It's a somewhat irritating fact that many of the local reserves (particularly those administered by the RSPB) seem to raise their water levels every Spring. I've occasionally asked the wardens why they do this during peak wader passage: almost always their reply is that it's done to stop foxes predating Bittern nests. Hmmmm... My usual response is that:
a) Foxes can swim!
b) Otters are widespread throughout Norfolk and I've often seen them take chicks
c) Marsh Harriers and Herons are far more of a potential threat to reed-nesting species.
d) Chinese Water Deer trample all over the reedbeds
e) It doesn't seem to be working!
I would have thought that shallow pools with muddy margins to allow hungry long-distance migrant waders to rest and refuel was a far more worthwhile strategy.