After consulting some more references, I would agree with Sally Hill's assessment that this bird is most likely a PACIFIC GOLDEN-PLOVER. I had the National Geographic field guide with me at the time and the bird looked intermediate between Pacific and American as far as the pattern of white on the breast, flanks and undertail coverts. I thought the white stripe did bulge forward on the sides of the breast as in the illustration for American and the undertail coverts looked more than half black. National Geo said that the undertail coverts should be mostly white on Pacific. Looking at photographs and illustrations later, the white pattern I saw most closely matches Pacific. The head and bill shape also indicate Pacific. I really couldn't see primary projection. The wings appeared just slightly longer than the tail to me, which also would point toward Pacific. Some illustrations I looked at showed Pacific having quite a bit of black in the undertail coverts, which is what I saw.