Months before skyrocketing to fame on the cast of "Glee," Mark Salling was a tree-hugging bird-loving acoustic-guitar-playing hippie-dippie park squatter -- and we got the footage to prove it!

You see, Mark -- an outspoken bird advocate -- is a member of the Echo Park Ornithology Club in Los Angeles.

And three years ago, right before Mark struck it big on "Glee" -- he expressed his pride in the animal organization by whipping out his guitar at a park meeting ... and jamming out an EPOC theme song he wrote.

You probably wouldn't guess from the lyrics -- but the song's called "Echo Park Ornithology Club."