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Intersex -- The condition of being intermediate between male and female
A few weeks back, a birding acquaintance of mine sent me photos of a Wood Duck that he had photographed at Dawson Corporate Park in Hillsboro, Oregon about 15 miles west of downtown Portland. The duck displayed a plumage that left him scratching his head. In his email he wondered, "is this a young male or maybe a hybrid?" It was neither. Almost immediately, I recognized that it showed a mix of both female and male characteristics, which led me to conclude that it was probably an intersex Wood Duck.
Intersex ducks are fairly well known and if you poke around you can find a fair amount of information and many photos of intersex birds on the Internet. However, in one aspect, gender-challenged ducks are different from other intersex birds.
In most bird families, the female plumage aspect is the default appearance. Like humans, the production of sex hormones wanes in birds as they start to age. This results in males taking on a more female-like appearance as they go through annual molt cycles. In ducks, it's the exact opposite, with older females gradually taking on the appearance of the male.
I visited the Dawson Creek ponds on 15 November 2013 and refound the bird. While there, I ran into another local birder, Steve Nord, who told me that this individual has inhabited this location for about five years. It's hard to know if there is a typical age when the transition starts to occur, but the account below, excerpted from a 1992 Dutch Birding article, chronicles the transition of a hand-raised female Mallard.
On 12 August 1977, four deserted Mallard Anas platyrhynchos ducklings were found and subsequently hand-raised [and ringed; RvdV]; they fully matured and lived for many years as garden pets in Goes, Zeeland. After c 10 years, one of the females gradually started to show male plumage and bare part characters. Within the group (now consisting of seven birds), however, the female still behaved as a females\ and was also treated like a female by the other ducks (which usually is not the case). The bird died on 3 September 1991. Dissection revealed that the left ovary and part of the oviduct were cysteous, possibly as a result of age. The right ovary, however, was still in its normal embrynic state. Birders are regularly confronted in the field with ducks that show incompletely developed adult male plumage characters. Although these are often moulting or juvenile birds, there is a possibility that they are females showing male plumage and bare part characters due to an ovarian abnormality." (http://fog.ccsf.edu/~jmorlan/feb00.htm)
Taking a closer look at the ducks at your local duck pond is likely to reveal a Mallard or perhaps another duck that exhibits the appearance of an intersex bird. Look for birds with male plumage and a female bill pattern. Over the past two years I've seen at least two presumed intersex Mallards. Both had some green on the head, some gray on the flanks and yet the bill was a mix of orange and black typical of a female, rather than a solid yellow-green straw-colored bill of an adult male. Several examples of presumed intersex Mallards can be seen at the link below:
Post J. N. P. and E. J. O. Kompanje 1992. Uitwendige geslachtsverandering bij vrouwtje Wilde Eend. Dutch Birding 14 (4): 131-134.