I am on the "wait and see side" of the fence. No one has yet proven anything, only having made a lot of well-worded and/or psuedoscientific statements.
The whole "chickens have 'chickeny emotions' which we can't understand (degree of emotion)," argument is open - I feel the jury is still out on that popularly vague, New Age fall-back. It's just like UFO's: I've never been in one, or even seen one, but I admit that the commonly held logic behind their existence has merit. Until one lands on my lawn, though, that is how it must remain.
Your statement about "no one having proven anything" is accurate to a fault; indeed no one can prove the existence or nonexistence of anything in the universe.
Any argument posed with the intent of proving or disproving anything, must rely on a set of assumed variables that are mutually or seemingly mutually understood as "true"
"I think therefore I am"
It's cliché' and yet, for your argument it is appropriate to point out that the only thing knowable is, ones own consciousness, anything else is subject to debate.
The question is, how far do we go in our attempt to determine, for our own reality, that which we accept as true and that which we decide to reject.
Since nothing can truly be known, we are left with the evidence that seems to exist within the physical universe we live.
If we assume that the universe that we perceive is in fact true, then it follows that we can not pick and choose to discard that which appears to be true when it serves our own purpose.
The majority of the evidence, that can be observed, suggests that chickens show emotions, to whatever extent is irrelevant.
If we are to assume that chickens exist, and we know that there is similar functionality within the brains of both chickens and humans, then it stands to reason that chickens would have emotions.
However, I can not prove the existence of chicken emotions, any more then I can prove the existence of you, and therefore, I shall have to accept that both exist or resign myself to a universe that cannot be accepted, for that which my perception of it confers to me.
Have a nice day.