Undead Fortitude:  If damage reduces the zombie chicken to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit.  On a success, the zombie chicken drops to 1 hit point instead.

The Flocking Dead:  If there are 6 or more zombie chickens in the flock, each creature within 30 feet of the flock and aware of it must succeed on a DC 12 Wisdom saving throw or be frightened for 1 minute or until the number of chickens drops to less that 6.  A creature can repeat the saving throw at the end of each of its turns.  If the saving throw is successful or if the condition no longer exists, the creature is immune to the Flocking Dead for 24 hours.

Peck. Melee Weapon Attack:  +2 to hit, reach 5 ft., 1 target. Hit: 3 (1d4 + 1) piercing damage.  If pecked, target must succeed on a DC 11 Constitution saving throw or be infected with Cackle Fever within 1d4 hours.