While mining out a stone block to replace with netherrack - with the intent of cooking chickens - he accidentally makes a hole in the floor, exposing the wiring of the Welcome Center's doorbell. Subsequently, an unfortunate chicken fell into the hole; Zack expressed his deep concern with a quick, "Well, see ya," and then proceeded to close up the hole with netherrack. This is seen at exactly 11:00 in the episode.
The chicken is still trapped under the floor of the Welcome Center, a survivor of the New Year's explosion. Its desperate clucks for help can often be heard when entering and exiting the building.
The Doorbell Chicken's ghostly clucks can be heard in Episode 384 - Bathroom Basilisk at around 12:39, as Zack walks through the Gay Bathhouse.