An orange-throated male Ruby-throated Hummingbird seen in late August 2009 in Virginia. Photograph copyright Masaharu Ishii, used by permission.
Update 16 August: a new post Progress on the orange-throated hummingbird mystery.
Update 14 Aug 2011: A follow-up to this post is now available, tempering some of these points and adding more questions – Orange-throated Hummingbirds: more questions.
Every year in August and September, a few perplexed observers in eastern North America send out questions about an odd hummingbird they have seen. The description is always the same: “similar to the common male Ruby-throated Hummingbird, but with an orange throat.”
The species involved – Ruby-throated Hummingbird – is quickly and easily confirmed, and if these birds generate any further discussion, it is simply to suggest that they are odd males, with worn or otherwise degraded throat feathers. Well, they are male Ruby-throated Hummingbirds, but I believe that the rest of that speculation is wrong. They are apparently typical males in non-breeding plumage! (Update: the orange-throated males seen in late summer apparently are worn – see my later post Progress on the Orange-throated hummingbird mystery – but the orange-throated specimens from the wintering grounds probably represent a nonbreeding plumage).
Hummingbird molt
Ruby-throated Hummingbird was thought to molt all of its feathers just once each year, in a complete molt on the wintering grounds that ended with the rapid replacement of all throat feathers just before the birds migrated north (Pyle, 1997). Then a model study by Donna Dittmann and Steve Cardiff (2009) used thousands of photographs of hummingbirds visiting their Louisiana backyard feeders to document a previously unknown summer molt in Ruby-throated Hummingbird. Wing and tail feathers are molted only once each year (on the wintering grounds) but the head and body feathers are replaced twice each year, once in the summer, and again in late winter.
When I read about this summer molt and the “extra” replacement of throat feathers, I wondered if there could be a connection to the odd orange-throated males that are seen each fall. If Ruby-throated Hummingbirds were only growing iridescent throat feathers once each year, just before migrating north in the spring, it’s hard to explain a different throat color in the fall. But, if these hummingbirds are growing a set of feathers that will only be worn in the non-breeding season, it makes sense that there would be less selection for brilliant red throats at that season. Molting the throat feathers twice each year essentially allows the male hummingbirds to “go casual” for the winter and grow feathers that are less bright, without paying any social penalty for it. Could the orange throat be a recognizable winter plumage shown by some or all males?
The nonbreeding plumage of Ruby-throated Hummingbird
Adult male Ruby-throated Hummingbirds - three in breeding plumage (left) collected in April in Florida, and three in nonreeding plumage (right) collected in Oct-Nov in Mexico. Specimen use granted by Museum of Comparative Zoology, Harvard University ©President and Fellows of Harvard College.
A survey of specimens at Harvard’s Museum of Comparative Zoology confirms that all of the winter specimens there have a significantly different throat color than specimens collected in spring and early summer. The photo above shows three spring Ruby-throated Hummingbirds in “breeding color”, with three winter birds in “nonbreeding color”.
The color difference can be tricky to see. Like other iridescent colors it changes slightly as the angle of view changes. For the photo shown here I chose the angle that showed the most dramatic color difference, but from some other angles the colors appeared much more similar. Nevertheless, the color difference is consistent and definite. No winter males show the deep red color of the summer birds, and only a couple of summer birds approach the more orange throat color of the winter birds.1
More questions than answers
Currently the biggest question is this: Is the color difference in the orange throat really the result of molt, or is there some other explanation?2
And there are a lot of other questions:
- Do any other North American hummingbirds have summer molts? If so, do they also have a recognizable non-breeding plumage?
- Besides throat color, are there other parts of the body plumage that show differences between summer and winter plumages?
- Do females show any difference between summer and winter plumages?
- What is the evolutionary origin (and correct terminology) for these molts?3
- Do Ruby-throated Hummingbirds undergo this summer molt wherever they are in eastern North America, or do they migrate to favored molting areas?
- Do all of them molt before migration, or do some migrate to the wintering grounds and molt there?
Many of these questions could be answered by careful observations by backyard birders. This is yet another reminder that new discoveries are still waiting to be made, even among the most common backyard birds of eastern North America. A well-executed study like the one by Dittmann and Cardiff can lead to new discoveries anywhere, for any species. All it takes is curiosity and observation.
References
Dittmann, D. L. and S. W. Cardiff. 2009. The Alternate Plumage of the Ruby-throated Hummingbird. Birding 41: 32–35. http://www.aba.org/birding/v41n5p32.pdf continued here: aba.org/birding/v41n5p35w1.pdf
Howell, S. N. G. 2010. Molt in North American Birds. 267 pp. Houghton Mifflin Harcourt.
Pyle, P. 1997. Identification Guide to North American Birds: Part I. Slate Creek Press. 731 pp.
Notes
Thanks to Masaharu Ishii for allowing use of his photo. Thanks to Peter Pyle and Donna Dittman for comments and discussion, and to Jeremiah Trimble and Harvard’s MCZ for access to specimens.
- The few summer birds showing orange throats may be one-year-olds in their first breeding plumage, or perhaps adult males that were less healthy or had lower hormone levels when their feathers were growing. Immature males in their first fall and winter also show this orange color on the few iridescent feathers they have acquired on their throats. [↩]
- Donna Dittmann (email) reports that during her study she did not see a color difference between old and new feathers on molting males in Louisiana. Furthermore, she says that comparing specimens of fresh fall males with worn summer males in the museum at LSU shows no difference in throat color, and that the only orange-throated specimens at LSU were collected in winter in Mexico (just like the orange-throated MCZ specimens). What does this mean? I don’t know. The throat color of the MCZ winter specimens (all from Mexico and Central America) shows a clear and consistent difference from the spring specimens. If it’s true that this difference is not apparent in Louisiana immediately after molting, then something must change in the next month or two as the birds travel to their wintering grounds. They can’t go through another molt, but maybe the existing feathers change somehow. Any hypothesis would also need to explain the occasional orange-throated birds that are noticed all across the eastern US in early fall. It remains a mystery. [↩]
- For those who really like molt studies: see Dittmann and Cardiff (2009), and Howell (2010) for some discussion of molt terminology, and the debate is ongoing. If the wing and tail molt in winter is connected with the complete body molt in late winter, that must be the annual complete Prebasic molt. In that case this newly-discovered summer molt of body feathers must be the Prealternate. But if the summer molt of body feathers is connected to the winter molt of wing and tail feathers (with a pause for fall migration and restart on the wintering grounds), then that whole process would be the Prebasic molt, and the body molt in late winter is a separate event unconnected to the replacement of wing and tail feathers, and would be the Prealternate. This latter scenario is a better match for the “standard” molt cycles of birds, but hummingbirds are an unusual group, so there’s no reason to assume that they comply with any standards of molt. The answer to those questions can probably best be found by studying the exact timing and extent of molts on the wintering grounds. [↩]
Related posts and pages:
- Progress on the orange-throated hummingbird mystery Thanks to Sheri Williamson (author of the Peterson Field Guide to Hummingbirds) and her recent...
- Orange-throated Hummingbirds: more questions Now updated by a new post Progress on the Orange-throated hummingbird mystery. This adds to...
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This is interesting.I myself have never noticed a difference in the throat feathers between spring and fall.I will pay close attention and document any that I see with a less brilliant gorget.
RTHUs molt contour feathers from approximately late July through October. Since the birds with the red-reflective feathers were collected in April, long after this fall molt, the surface of their feathers is not worn so shows red. The yellow-reflective feathers are obviously worn in the photo, so those feathers were not yet molted in the year the birds were collected, but were the feathers from 12 months before. The surface of those feathers is worn so refracts a yellow color. The feathers act as a prism and are not any of these colors, but are black.
Hi Cathie, I do not agree. I don’t think it’s as simple as “worn feathers”. Ruby-throated Hummingbirds molt the throat feathers in late winter, in an extremely rapid and nearly simultaneous molt just before they migrate north, so in April they are all fresh. Dittmann and Cardiff discovered an additional molt in the summer, in which the birds molt their throat feathers again near the end of the process, so the fall and early winter specimens should also be in fresh plumage. As you point out, the only pigment in the throat feathers is melanin, and it is the microscopic structure of the feather surface that reflects only a very intense and pure color. It’s not at all clear how wear would impact the visible color of those feathers. There are no pigments that can fade the way the red color of a cardinal fades over time, and no evidence of any gradual change in color as the feathers get older. I would expect that any damage to the surface of the feather would simply result in “lights-out” and a dark gray or blackish appearance. I can’t see how wear would produce a slightly more orange color, as that requires a very precise alteration of the structure. It could happen, I guess, but many things argue against it in this case.
I catch many AHY-M birds (actually all AHY birds molt all contour feathers at this time) in all stages of molt between July and September, and I have never seen males with gold colored throats after they molt, always red. Perhaps the birds you found with non-red throats are simply an aberrant group and not standard. I have banded about 36,000 RTHUs since the year 2000, and have not seen the golden gorget feathers that your birds show. I work in southern Illinois and southern Indiana where the population of RTHUs is extraordinary (as well as in southern Missouri) and we get migrants from farther north as indicated by recapture data. I have seen a golden color refracted from the feathers, but when the angle of view is changed, they show red.
OK, it’s good to know that you speak from some experience. And thanks for answering one of my questions – that all adults molt in late summer. Regarding the next question on my list – do your “migrants from farther north” arrive and then molt in your area, or are they passing through after already molting on their own summer grounds?
On the orange-throated question, you are saying basically the same thing Donna Dittmann said – that males look just as red after their fall molt as before, and that maybe the MCZ specimens are an “aberrant group”. I am resisting that explanation because it’s just too easy, and seems very unlikely. There are about 20 winter specimens at MCZ and all of them are orange-red.
One possibility, as I said in my post above, is that something happens between the summer molt and the wintering grounds. Maybe the new feathers grown in the fall look the same red color at first but lack some critical structural feature, so that over a matter of weeks the color changes to more orange?
I would also submit the idea that the color difference is subtle enough to be missed if you are not actively making comparisons and looking for it (with all due respect). All of the specimens look “red” at a glance, until you sort them into summer and winter and then the differences are apparent. As I said above the photo shows the most dramatic difference, and it might be misleading you. The difference is not that obvious most of the time. The fact that you’ve never seen an orange-throated male in all of your banding suggests to me that you might be missing them, since birders do report them occasionally in the eastern US in late summer. Maybe this fall you could use some color swatches and match the throat colors of adult male hummers, to see if there is any variation in throat color and if it correlates with molt?
I said that I never see a gold colored gorget in a male in the fall AFTER it completed its molt, only red gorgets. Indeed, the ratty looking males sometimes do have a gold cast to their gorgets, but those are the feathers they are loosing, not gaining. I don’t think that I would miss an orange colored gorget on a RTHU AHY-M, even if I was very busy; I have often been accused of having extraordinarily good vision, both in detail and color. I do see gold gorgets occasionally, but as I said, only on those molting and only the old feathers. I cannot tell, as you know, a migrant from a local bird by molt, time of year, or anything else. I (and my co-authors) have published 2 papers using isotope analysis to figure out where HY birds came from and what migration pattern is suggested by testing feathers from HY birds caught in Central America, but with adults, one can’t be sure where the feather is actually molted, unlike HY birds which grow feathers while in the natal area. Your question might be answered by contacting my fellow RTHU banders across the range to find out what they see in the fall contour molt in RTHUs. You could also contact the MOSI group to find out about the occasional AHY-M RTHU they capture during their banding program in Central America. I do agree with Donna; sometimes simple is right.
Thanks Cathie. I agree simple is often right. In this case I had a simple explanation (fresh feathers after the fall molt) but that doesn’t match your observation that freshly-molted birds are red-throated. You also have a simple explanation (worn feathers before molt) but that doesn’t fit my observation that all winter specimens are orange-throated. So maybe neither one of us is right, and the answer is something else. I think it remains a mystery.
I have had an orange throated one that is protecting the feeder, he perches on top and runs all the others that come close he even runs the starling from my other feeder, it’s cute to watch he doesn’t seem to drink any, just perches on top of the feeder.
Hi Debbie, That sounds like the behavior of a male Rufous Hummingbird, which do have orange throats, and are common in the west (if you are east of the Rockies it would be very rare). Are you sure of the species?
I’ve been thinking about the difference in gorget color, and wondering if it’s based on a lower hormone level in the fall. I don’t know if that kind of chemical test is available and whether feathers from the specimens you cite could be used in a test, but it would be interesting. That might explain a slight color difference since the males are not concerned with breeding during that molt. My other question is when would the second molt occur? Around here it seems the males molt in the late part of July and in early August. However, the first birds returning to US are in late February, which would have them beginning a molt in December or January. Maybe they molt just the gorget to look beautiful for the girls. Perhaps a check with those southern hummingbird banders who see so many wintering birds to ask about a gorget molt would be helpful. They of course deal with mostly western species and not RTHUs, but it might lend some insights.
Good suggestions, thanks! Most feather color is controlled by hormone levels during feather growth, and I assume iridescent colors would be the same. I don’t know if those hormone levels show up later in the feathers, though. In the case of most bright colors the birds have to create the red and yellow pigments, and therefore it is “easier” or less costly to grow feathers with drabber colors. In the case of iridescent colors I’m not sure how that system translates, that is, in the summer molt they grow feathers that are “easier” and take whatever color they end up with, but are orange feathers easier to grow than red ones?
I have no personal experience with the winter molt, but it does seem possible that (like many other brightly colored birds) they molt only the flashy feathers for the spring. However, that should show up on one-year-old males, which would retain their juvenile body feathers through the spring migration and should look very worn. (And note that Sheri Williamson has just suggested on her blog that the summer molt might be very limited and the spring molt complete?) If you know anyone who handles winter hummingbirds it would be great to get their insight on this.
I found an orange throated humming bird dead the other day here in ohio. I just thought it was alittle less red but it had an orang throut. Are they still the same species or are they conciderd differend species?
It’s all the same species – Ruby-throated Hummingbird (unless it was orange all over and in that case it would be Rufous or Allen’s Hummingbird). Did you take photos of the throat, or save the specimen to donate to a museum?
May 12, 2012…..just saw my first orange-throated hummer. Rushed to your website and found that it was not so unusual. We are in Holden, MA. Will keep watching to see if he stays the same all summer.