The answer to this question can be found in the preface of this excellent novel concerning growing up and the loss of innocence. Alan describes a particular tree that attracted hummingbirds to it. Note how this tree is described as they see it and in particular the kind of language that is used to describe it:

In the morning as we walked up to the mountains we saw the hummingbirds around it and they gave delight to the whole day. We clutched the immortal morning in our hearts and knew there could be no sourness or disaster or stale disgusts in life while the jewelled birds flew in the air and made lovely bracelets around the cassia's branches.

The way in which the hummingbird tree is associated with an "immortal morning" and how it gives "delight to the whole day" is something that shows the hummingbird tree symbolises the innocence and beauty of youth. Focusing on this hummingbird tree helps the narrator and his friends to feel no evil or darkness. However, as the preface continues,  at evening, the hummingbirds had all flown away and the "day had died." This of course is symbolic of the death of that childlike innocence and the way in which innocence is exchanged for pain and experience of adulthood. The title draws attention to the key theme of the novel, the coming-of-age of the principal characters, and how they exchange their childhood for an adulthood which is characterised by the "sourness" and the "stale disgusts" of adult life.