I know from my experience that major publishers usually split these mechanical fees with their songwriters, after all advances (staff writer 'draws') are recouped, but I didn't know if this was the norm. The answer came from Bret Teegarden, founder of the social network for professional musicians NashvilleMusicPros.com :
Rule of thumb is a 50/50 split and most publishers pay within 30 days of a Qtr end. Some are different and terms are usually stipulated in publishing agreements or single song contracts. Any recoup of advances would come out of the writers share.
Say a song earned $1000 in mechanical royalty income in the first quarter of 2011 and the writer was advanced $300 for the song. On April 30, 2011, the publishing company would ideally issue the writer a check for $200. It gets more complicated with multiple writers and publishers on a single song, but generally is just dealing with fractions. Also, royalty payments are combined for a writer's catalog of songs.This is indeed what I've experienced. Much obliged, brother Bret!
(Community rocks... if you're in Nashville doing music business, you need to be on NashvilleMusicPros.com.)