I think I better explain if for the people who don't understand the video...

The Chicken looks at the castle when Theresa says "freedom is nothing but a dream", the chicken is thinking of being free like the birds flying by the castle.

Then Theresa says "It's time to make a stand" and the Chicken does so against the oppressor - in this case the Chicken Salesman. He 'leads the rebellion' by escaping.

Next, Theresa says "the city of the downtrodden." This is represented by the chicken as he gets literally trodden on the ones who work for the guard, or the Monarchy.

Afterwards, Theresa says "those who speak out are punished". This is shown by the dude with the 'traitor' badge on, and then the chicken represents it by being shot itself.

Then a rather important part - Theresa says "Those who dare to hope find nothing to hope for". This is shown by the chicken flying, almost reaching his wish - his dream, but then he falls and is returned back to the working life (shown by the factory).

Just before the end, Theresa says that 'true rebels fight against all odds', and then the Chicken is shown dodging cleavers and other knives and such and then he doesn't give up as he carries on running.

Lastly, Theresa says "they cannot triumph alone" and then it shows other birds - who don't help the chicken. He is then killed because... he was alone.

So the chicken is an image for a rebel, leading the rebellion against the Industry and such... Poor Chicken ;(