Why is 'Birdsong' by Sebastian Faulks called Birdsong?

It is a book of contrasts. In the nice bits, like the Norfolk countryside or the French village, you can hear the birds singing. In the nasty bits, like underground, it is unnatural, so there are no birds singing. The author is saying, things are good in peacetime with nature allowed to flourish. They are bad in wartime with guns scaring the birds and terrible conditions underground where are no birds or anything natural.