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Chickens are 
level 1 critters found in many areas of Azeroth. They are particularly numerous near human settlements in the Hillsbrad Foothills, Westfall and Duskwood.
If a player targets a chicken and uses the emote "/chicken" numerous times, the chicken may eventually offer the easter egg quest [1] CLUCK!.
Other forms of chicken come in mechanical and beastly form.
Other chickens
- "Plucky" Johnson - a polymorphed human apprentice in the Thousand Needles
[Ancona Chicken] - a small pet purchased from Magus Tirth- El Pollo Grande - a giant black chicken summoned in Felwood
[Gnomish Battle Chicken] - an engineered mechanical attack chicken
[Mechanical Chicken] - a small pet reward from the quest 
[50] An OOX of Your Own- Prairie Chicken - a small pet reward from the CLUCK! quest