The title is not in reference to a bird song, but a Bible verse:
“Are not two sparrows sold for a penny? Yet not one of them will fall to the ground apart from the will of your Father. And even the very hairs of your head are all numbered. So don’t be afraid; you are worth more than many sparrows.” (Matthew 10:29-31)
Referencing how Sandoz felt he was allowed to fall, thereby him losing his faith in God.
Excerpt from The Sparrow:
“There's an old Jewish story that says in the beginning God was everywhere and everything, a totality. But to make creation, God had to remove Himself from some part of the universe, so something besides Himself could exist. So He breathed in, and in the places where God withdrew, there creation exists."
So God just leaves?"
No. He watches. He rejoices. He weeps. He observes the moral drama of human life and gives meaning to it by caring passionately about us, and remembering."
Matthew ten, verse twenty-nine: Not one sparrow can fall to the ground without your Father knowing it."
But the sparrow still falls.
Later on, Sandoz speaks this to the Cardinals:
“That is my dilemma. Because if I was led by God to love God, step by step, as it seemed, if I accept that the beauty and the rapture were real and true, the rest of it was God’s will too, and that, gentlemen, is cause for bitterness. But if I am simply a deluded ape who took a lot of old folktales far too seriously, then I brought all this on myself and my companions and the whole business becomes farcical, doesn’t it. The problem with atheism, I find, under these circumstances...is that I have no one to despise but myself. If, however, I choose to believe that God is vicious, then at least I have the solace of hating God.”