This striking duck breeds primarily in freshwater ponds and wetlands in Western North America. It replaces the Blue-winged Teal (A. discors) throughout much of western North America, especially in the Great Basin south of the Prairie Pothole region. Its North American population has been estimated at fewer than 300,000 making it one of our least common dabbling ducks. Birds breeding in North America and Mexico are the race, A. c. septentrionalium. There are four additional races (one likely extinct) confined to South America, most of which have more black spotting on the breast.
The Cinnamon and Blue-winged Teal are closely related. In fact mtDNA is virtually identical between Blue-winged and South American Cinnamon Teal (subspecies A. c. cyanoptera). Interestingly cyanoptera has black spots on the sides and breast similar to the pattern of Blue-winged Teal suggesting a closer relationship between it and Blue-winged than there is between Blue-winged and North American Cinnamon Teal. The females of the two species are tricky to distinguish. This photo shows the dark blended face and dark belly characteristic of female Cinnamon. In the Bay Area, these birds are primarily migrants, especially in the Spring, but they also winter locally. Some authors (IOC, H&M4) place this species in the genus Spatula. Digiscoped with Panasonic DMC-LX5 | Nikon FS 3 | 30XWA | hand-held, no adapter.
References:
Carboneras, C., Christie, D.A. & Kirwan, G.M. (2017). Cinnamon Teal (Spatula cyanoptera). In: del Hoyo, J., Elliott, A., Sargatal, J., Christie, D.A. & de Juana, E. (eds.). Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona. (retrieved from http://www.hbw.com/node/52892 on 19 April 2017).
Gammonley, James H.. (2012). Cinnamon Teal (Anas cyanoptera), The Birds of North America (P. G. Rodewald, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America: https://birdsna.org/Species-Account/bna/species/cintea
Johnson, K. P. and M. D. Sorenson. 1999. Phylogeny and biogeography of dabbling ducks (genus: Anas): A comparison of molecular and morphological evidence. Auk no. 116 (3):792-805.