Late swift brood?

Hello,

we regularly have swifts visiting in the summer - 3 nests in all. We had thought they’d left some weeks ago, though have seen the occasional solo swift nearby since then, perhaps two or three times. This afternoon we were surprised to see what appears to be a young swift poking its head out of one of the nesting holes, quietly calling. 

We’re wondering if it’s usual for young to hatch so late? Could there be just a single adult left behind with the young? 

Thanks for any info!