John David "J.D." Souther
Eagles writing/co-writing credits
The Best of My Love (with Frey and Henley)
Doolin-Dalton (with Jackson Browne, Frey, and Henley)
Funky New Year (with Seger, Frey, and Henley)
Heartache Tonight (with Seger, Frey, and Henley)
James Dean (with Browne, Frey, and Henley)
Last Good Time in Town (with Walsh)
New Kid in Town (with Frey and Henley)
The Sad Cafe (with Frey, Henley, and Walsh)
Teenage Jail (with Frey and Henley)
Victim of Love (with Frey, Henley, and Felder)
You Never Cry Like a Lover (with Henley)
Involvement with the Eagles
J.D. Souther, most famous for the hit "You're Only Lonely," was first introduced to Glenn Frey in 1968 through his girlfriend. Glenn was dating her sister (read more about the sisters here). They hit it off and before long they had become roommates and formed a duo named Longbranch Pennywhistle. After that group's failure on Amos records, they were signed by David Geffen - but not as a duo. Souther was made a solo artist and Glenn was told to go find a group to join.
That group became, of course, the Eagles. Don Henley also knew Souther from their favorite bar/hangout the Troubadour and he and Glenn considered making Souther an Eagle as well, but that didn't pan out. Regardless, they continued to collaborate with him and he was co-writer of several of their hits. See the entry on Jackson Browne for the tale of how the guys wrote James Dean and Doolin-Dalton.
Glenn appeared and sometimes co-wrote songs on Souther's albums John David Souther (1972), Black Rose (1976), and You're Only Lonely (1979). He also appeared on the Souther-Hillman-Furay Band album Trouble in Paradise (1975). The song Bad Man, performed by Randy Meisner on the FM soundtrack (1978), was co-written by Glenn and Souther.
Surprisingly, post-Eagles, it was Don whom Souther collaborated with rather than Glenn. On Building the Perfect Beast, he co-wrote Man with a Mission. A few years later, he co-wrote Little Tin God, If Dirt Were Dollars, and The Heart of the Matter off of The End of the Innocence.
Read an interview with Souther where he talks a bit about the Eagles here.