When all else fails, find some dead alcids to look at...

http://home.pacifier.com/~mpatters/obrc/dead_alcids.JPG

Most guides show Cassin's Auklet with a very gray breast and
belly (including Sibley), but if one has enough photoguides
(enough in the relative sense, one can never have enough in the
absolute sense), it turns out that white-bellied Cassin's
Auklets are pretty common.

It's always important to start by assuming that whatever it is,
it's probably common (I think that's Paulson's law) and work
toward rare. I still feel fairly confident that yesterday's
bird was a Xantus'. I saw it well. It was not hooded and had
a clean white throat. The only other possibility is a basic
plumage Long-billed Murrelet that wasn't showing the white
scapulars (how likely is that? I don't know).

I think I let myself get swept up in the moment today. The
birds seen today were almost certainly white-bellied Cassin's
Auklets, though there were several gray-bellied birds (just
like the one's in the field guide) which helped me justify my
delusions.

--
Mike Patterson Alas, to wear the mantle of Galileo,
Astoria, OR it is not enough to be persecuted
celata at pacifier.com by an unkind establishment,
you must also be right.
---Robert Park
http://www.pacifier.com/~mpatters/bird/bird.html