Hi all: I thought that I'd wade into this sticky discussion because I'm apparently a masochist in this regard. I have been intending to write an In The Scope essay (Colorado Birds) on dark Mallard-like ducks, but I've never gotten the photos that I'd like to use for the article, so it waits on simmer on the back burner. Here in southern NJ, American Black Duck (ABDU) is a reasonably common breeding bird and a very common wintering species in the back-bay marshes that are so extensvie here, so I see 1000s of them a year, though really only study a few hundred of those. Additionally, on nearly any given day, I can find a few American Black Duck x Mallard (MALL) hybrids in Cape May that fairly well cover the gamut of intermediacy between the two parental species; these hybrids are not rare here. In my experience, a fairly high minority (say 20-25%) of the American Black Ducks that I see reasonably close and in good light exhibit at least some thin, trailing white edge to the wing, with that edge often being quite thin and probably not detectable at long range. I've wondered about that fairly often -- thinking that the percentage seemed too high to me to be due to genetic introgression from Mallard, so when Mark Peterson alerted me to this thread, I called Michael O'Brien and asked him what percentage of otherwise normal-looking ABDU he sees with at least a thin, white trailing edge. He responded, "sort of a lot," which I took to mean agreement with my experience in the field. When Mark sent me a note about this thread, he also provided me with a link to a great paper concerning the subject, I delved into it, at last, to answer my own questions on the subject. This paper,
Description and Identification of American Black Duck, Mallard, and Hybrid Wing Plumage by R. E. Kirby et al. (http://www.npwrc.usgs.gov/resource/birds/diplume/index.htm) was a real eye-opener. The premise is that ABDU naturally exhibit a variable-width white trailing edge to the secondaries, generally thin, but it can apparently be as wide as that on MALL. I will allow them to tell the details: "Dorsal: Black Ducks can have white posterior to the speculum, but they never have white anterior to the speculum. White posterior to the speculum may vary from none, to a thin white edge, to that present on typical Mallards. The lack of white anterior to the speculum on the greater secondary coverts strictly separates Black Ducks from Mallards. However, Black Ducks often do have a pale gray subterminal line (terminology of Cramp and Simmons 1977) on these coverts, and on some birds this gray line is suffused with one of several shades of "brown" or "tan." The overall effect is edifficult to describe precisely, since several colors (#23 [Raw Umber], #24 [Buff], #36 [Amber], #37 [Antique Brown], #38 [Tawny], #39 [Cinnamon], #40 [Cinnamon Rufous], and various combinations with one of the Neutral Grays [terminology of Smithe 1974a, 1974b]) may be present. The result is a line of variable width adjacent to the terminal black that is distinctly not white in comparison with the white on the dorsal surface of a Mallard wing." This means that the critical feature about white in the speculum separating "pure" ABDU from MALL and ABDU x MALL hybrids, is that ABDU never have white on the leading (forward) edge of the speculum, but that they can show such on the trailing edge. The paper also cited the description of ABDU from The Handbook of North American Birds (Palmer 1976, see citation in online version of Kirby et al.), which included this quote, " ... secondaries have broad black ends, which vary from bird to bird from white at the very tip to ends solid black...." More importantly, particularly as it deals with the subject prior to the great swamping of ABDU by MALL, they referred to a description by Nuttall from 1834 that also noted that ABDU exhibited white "occurring on the terminal edge of the secondary feathers of Black Ducks...." Like Christian's comment, I have heard others state that ABDU cannot have white on the speculum, so they ID all apparent ABDUs with white there as hybrids, this when there was no way to determine that the ABDU that they saw with white trailing edges were actually hybrids. I don't mean to pick on Christian at all -- his field skills are exemplary, but that belief is widespread out here and it's easy to absorb dogma as gospel; I certainly did when I was living in NYC. But, during my first stint of living in Cape May, I began to doubt it, as I saw so many white trailing edges on ABDUs and because I realized that no one had ever been able to provide me with any proof that the belief is true, rather than simply a circular argument. Of course, no one has done the genetic testing on such birds that could help solve the problem once and for all, but the weight of evidence has me leaning strongly toward it being true that ABDU can sport white trailing edges and still be perfectly fine ABDUs. There are numerous other characters that can indicate the presence of MALL genes in apparent ABDUs, chief among them being green on the head and curly uppertail coverts in males, more-patterned side and flank feathers and orange in the bill in females, and white in the tail and on the leading edge of the speculum in both sexes. Cole's description hit most of the key points for ABDU, with the yellow bill indicating that the bird is a male. However, most of that description does not rule out two other possibilities, Mottled Duck (MODU) and Mexican Duck (MEDU), the latter being currently considered a subspecies of MALL, though recent genetic evidence suggests that MEDU is more closely related to ABDU and MODU than it is to MALL (I expect a split in the near future). Yes, the "fairly gray colored [sic] head" is indicative of ABDU, but that is the only character noted that may do the trick, making it a one-character identification, which are usually fraught with peril. In fact, the bill being described as "yellow" is at least a bit questionable, as male ABDU nearly always have some olive tones in the bill color; truly yellow more accurately describes the color of bills of males of both MODU and MEDU, both of which have dark tails and contrasting bright white wing linings. Yes, MEDU shows white on both leading and trailing edges of the speculum, which should enable elimination of that species from consideration. Yes, MODU has a buffy/peachy face that is fairly distinctive, but the species often lacks a white leading edge to the speculum. So, while I personally believe that Cole and Nick saw an ABDU at Jackson Res., whether I'd vote to accept it were I a member of the CBRC might be a different beast altogether. However, I also remember the two ABDUs that I found in CO during my tenure there -- both were with gigantic flocks of MALLs, which is the situation that Cole and Nick encountered. I don't know that that is germane to acceptance by the CBRC, but.... This venue also provides me an opportunity to step up on my soap box, the one from which I expound that we really need to nail down the IDs of the increasing number of dark MALL-like ducks being found in CO. Both MODU and MEDU are currently expanding their ranges to the north, with MEDU being no rarer than uncommon at Bosque del Apache NWR and MODU breeding in OK and KS (I believe that the latter is correct). While the CBRC has recently accepted a number of records of MEDU, we still await the first acceptable record of MODU for CO. Sincerely, Tony Leukering Villas, NJ -- You received this message because you are subscribed to the Google Groups "Colorado Birds" group. To post to this group, send email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/cobirds?hl=en.
