Our friend Dr. Robert Askins sought out the opinion of Dr. Alan Brush who has done extensive research on the structure and pigmentation of feathers. His response was the following:
My thought is that on adult bird, the plumage would have been replaced pretty much completely by the end of summer, eg., this is the basic plumage. There is nothing in the literature I can find, that reports this change in this species. So here are the alternatives. 1) dietary, as in HOFI or CEDW. That is, some new or different food source during molt If that were the case, I would expect it to be a little more widespread or common. While HAWOs are not as common and feed over wider areas, the frequency in the NE would be expected to be lower. Simply a sampling problem. My guess would be that a dietary change is not the case. 2) a mutation or other type of change in the metabolic pathways that changes the precursor carotenoid into the form deposited in the feather. Although this species mainly eats animal food (although I can't think of it as a 'predator'), mostly beetles, grubs, spiders, etc, they also take fruit and seeds. That means they get not only a variety of carotenoids in their diet, it's available year round. Where the 'error' resides in this individual may be the malfunction of an enzyme in the pathway from precursor to product or in the mechanism that transports molecules or incorporates them into the follicle. A change in any of these could produce the change in the nape feathers. Since carotenoids are not deposited in any other tract, it's not a more general problem. One could, of course, discriminate among these possibilities with a chemical analysis of the feathers from this individual and a 'normal' feather. That's the best I can do from here (keyboard rather than a lab).
My thanks to both of them for their time, thoughts, and allowing me to post everything! I have heard from a grand total of one person thus far - Jay Kaplan - who has actually seen a bird like this before. He described it as a color aberration that occurs from time to time. He told me that sometimes people think they have American Three-toed Woodpecker when it is a Hairy Woodpecker with yellow markings rather than red. He has seen two of them in his time in Canton, Connecticut. This is interesting to me for a few reasons, the first being that it is very difficult to find anything in literature describing my bird precisely, and that more modern search techniques (Google images anyone?) yielded nothing in this exact manner either.
The second is that his statement on people's mistakes concerning the identification of species leads one to believe that this occurs more frequently than it seems to in my experience thus far. I have had many experts, doctors and scientists and birders with decades of birding in the state, tell me they've never heard of such a bird, let alone seen one. I think Jay's time on the Avian Records Committee of Connecticut predisposes him to receiving reports of birds such as misidentified American Three-toed Woodpeckers. Even with that variable in mind there may be more birds like this occurring at a higher frequency than I had imagined (albeit still extremely low) not sufficiently recorded or disseminated widely enough.
A few people wrote in concerning juvenile Hairy Woodpeckers that have red replaced by yellow, but there still seem to be some problems here. Rich Huck wrote:
I happened to have a copy of "The Birds of North America, No. 702, 2002 publication on the Hairy Woodpecker. On page 19 of that publication, there is a section on APPEARANCE. Under the MOLTS AND PLUMAGES, subsection on "Juvenal plumage", the second paragraph reads: "Juvenal plumage similar to Definitive Basic (adult) plumage for each sex, but red patch extending from rear of superciliary stripe absent in males; white often tinged with a SULPHURY YELLOW (my emphasis) (Maynard 1881),....paragraph continues. The reference to Maynard is to his book: Maynard, C.J. 1881. The birds of eastern North America. C.J. Maynard & Co., Newtonville, MA. This was apparently a private publication on his part based presumably on his research.
The Birds of North American Online makes the same reference. On a similar topic Ed Kanze sent me an email with the following:
First thing I did was take a look in Sibley (2000) and there I found this in the descriptive text about the hairy: "Juvenile rarely has red plumage replaced by yellow" The accompanying illustration shows a juvenile with red on the forehead. Not sure how common adults are with yellow rather than red at the back of the head, but I imagine the same process is at work, whatever it is!
I think that is precisely it for both - the description is referring to the top of the head or crown of a juvenile bird. This area is usually covered with some thinner and less vibrant red. However, in a small number of birds, it is rarely a sulphur yellow as mentioned above. You can Google such juvenile birds and see exactly what I am describing. I do not believe this refers to traditional red patches on the back of the head. This is also a bird in January, not the end of the summer or the fall season.
I have seen some photos of Hairy Woodpeckers with a little more orange than red patches, and others with an odd shade of red seemingly because of the angle of the view. The dorsal view of a male in February in the Birds of North America Online Hairy Woodpecker account was mentioned to me a couple of times, but it looks like a typical red bird at an odd angle which makes the shade look lighter and brighter (I've now done a lot of staring at the yellow and a classic red Hairy Woodpecker male in my neighborhood and this sort of distortion happens). David Lawton mentioned to me that there "are examples of Hairies with yellowish outer tail feathers in northern California" and indeed there are! These birds have a strange and unique look to them, and while this may not have much in common, it is another example of an odd Hairy Woodpecker.
Whatever the precise answer is it seems that Hairy Woodpeckers as a whole have an intriguing habit of finding creative ways to show some unexpected yellow. Please keep the thoughts and theories coming! I'll keep an eye on my friend. Thanks again to everyone who contributed ideas and effort.