'The swift's black bow'

It looks very much as if the swifts have flown, from Lichfield at least – a notably early departure. It's been a strange, but ultimately successful, season, beginning with the newly arrived birds absenting themselves for long periods from the cool, wet weather of late spring and early summer (and who could blame them?). Then, once the weather had properly warmed up, back they came in force, as numerous as I have ever seen them in Lichfield. If they have truly gone now, it has been a short season indeed – down south I often saw my last swifts in mid-August. I must have seen my last of this year a couple of evenings ago – a pair in fact, lazily circling high up over the garden.
  Unlike Edward Thomas in this lovely short poem, I never know when I have seen the last of the season...

'How at once should I know,
When stretched in the harvest blue
I saw the swift's black bow,
That I would not have that view
Another day
Until next May
Again it is due?

The same year after year –
But with the swift alone.
With other things I but fear
That they will be over and done
Suddenly
And I only see
Them to know them gone.'