An act of eroticism or love where one partner regurgitates recently-ingested food directly into the mouth of the other partner. Meant to be a sign or symbol of extreme affection. The practice is derived from the nurturing of a mother bird for her hatchlings, where she chews up her food and spits it directly into the mouth of her babies to nourish them while they are still unable to feed themselves.
Friend: "Last night after dinner, my girlfriend and I did birdie style."
Me: "You must really love each other."