Tuesday, February 4, 2014

American Black Duck

Having errands to run in Northfield on Monday, 3 February 2014, I looked over the flock of some 350 Mallards loafing about the only remaining patch of open water on the Cannon River. eBirders have reported an American Black Duck from the river and local lakes all winter. I found the duck almost immediately.
American Black Duck field marks include an olive bill, unlike the orange and black bill of a female Mallard. Black Ducks have very dark sides and lack the Mallard's buffy breast. Most similar are eclipse male Mallards, which also have olive bills. But Black Ducks do not have buffy breasts and have much darker sides. The situation becomes more confusing since the two ducks often hybridize. Hybrids, however, often have white in the tail, curly upper-tail coverts, and greenish on the head (see my previous post on these hybrids). The shiny speculum on the bottom photograph seems a bit blue to me, but iridescence color is always an unreliable field mark. The single white line trailing the speculum is occasionally seen in American Black Duck.
American Black Duck populations recently severely declined. Between the 1950s and 1990s, numbers more than halved. Habitat destruction on wintering grounds played a role in the decline, as did heavy hunting pressure. In the 1980s, both the United States and Canada placed hunting restrictions on the killing of American Black Ducks and numbers increased. Hybridization with Mallards does not help the situation. Hybrids may account for over 13% of birds taken along the Atlantic Flyway—but this number is just an estimate, since backcrosses can be indistinguishable from pure American Black Ducks or Mallards. Longcore et al. (2000) warn, “hybridization of an abundant species with a less abundant and declining one can lead to genetic extinction of the species with the smaller population.”  These authors are not overly concerned about hybridization in this case, because the Black Ducks’ numbers are increasing and because the ducks’ 
range is not small or fragmented. 

Considering all of this genetic mixing, I am not sure why American Black Ducks and Mallards are considered to be separate species.  I did note, however, that often, when this Black Duck swam close by the male Mallards, the drakes aggressively chased the Black Duck away. 

2 comments:

  1. Hi Dan, am just downloading some photos that I took on the southern shore of Nova Scotia, where I see about 45 American Black Ducks daily. Today I noticed that there was one that an extremely what you call buffy breast. Have never seen this before, and wonder what happened to this duck. Would be happy to send you a photo if you wish.

    Thank you,

    Christine

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    1. I would enjoy seeing your Black Duck photo…can you post it on a website? dan

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