Brent, on any given tree the berries are in different stages of fermentation as long as there's sugar left in the berry. Fermentation takes place not just on the surface but throughout the berry. It doesn't take much for a robin to get snockered.
In reading James Lee Burke's Robicheaux series set in Cajun country Louisiana, there's often mentioned a pot of robin gumbo in small knife and gun clubs (bars) for patrons' consumption.
Gil