Trees Full of Life

A few days ago when I was walking it was very foggy - even nearby trees were invisible. I heard an immense clattering, almost a machinery-like sound - very loud and very complex - from where I knew there was a line of trees, although I couldn't see them. After a moment, I realized I was hearing an enormous flock of birds that were migrating through - I'm not sure what kind as I never saw them - I'm suspecting a mixed flock of blackbirds, grackles and starlings. There must have been hundreds of birds to make such a clamor.