Excerpt from: Don Roberson "CREAGRUS" website, 2011, The Creagrus California Counties Project, San Francisco County, www.creagrus.com, web posted to his site in 2006:    ... link to website: http://creagrus.home.montereybay.com/CA-SF.html "... Note that the left-hand coot has an entirely white shield [note, image not shown here], a character once thought to identify Caribbean Coots in North America. The late Luis Baptista and I published a paper [Roberson & Baptista, 1988, White-shielded coots in North America: a critical evaluation. Am. Birds 42: 1241-1246] debunking this theory and all U.S. claims of "Caribbean Coot." Rather, male American Coots with excess testosterone may possess the all-white, high shields that match Caribbean coots. The San Francisco coot was one of the ones we featured in our article. ..." Excerpts from: Roberson, D., and Baptista, L.F., 1988, White-shielded coots in North America: a critical evaluation: IN: Am. Birds 42: 1241-1246:    ... link to SORA Searchable Ornithological Research Archive, scroll to article: http://elibrary.unm.edu/sora/NAB/v042n05/ ... or directly to report (800k, 6 pages): http://elibrary.unm.edu/sora/NAB/v042n05/p01241-p01246.pdf, National Audubon Society Field Notes, v.42, no.5, winter 1988. "... In November 1983-January 1984, we surveyed coots wintering on selected small ponds in California. For the most part these were ponds where we had found atypical coots in previous years, though half were selected without knowing what was present. We rather arbitrarily divided the shield characters observed into five categories: A = Normal American Coots with typical red callus. B = "Intermediates," with red callus but washed with yellow on shield below callus, some having a very reduced callus. C = Bulbous shields strongly washed yellow sometimes with reddish splotches. D = Bulbous white shields, with obvious reddish spots, streaks, or splotches, but not a callus. E = Bulbous white shields with little or no reddish staining, limited (if present) to extreme upper edge of shield. Although there is some subjectivity in assigning birds to these categories, they provide a convenient means of quantifying the frequencies of the morphs in various populations sampled. ..." Article of interest, "The 'Caribbean' Coot in North America", IN: Sibley Guildes, Identification of North American birds and trees website, 2011.