One way to approach a question like this is to consider some of the central themes of the story and which quotes help to present those themes. One of the central aspects of the story is the way that human life is compared to bird life. Note the way that Michael's Dad repeatedly refers to his baby sister as "Chick," and also Michael and Mina's efforts to care for Skellig are paralleled by the owls in Mina's attic, that likewise help to care for him and restore him. This parallel is explored further in Chapter 36, when Mina and Michael have just heard that Michael's baby sister needs heart surgery. They talk about the blackbird fledglings and what they dream about. Consider the response that Mina gives and how it relates to their situation:
"Sometimes they'll be very scared," said Mina. "They'll dream about cats climbing towards them. They'll dream about dangerous crows with ugly beaks. They'll dream about vicious children plundering the nest. They'll dream of death all aroudn them. But there'll be happy dreams as well. Dreams of life. They'll dream of flying like their parents do. They'll dream of finding their own tree one day, building their own nest, having their own chicks."
Such a quote helps to cement this parallel, and Mina goes on to make this parallel explicit, saying that she and Michael are like chicks, "Happy half the time, half the time dead scared." There is something in this comparison that seems to express the uncertainty and the joy of the human condition.
I would also want to highlight what Doctor MacNabola says to Michael when he is asked if love can help somebody get better:
"Love," said the doctor. "Hmmm. What can we doctors know about love, eh?" He winked at the student with the notebook and she blushed. "Love is the child that breathes our breath / Love is the child that scatters death."
Although Doctor MacNabola is clearly presented as a figure who is more interested in chatting up his medical students than seriously responding to Michael's question, he unwittingly speaks great truth, for this novel is, above all, a testament to the power of love and how love can heal and restore.