Most swifts have gone by mid-August although swallows are often seen much later.
With swifts you just realise they are no longer here. Sad, but they will be back. To me, the screaming swifts are a mark of an English summer. When I see my first swift each spring my heart is lifted. They have been making their perilous journey for thousands of years and, hopefully, they will still be doing so long after I am dead and gone.