Law enforcement sources tell us ... Birch was found around the corner from his home, dead from a gunshot wound to the head. We're told all signs point to a self-inflicted wound.
We're told authorities have not found a suicide note and there were no apparent signs of drug use.
Birch has been playing with Elton John since 1992, and was also involved in the biggest single of all time, "Candle in the Wind 1997."
Birch has also performed with legends like Tina Turner, Stevie Wonder, Eric Clapton, John Mayer, Cher, Sting, The Backstreet Boys, The Spice Girls, Clint Black and many others.
Here's Birch with Elton last year on David Letterman -- one of their final live performances together.