no one can live a full/joyful life if they are denied freedom
Explanation:
the author describes the outside world in the first few stanzas, and we learn that the bird beats its wings because it wants to escape the cage. the bird's song was a cry for help as it said "is not a carol if joy or glee"
It's certainly sensory. And it's figurative too. I think I'd pick figurative because the central piece of language is a simile. That's pretty good use of language when you compare the bobbing heads of flowers to helmeted soldiers.