At one time The Byrds were considered the "American Beatles" – my inclination is to split that title with The Beach Boys...
Though he was an accomplished folk singer and guitarist, Chris Hillman was a novice when The Byrds took flight in 1965 and was usually replaced by studio players, most notably Larry Knetchel, for recording sessions which required a skilled bassist.
Hillman’s successor, John York, who had previously worked with the Mamas & the Papas among others, was a vast improvement harmonically and rhythmically. York appeared on Dr. Byrds & Mr. Hyde and the classic Ballad of Easy Rider, both in 1969, which was his sole year of service to bandleader James Roger McGuinn.
The band’s third and final bassist, the late Clyde “Skip” Battin was considerably older than his band-mates with an impressive career that stretched back to the 1950s. Skip contributed a far more fluent rhythmic expertise to The Byrds influential canon as evidenced on the flawed masterpiece Untitled (1970) and the archival release Live at the Royal Albert Hall 1971. Battin also waxed several albums under his own name, and was a member of various incarnations of the New Riders of the Purple Sage and the Flying Burrito Brothers.
Note: Once again, the Rock ‘n Roll Hall of Fame blew it big time by not including Messrs. York, Battin, Clarence White, Gene Parsons, Kevin Kelly, and Gram Parsons when The Byrds were enshrined in 1991.