Two pairs of goosanders on the Severn by English Bridge at Shrewsbury, in the middle of December. The females (especially) seemed to be getting fairly aggressive; one or the other threatening one or other male, or the other female.
Females also adopting an odd posture. Stretching their necks out low, so their whole body from beak to tail was just above the water line.
The males in contrast were raising their heads as high as they could above the water.
Finally realised this must have been part of their courtship routine when one of the pair started mating midstream (not pictured).