First off, I'm not that emotionally involved with the rights of chickens. I'm saying this so you don't read any vehemence into the tone of my writing. So don't take any offense, please, I'm just jumping around with words and ideas, alright?Anyway...
Why must chickens or any other sentient being have to serve a purpose in the welfare of humans? Can't chickens just enjoy being chickens without us having to have a use for them? One way you could reframe this line of thinking is that the weak (chickens) should serve a purpose for the benefit of the powerful (humans). You could extend this line of thinking beyond chickens with barbarous results. What purpose do people who are seriously mentally handicapped serve? How about the homeless? What if science uncovered a method for extracting the vitality out of them and transferring it upon the wealthy? If the weak have no purpose but to serve the powerful, then there should be no complaints. But let me assume that someone will raise the objection that chickens are a different species than humans, and a vastly stupidier one at that. Alright. If another life form, that dwarfed our intelligence landed on earth, discovered that human brains were the most incredible delicacy in the entire cosmos, why should we be any different than chickens? This hypothetical race of beings could experience love and thoughts so much deeper and richer than our capacity, that humans would by comparison be closer to an earthworm than a noble chicken.
Once again, this is all just for the sake of discussion. I enjoyed a rather delicious buffalo chicken sandwich for lunch today without any thought to the relationship of my power to the chicken's.