If your contention is that chickens are not sentient, can you provide your own definition so that your perspective might be clear? Are you suggesting that birds in general lack the capacity to feel, or is this something more specifically in regards to chickens?
The ability to perceive things subjectively as an entity with self-awareness.
There would be more evidence that chickens are sentient than not using that definition. The alternative viewpoint essentially would suggest that chickens operate rather mechanically, and any demonstration of self-awareness, or indeed, their documented capacity to suffer, is merely a reflection of the human observation rather than the chicken itself. Given that other birds are self-aware beyond any reasonable doubt, I suspect you will have a hard time finding any scientific evidence that suggests that chickens are unique among birds in somehow lacking the ability to feel.