This fall's unprecedented and incredible showing of Common Eiders has enabled me the opportunity to really study plumages of the species, as one can get quite close to them as they loaf and forage near shore at Cape May Point, particularly near St. Mary's. Last night, Dave Czaplak and I pored over my 100s of photos of the birds attempting to make sense of the great variation in plumages. We referred to a variety of field guides, but primarily used the Identification Guide to North American Birds, part II by Peter Pyle (2008, Slate Creek Press; Note: I have no financial interest in the product). Though the two volumes of Peter's tour de force is primarily a guide for banders, I consider it an indispensible tool for birders.
First up in our efforts to understand the bewildering variety of plumages is to understand the timing and progression of molt in the species. Turning to pg. 128 in Pyle, we find that young-of-the-year conduct a partial pre-formative molt away from breeding grounds during Oct-Mar. As juvenile males are similar in plumage to that of adult females, this is a critical point. Also on that page, we find in a boldfaced note that (and I paraphrase here) some juvenile males don't initiate this molt (or, at least, obtain some white feathers) until Mar-Apr. In combination, these two pieces of information let us know that while some juvenile males have started down the road that makes them, plumage-wise, obviously males, some may not have and that we have to be careful at assigning all the juvenile birds as females.
Looking directly at plumage now, juvenile females are described as having "head and body pale brown"while both adult females and juvenile males have "head and body dark brown." This, then, would suggest that this bird is a young female.
[Click pictures to view a larger image size.]
If we look at the inner greater coverts (gc) and inner secondaries (ss), we can see indistinct whitish tips, a feature of juvenile/immature females; juvenile/immature males should show virtually no white here. I do, however, have some concern about the two worn nearly-white feathers on the back that might indicate some other age and/or sex, but I cannot reconcile all the other features with anything but a young female.
The next bird is darker-headed and -bodied and shows substantially more obvious white tips to the inner greater coverts and secondaries. These two features make me quite confident that this is an adult female.
I will treat the ageing of males, a much more complex aspect of this task, in a day or two. Right now, the sun has finally come out (after 5.5 days of gray) and I've got more photographs to take! Also, if I've gotten any of this wrong, please feel free to drop me a note and let me know, as Im not at all an expert in this!
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