I walked round Alwen Reservoir at the weekend (near Betws y Coed) at the far end of the reservoir a bird flew underneath the bridge, skimming the water. I'm usually ok identifying birds but nothing comes to mind, searches for the usual British birds come up with nothing.
It was like a large swift but probably sized nearer to a slimmer Blackbird, it looked black on the back with some white near the rump, mostly white underneath as well. Wings and proportions were similar to a normal swift.
Any ideas what I've seen?