In chapter 13 of the novel, Hannah notices in the sky above her "a quartet of swallows ... twittering madly." In chapter 18, she notices them again and is momentarily transfixed by them. She watches them flying in circles overhead, dipping occasionally to the ground to catch bugs, and she thinks to herself about how these swallows simply carry on with their ordinary lives, regardless of and utterly indifferent to the atrocities being committed in the camps immediately below them.
Looking up at these swallows, Hannah thinks that it is "as if all nature ignored what went on in the camp." This thought, that nature is completely indifferent to the sufferings of humans, is a profoundly depressing thought, and it emphasizes the hopelessness and loneliness of prisoners like Hannah.
On the other hand, the swallows also symbolize the basic and essential human yearning for freedom. Birds in general often symbolize freedom, and it is perhaps ironic that Hannah and her fellow prisoners must look up at the birds and be reminded of the freedom which they do not have. The swallows fly overhead, just out of reach. They symbolize the natural free state of all animals, but they also remind us of what Hannah and her fellow prisoners are cruelly being denied. The prisoners watch the birds circling overhead, just out of reach, and are reminded that their freedom, likewise, is out of reach for them.