Collared Dove, Confused

Diligently
She flies down to the garden,
She selects the ideal twig,
She flies up, twig in beak,
She disappears into the birch tree,
She reappears, and repeats.

Classic nesting behaviour,
I hear you say,
All birds do the same,
Every spring – nothing
Remarkable about that!

So why is this dove
Building a nest so diligently
In September?