a photo of gorget replacement in progress to my Flickr account. If Anna’s completely replaced their gorgets twice a year, we’d know about it.

      Browsing through the thousands of Black-chinneds we’ve banded on the San Pedro River between July and October indicates that body molt is common in birds of all ages and both sexes, but gorget molt in adult males is rare—seldom more than a few random pinfeathers that may represent adventitious loss. It’s more common to see gorget feathers missing or regrowing in asymmetrical clusters, most likely knocked out in combat. Even juvenile males seem to suspend gorget molt, seldom showing more than a handful of adult feathers before the end of the banding season (late Sep/early Oct). Occasional males in spring can be aged as second year by retention of a few pale, worn chin feathers, which also argues against a complete summer/fall molt (at least in juveniles).

      My experience with Ruby-throated is limited, but checking our banding records from the 2002 Hummer/Bird Celebration (September 12-14), no gorget molt was noted in any of the 13 adult males, but 4 were in light to moderate body molt. Of 18 adult females, 4 were in light body molt. No adults of either sex were in spring-like fresh plumage with distinct buffy fringes, but 3 adult females showed faint buffy fringes suggesting late completion of the normal annual molt (possible SYs); 2 of those were among the 4 in light body molt.

      Bob and Martha Sargent, Nancy Newfield, and other southeastern hummingbird banders have many, many years of data on RTHU in fall migration plus some overwintering individuals. If they routinely collect data on molt, they should be able to give you a far more complete picture.

      The simple explanation for the apparent disparity between the frequency of orange gorgets in late summer vs. winter is three to six additional months of wear. Our Anna’s males are also sometimes coppery in late spring (analogous to late summer in their annual cycle) and always so just prior to gorget molt in late summer (analogous to midwinter). We suspect that elevation plays a role in accelerating wear, as Anna’s in coastal California seem to retain pinker gorget color at the end of the cycle than those in Arizona. (A common question from visiting California birders is “What looks like an Anna’s but has an orange throat and crown?”)

      So, are you up for some microscopy?

  2. Thanks. The idea of a very limited summer molt may fit your experience, but it doesn’t fit other evidence. Dittmann and Cardiff report that they saw lots of throat feather molt in July and August, and Cathie Hutcheson said in comments at my blog “all AHY birds molt all contour feathers at this time”. Most of the September birds you were handling in Texas had probably finished molt. Also, if there was only a limited replacement of throat feathers in summer then the MCZ specimens I looked at should have shown a contrast between a few new red throat feathers with old and worn orange feathers, but their throat feathers are all uniform in that respect. It’s not surprising that Anna’s (nonmigratory) would have a very different molt strategy than Ruby-throated, and there is a lot of evidence that Ruby-throateds have two significant molts each year.

  3. hi Sheri!

    Fascinating post. My name is Rafael and I am a grad student at the University of Akron, studying feather iridescent color development and evolution. I am currently studying color production in hummingbirds, using microscopy and optical models to understand how variation in structure results in variation in color.

    The mystery you guys have here seems fascinating to me. Definitely the differences in color must be a consequence of differences in morphology, but I can’t tell for sure if that could be caused by degradation. Theoretically the keratin cortex should have very little influence in the color (though we have results that question this), and a shift from pink to orange would have to be a consequence of the melanin platelets either becoming smaller or having a greater proportion of air inside them. Melanin is a pretty tough pigment, though, and these colors are usually very stable over time (though they can be degraded by UV light).

    If there is indeed no doubt that it is indeed degradation and not molt, it would definitely be interesting to see what the mechanism of color change is. Nothing similar is reported on the literature for interference colors produced by melanin platelets. We here in Akron would have the interest and the resources to do an EM investigation of the structural differences between these feathers (though it probably wouldn’t help in settling the molt x degradation debate), if you have samples available. It would be a great addition to our understanding of the physics and ecology of iridescent colors. If it’s something you would be interested in pursuing, don’t hesitate to contact me.

    Thanks!

    • That’s awesome, Rafael! You’re just the person we’ve been looking for. I’m sure I can arrange to collect some fresh feather samples from the birds we band, and perhaps David can persuade the MCZ curator to part with a feather from one of the specimens he’s been looking at. Please e-mail me at my work email, sheri@sabo.org, and we’ll see what we can get going before the end of the field season.

  4. I’m confused. Is the bird considered an orange-throated hummer or a ruby-throated hummer? I have a ruby (which you kindly identified for me), and a couple of days ago an orange male appeared. I’m hearing them referred to as the same bird or separate species. He looks EXACTLY like my ruby–only with an orange neck.

    I can say when the sun hits his neck it’s just gorgeous!

    Thanks!

    • It’s confusing because a Ruby-throated Hummingbird (capitalized) is a member of a particular species, and an orange-throated hummingbird (not capitalized) could be any of several species. (Don’t get me started on people who don’t capitalize the names of bird species.)

      There isn’t a species that goes by the name Orange-throated Hummingbird. Instead, we’re talking about the usually red gorgets of male Ruby-throated Hummingbirds taking on an overall orange hue (not just from some angles).

  5. I live in Maine
    I have a ruby throat hummer that has been here all summe it’s a male with a orange gorget I have been waiting for it to turn red. My other male hummers have left nut the orange is still here. Can you please tell me if he is rare or it just didn’t turn red.
    Thank you Judy

    • Judy, if you re-read the post, you’ll find that the iridescent feathers on a male Ruby-throated don’t change from orange to red, only from red to orange. The bird’s annual molt, which is normally complete before spring migration, replaces worn orange feathers with fresh red ones. If the bird’s gorget was orange when he arrived last spring, one possible explanation is that failure to complete his annual molt caused him to retain his previous year’s gorget. This might also explain why he is still present when the rest have migrated, as it may be linked to a health issue.

  6. Today, May 21,2013 I saw a hummingbird with a brilliant orange throat at Ebey’s Landing State Park on Whidbey Island WA. Unfortunately I did not have a camera with me. So I found your article extremely interesting & wanted to share the sighting with you.

    • That would be a Rufous Hummingbird, Anita, a species in which males normally have orange throats.

  7. I just saw a hummingbird with a coppery orange gorget with an all green back in Portland, OR. Could Anna’s have an orange color at this time of year? I’ve never seen this before and all the other Anna’s are fuschia currently. I have pictures of it. Allen’s are not typical this far north but it sure doesn’t look like a Rufous either.

    • It’s not too early for Anna’s Hummingbirds’ gorgets to be showing coppery tones here in the high desert, but the wear-related color shift isn’t nearly as conspicuous in lower, less sunny parts of the species’ range. Adult male Rufous can also have green backs, but they don’t look (or sound) much like Anna’s in any plumage. Since hummingbird hybridize rather frequently, the question may not be as simple as Rufous (or Allen’s) vs. Anna’s. Photo and/or video documentation would help solve the mystery.

      • Thank you for sharing your follow-up and insight. A local expert believes this is an Anna’s. The coppery gorget definitely confused me. It’s good to know they can change color and that hybridization occurs.


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