Larry, et al.,
I've observed this before in BWTE at Eagle Bluffs CA. It is a striking feature. I wanted to learn more about it.
Fortunately, I have a copy of THE BLUE-WINGED TEAL, Its ecology and management, by Logan J. Bennett, Colletiate Press, Inc., Ames, Iowa 1938. This book is based on Dr. Bennett's Ph.D dissertation research.
Below is what I found.
Kennard (1919) described a sub-species of the Blue-winged Teal. The promulgated name was Querquedula discors albinucha. The basis for the description was the white crescent of the male in nuptial plumage extending over the eyes and joining at the back of the head to form a white nuchal spot in birds that apparently nested in the southern part of the United States. This subspecific determination was not accepted by the American Ornithologists' Union (Oberholser, 1921) on the basis that the evidence presented indicated a difference of age or a color phase. The author examined several hundred specimens from Mexico and the United States. It was common to find specimens with the white crescent extending back over the eyes and joining. On some specimens the feathers had to be parted before the few white feathers extending backward could be seen. On other specimens the markings were quite distinct. The author confirms the opinion of the American Ornithologists' Union that the presence of the white nuchal spot is merely a variation in plumage coloration.
Now, let's find a Garganey in Missouri!
Bodacious birding to all,