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In 1973 Birk began researching the fur trading fort along the Snake River long identified as "Connor's Post" after local fur trader Thomas Connor. Birk's research disproved that Connor, who was illiterate, could have authored the journal that described the construction of the fort, indicating instead the British North West Company trader John Sayer as author. In 1976 the MHS updated the site's name to the North West Company Post in recognition of this fact. Birk's research culminated in a nearly 500-page report on "the history, ecology, and archaeology" of the fort site published in 1980, by far his most ambitious publication to that date.

In 1989 Birk published a re-edited transcription of Sayer's 1804-05 journal, part of a series of regional history publications for the Cross Lake Association. He returned to Sayer again for his Master's thesis, completed in 1999, which incorporated much material from his 1980 publication. In 2004 Birk published a short treatment of the same material called "The Messrs. Buid Comodiously," written for a popular audience. Birk turned to Sayer once again in 2014 for an improved and expanded version of his thesis to be published as "Life at Sayer's Fort," a manuscript left finished but unpublished at the time of his death.