Explanation: it is a bird and birds are supposed to fly free in nature. To aggravate this even more the cage is narrow which gives the feeling of suffocation.
It could be many things. It could mean a loss of freedom. It could mean anger at being where he is. It could be anything but something pleasant and positive.
He's not indifferent. He's stalking. He doesn't like it there.
He doesn't find it monotonous. If he did, he wouldn't be stalking. And he wouldn't find the cage narrow.
He is anything but satisfied. That would be far too positive.
