Late Departure

Hi everyone. Every year we have a pair of swallows nest in our porch. They have two nests and produce a clutch in each nest each year. 

I am concerned that all the birds have flown except one. I’m assuming it’s one of the parents. I’m sure by now they have usually fledged and set off on their migration by now. It’s definitely unusual for there to be a single bird left. 

Am I being to concerned or should I look to do something for the one that remains, and if so, what?

Thanks in advance. 
Mike