When a girl is finished giving oral sex, and the man ejaculates into her mouth, she spits it back into his mouth, much like a mother bird spits up worms into its babies mouth to feed them.
Somewhat like snowballing.
David likes it when I start motherbirding in his mouth. Davey like it.
When you performe the act of fellatio on a male, he then ejaculates into the mouth of the sexual partner. From this point the partner of the pleasure receiver contains the semen in their mouth and spits or kisses the semen in his mouth. Hence mother birding.
"I'm think of mother birding Josh to spice up the relationship" "I mother birded Dan and he totally loved it"