In reply to Flinticus:
I see lots of them when walking on Romney Marsh. They build their nests in the reeds on the banks of the dykes. Best to pass on quickly. If you stop to take photos , they can get stressed and agitated, as they regard you as a threat to the nest, eggs or young. It's particularly important to keep dogs well away. They will attack a dog in self defence, and try to drown it.