>Is it safe to assume that dowitchers in northern Idaho and eastern
>Washington are Long-bills?


No. At least not in Benton County WA where I say Short-billed Dowitchers
are occasional; not rare.
I've seen about ten so far this summer, most were ID by call.
All were either at the Yakima River Delta or on the Umatilla NWR.

I'll let other's talk about plumage, but one thing you might look for is:
Short-billed's seem to prefer "dryer" parts of the mudflats than
Long-Billed.

A Short-billed Dowitcher was at the Yakima Delta late August 28.
It called almost constantly for the 20 minutes we were there.
I thought that strange. It was getting dark & I assume it was about to
migrate.
Didn't see it today Aug 30. About ten Long-billeds were there.
Also a FRANKLIN'S GULL was there today for a short rest before heading south
about 4 pm.

Good Birding,
Craig

Craig Corder
Hermiston, Oregon, USA
Umatilla County in north/central/eastern Oregon
ccorder at eoni.com