Washington Birder has an excellent website, with lots of interesting content, including checklists for each county. Washington Birder Home Page
Washington State and County Checklists - Click on the map directly for a county list.
County by county checklist, especially useful in learning where to go to find birds you haven’t seen.
Excel County Checklists - Click on “2014 Traditional, non-macro checklist.” This is nice, graphic representation of bird abundance by county.”
North Cascades National Park, prepared by Robert C. Kuntz II, John R. Douglass and Jonathan Bjorklund, North Cascades NPS Complex, Sedro-Woolley, Washington (online version provided by the Northern Prairie Wildlife Research Center).
Padilla Bay National Estuarine Research Reserve, provided by Glen "Alex" Alexander, firstname.lastname@example.org, Padilla Bay NERR 1043 Bayview-Edison Road Mount Vernon, Washington (on-line version provided by the Northern Prairie Wildlife Research Center).