Broad white trailing edge to the speculum, but no obvious white at the front of the speculum - suggests it might be a Mottled × Mallard hybrid / intergrade (Mallard has prominent white bars both sides of the speculum, Mottled has little/no white either side). But hard to be certain!
Thanks, Resin. I wish I had been quicker on the draw and got the exposure right. I needed to compensate for all that bright sky or switch to spot meter, but didn't do either. I just put the camera up and started following the bird and shooting. It was all alone and flying fast.
I hadn't thought about the possibility of a hybrid. I couldn't find a bird that had the broad white on the trailing edge with nothing forward, so hybrid makes sense. I wonder if it could be the Black Duck/Mallard hybrid which is more common? I'll see if I can find images or drawings of the wings on the hybrids. Nothing on that in Sibley.