For the poem 'Forgive My Guilt' by Robert Coffin, what is the one sin the speaker is sure of?

The speaker notes that one sin he was sure of was having shot the wings of two birds as a youth.

I am sure of one sin I have done.
It was years ago, and I was a boy,
I lay in the frostflowers with a gun,
The air ran blue as the flowers, I held my breath,
Two birds on golden legs slim as dream things
Ran like quick silver on the golden sand,
My gun went off, they ran with broken wings
Into the sea, I ran to fetch them in,
But they swam with their heads high out to sea,
They cried like two sorrowful high flutes,
With jagged ivory bones where wings should be.