Zone Zero is the Outertale equivalent of the True Lab and therefore is only accessible during a True Pacifist Route. It is a mysterious pocket dimension accessible through a portal in Alphys' lab 'toilet', and is the only place outside of the Ebbot Quadrant that can be accessed from within the EM field. Zone Zero is where Alphys conducted her DETERMINATION experiments.
Crows appear in Zone Zero only. It is not known whether these birds are a result of the DETERMINATION experiments or naturally occurring. They have a chance of appearing when Frisk enters any scene in Zone Zero, and fly away if walked too close to. While flying, faint flapping noises and chirping will play so long as they are on screen.