Why have the birds stop singing?

Sadness washes over me in times like these.

Around the birdbath I see the birds bath -

they wiggle and snuggle and wobble and preen... 

but why is it that 

I can't hear the birds sing?

I see the birds - all of them,

happy and free. 

All of them chirping so happily.

And although I try to hear them sing,

I cannot hear anything.