Well, some fine guesses on this one: Dennis himself thought Bufflehead, initially, except for a wing chord of 22cm–certainly too large for the diminutive Bufflehead, which averages 14-18cm. This is if Dennis’ wing chord measurement is to believed, which it is, given his consistently accurate measurements.
So, some other possibilities must be entertained. Diana Gaumond suggested American Oystercatcher, which certainly does sport stark black and white wing plumage. But the Oystercatcher’s white wing patch extends to cover more of the upper wing than our specimen. John Stanton asserts Red-breasted Merganser, hedging with Bufflehead if the specimen were too small to be a Merganser. Well indeed, the 22cm measurement places Dennis’ specimen squarely in the Red-breasted Merganser range. Dennis expressed some trepidation over this identification, as he did not detect much of a black wing bar within the white wing patch, which is usually rather striking in the merganser. Agreed, the bar is not extremely prominent in this bird.
I will offer some side by side shots of our specimen and the Red-breasted Merganser (photos borrowed from the excellent University of Puget Sound’s web archive of spread wings). Any thoughts? Please also check out the other images at the site and suggest any other additional ideas. We, here at SEANET, learn a great deal from these discussions, not being experts in this field ourselves, but being exclusively self-taught.