|Intergrade drake American x Eurasian Green-winged Teal. Tijuana River Valley, California. January 18, 2015. Greg Gillson.|
The Eurasian form of Green-winged Teal "Common Teal" breeds as far east as the Aleutian Islands of Alaska. While most of this population winters southward in Asia, some individuals are regular vagrants that are found annually in winter south to California amid flocks of American Green-winged Teal. Additionally, intergrades (if one considers them subspecies, or "hybrids" if one considers them the same species) are equally frequent--rare, but if you look, you can often find some annually.
Drakes of the American form have a vertical white bar on the side of the breast. The Eurasian form lacks this vertical bar and instead has a white horizontal bar at the edge of the wing. Additionally, the green eye/ear patch on the Eurasian form is surrounded by bolder white lines.
I photographed an intergrade at the Dairy Mart Road "Stick Pond" at the Tijuana River Valley recently. The out-of-focus documentation photo is above. It shows both the vertical and horizontal white bars, and the face markings are midway between the American and Eurasian populations.
Field guide author and artist David Sibley wrote an identification piece and drew artwork on his blog in 2011 about the two forms and intergrades between the two. If you look at his references, he used my earlier photos and discussion from my web site in 2004. Pretty neat!