A Robin Red breast in a Cage Puts all Heaven in a Rage.
How many birds are in cages all over the world? The answer is: a lot. It's a pretty everyday sort of thing. Most people probably don't think this is particularly horrible, though plenty of animal rights activists would beg to differ. One on the one hand, the speaker seems to be on the animal rights' people's side—but he also might have a little more going on than an enthusiasm for the rights of birds.
The caged bird is a tiny example of injustice—of someone wrecking someone else's freedom. But the speaker argues that these tiny injustices have bigger, cosmic consequences. That's why heaven itself gets in a rage: a small example of freedom being crushed is really an example of freedom getting crushed everywhere, on a global or universal scale, and with human beings and not just animals. It's like what Martin Luther King Jr. said, "An injustice anywhere is an injustice everywhere." Our speaker couldn't agree more.
Heaven in this case is made up of the powers that control the universe—a higher, divine world, not the sky-and-clouds heaven.
The rhyme here directly connects heaven's "rage" with its source, the "cage."
Also, now we're in full iambic tetrameter mode at this point. For more on this, see the "Form and Meter" section.
A dove house fill'd with doves & Pigeons Shudders Hell thro' all its regions.
Basically, we're still in the same territory as in the last lines. Birds are being imprisoned—unjustly. But this time, it's not heaven that's being upset—it's hell.
When Jesus dies during the crucifixion (in the Gospel of Matthew's telling of the story) it causes earthquakes to break out and it opens tombs. Blake isn't necessarily making this reference when he talks about the imprisoned doves causing hell to shudder (since hell is, metaphorically, under the earth)—but he is suggesting that suffering (any kind of suffering, whether it's doves, robins, dogs, people, or whoever) causes the same kind of universal reaction. It's a diss to Mama Earth herself, and even hell is disturbed.