The weather on Wednesday was near ideal for shooting swallows - windy, sunny, blue skies.
The wind direction is always a factor and it was coming out directly from the west. It was so strong at times on my back that I nearly fell forward into the water. The trick is to find an obstruction that the birds have to fly around/through as they come towards you upwind. In my case there is an irregular weedbank that they come around before coming towards me.
So the wind acts to brake their approach (relative to you) and also orients them for a head shot (if that's what you're after) and it forces them to use a somewhat regular approach path.
In short order I was able to capture all three swallow species (tree, rough-winged, and barn). Still, these are significantly cropped. All with the 500mm.