Some migrants still moving up river at Sabaloka

I was out at Saboloka today for a family picnic. I was surprised to see large numbers of hirundines still moving up the river. The vast majority (many hundreds) were Sand Martins, but there were a few House Martins and Barn Swallows amongst them. There were also a couple of Spotted Flycatchers. We wondered at the lateness of this movement, as these birds will be arriving very late on their breeding grounds.