Saturday, February 5, 2011

Immature male diving ducks

[How many male Buffleheads are in this picture? See below for the answer. Click on picture(s) to see larger version(s).]

This essay deals with molt, so all those true moltophobes, skip down the blog now. However, if you read on, you might learn something that will make you a better birder, so, please, stick around.

‘Tis the season for birder confusion over the age, thus the sex, of individual diving ducks. Unlike most dabbling-duck species that we find in winter in New Jersey, many immature males of various diving-duck species delay transition from their female-like juvenile plumage to male-like plumage. This delay can be short or quite long. The Cape May birding community will probably long remember the “female” Hooded Merganser that summered at the Cape May Migratory Bird Refuge in 2008 that molted into male plumage in July-August of that year!

First off, a quick synopsis of the nitty-gritty of plumage transition. These ducks leave their natal grounds (waters?) in mostly Juvenal plumage and transition to a more adult-male-like plumage (Formative plumage) via their pre-formative molt, which is conducted during fall migration and/or (primarily) winter. In this essay, the timing of the pre-formative molt in the various diving-duck groups are presented in month spans (e.g., Nov-Mar) within the genus parentheticals, and these data are taken from Peter Pyle’s Identification Guide to North American Birds, part II (2008. Slate Creek Press). Note that all adult males of all of these species will have attained Definitive Basic plumage (colloquially known in ducks as ‘breeding plumage’) before arriving on winter grounds. To wit, they will look like males! Beware, though, that a not-insubstantial minority of male eiders and Harlequin Duck are diagnosable as sub-adults in their second winter, as they do not quite achieve Definitive Basic in their second fall. Finally, beware that some older females of many of these species can obtain some male plumage traits, with older female Surf Scoters being particularly prone to this phenomenon and molting in white or whitish nape patches.

Bay ducks (genus Aythya, Sep-Mar) do not typically delay very long, with virtually all immature males sporting at least some obviously male-like plumage before December.

Eiders (genus Somateria, Oct-Mar;’ we’ll skip Steller’s Eider, which is in a different genus) are quite variable in this regard, with many immature male Common and King eiders exhibiting, at least, white chests by December; however, a largish minority delay onset of their pre-formative molt until mid-winter.

Harlequin Duck (genus Histrionicus, Sep-Mar) molt timing is more like that of the bay ducks, in that immature males have molted enough before January as to be obvious.

Scoters (genus Melanitta, Oct-May) are like eiders in their molt strategy, with most immature male Surf Scoters and many immature male Black Scoters diagnosable as such before December (White-wingeds presumably follow this pattern, too, but there are many, many fewer of them here in Cape May to study).

Long-tailed Duck (genus Clangula, Nov-Mar) can delay their pre-formative molt (or, at least, obvious aspects of their pre-formative molt) fairly long, though the bills of most immature males start sporting pink by December, whether their plumage has changed appreciably or not.

Goldeneyes and Bufflehead (genus Bucephala, Aug-Mar) are quite variable in the timing and speed of the pre-formative molt, with most immature male Common Goldeneyes exhibiting at least something of the white loral patch by November, but with many (most?) immature male Barrow’s Goldeneyes and Buffleheads not being easily diagnosable as males until January or later.

Mergansers (genera Lophodytes and Mergus, Aug-Mar), like the members of Bucephala, are quite variable in pre-formative molt timing and speed. I have already noted the truly laggard Cape May Hooded Merganser example, above, and, despite Pyle (2008), many immature male Common and Red-breasted mergansers are still sporting primarily female-like plumage well into spring. Such birds can be sussed by the dark green patch around the eye that many seem to obtain before any of the rest of the male-like formative plumage is attained.

Here is a picture of an immature male Lesser Scaup from San Diego Co., CA, on 28 December. Notice how the bird's eye color has already made the transition from muddy yellowy-brown to bright yellow, despite that the plumage has only just started to change. We can see that it has some adult-male-type scapulars and side feathers, but that more than a quarter of its head and all that we can see of its chest are still clothed in brownish female-like Juvenal plumage.

And here is our opening picture, again.

So, how many male Buffleheads are represented in this picture? The best answer to that question is '2 < x < 5' (that is, more than two, fewer than five). This is because there are certainly three, but there might be four. The bookend individuals are both adult males, with crisp black-and-white plumage and with the white wing patch extending from leading edge to trailing edge. The second in line is also obviously a male, but it's also obviously not an adult, as its white head patch still has a bit of dark Juvenal plumage mixed in and it still sports its Juvenal wing plumage.

The third bird is the problematic one. Almost any birder will identify this bird in the field as a female without giving it a second thought; and in May or June, that birder would be right. However, recall that timing of the pre-formative molt in immature males is individually quite variable and though this picture was taken on 4 February (at Sunset Lake, Wildwood Crest), this might still be a male. In fact, looking more closely at the greater coverts (that tract of feathers that forms the narrower band of white in the wing), the extent of the white in those feathers is in the overlap zone between adult female and immature male (see Fig. 98, pg. 144, in Pyle 2008), but on the extensive end of that overlap, which might suggest that the bird is an immature male. There's just no certain way to tell, short of getting the bird in hand or incredibly-better pix of the bird than this.

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