With good numbers of Shelduck on the marsh and river at the moment and some fine springlike weather it is not too surprising that they have been getting frisky. The males seem to outnumber the females about 5:1 and are often seen in hot pursuit of the ladies whistling and grunting! 

They will not settle down for some time yet and have weeks of preliminary courtship to go before she choses a mate and finds an old rabbit burrow in which to nest.  As you will see in the photo by Mark Hart below, the females are about a third smaller than the males and I was always led to believe that this allowed them to incubate without intrusions from the more robust amourous males!  If this is wrong please tell me!