How did Dave find talking chipmunks?

In the cartoon from the 80s they were abandoned on his front doorstep, and he adopted them. i don't know their origin in the movies. I just remember when my sister used to watch the TV show. This detail stuck with me for some reason. I'll check to see if I can get an episode number and air date...

update(according to wikipedia):

"A Chipmunk Reunion" (1985) Written by: Janice Karman & Ross Bagdasarian Co-Written by: Cliff Ruby & Elana Lesser When they cannot determine when their birthday is, the Chipmunks set out to find their mother, Vinny, who abandoned them as babies.

Songs: “Mother and Child Reunion” originally performed by Paul Simon, "Vinny's Lullaby" (owing to Dumbo's "Baby Mine") "Vinny's Blues", and “Shake It Up” originally performed by The Cars.

Broadcast April 13, 1985

First appearance of Vinny, the Chipmunks' mother. In this appearance, she is voiced by June Foray (uncredited).

This special was syndicated as a regular episode.

It is revealed that the Chipmunks' birthday falls on the 14th. Prior to the time the Chipmunks found out, Dave always celebrated their birthday around the time they met.

Was released on VHS along with The Chipmunks Story in The Chipmunk Family Tree.
Originally aired as a primetime special.

Part of the DVD "Alvin and the Chipmunks – The Very First Alvin Show".