Animals
The more northern the marsh, the less difference the tide makes to animals in the winter. They are all resting, made torpid by the cold, or have departed to take refuge in warmer places. Crabs have retreated down into the mud, too cold to respond to any stimuli. Insects rest as eggs or larvae in the mud, or are burrowed in the ground above the high tide mark as adults. The birds have gone south.
Plants
Not only the animals, but the plants are affected by the cold. If the winter is severe enough to freeze the seawater and there are storms to push cakes of ice up onto the frozen mud, anything protruding above the surface will be ground to bits or sheared off and carried out to decay in the sea. Even if there is no ice damage, the stems and blades of the grasses are killed. Only the underground parts can resist freezing. They remain protected in a blanket of wet or frozen mud, until the spring sun warms the marsh.
Birds
Birds must have a continuous supply of food to keep their bodies warm.
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The northern bird population is restricted to ducks that feed in the water. They chase the fish slowed by cold and they poke and dab in the mud, sifting out the little worms and clams hiding there.
Animal life in the water eases itself out of the marsh. Some inhabitants move away. Some burrow down deep. A quietness settles everywhere. There are no loud territorial disputes between birds. The fiddler crabs do not rustle through the plant stems.