Dear Cherie, there are a lot of Wildfowl that use the plants for nesting! There is also a lot of Red Foxes and crows. So to protect the plants and the wildfowl from damage this is done, it gives the wildfowl a chance when they have eggs in a nest. For if you look at the picture of the two Drakes together the water is very shallow. A red Fox would go straight through that and raid a nest or eat the young or worse kill the parents. Also with some plants you don't want the wildfowl in, as they can damage them too much, so that can stop them and give them a chance.
Hope that sums it up for you, although you do not get the trouble with red Foxes we do here. They are not liked by most of us as they kill for pleasure, not to eat. If they get in a chicken run they go into a frenzy and will kill every single chicken, but never eat one, just blood lust! They do the same with new born lambs and no one can understand why they do it.
Crows are clever and magpies, jays as well, They will raid nay birds nest for the eggs, which is not good if you are trying to look after wildfowl.
Regards.
Neil.
