Knots can be seen in huge numbers on the Stour Estuary
An area of saltmarsh behind a supermarket in Manningtree makes an ideal location for birdwatching according to the local RSPB.
"It is a first class spot, a bit of a hidden gem on the Stour," said the RSPB's Mark Nowers.
"We get about 8,000 knots here on the Stour Estuary in the winter. It is a large proportion of the 30,000 or so birds that we get on the estuary."
The site known as Hogmarsh is owned by the Essex Wildlife Trust.
Like many species, the birds on the saltmarsh can struggle at this time of year to find enough food.
"The food supply is always there on the mudflats. The real danger for them is ice cover on the mudflats," said Mark.
"If they can't access that food, they have to do something about it, and the normal solution is that they move.
"They may have come thousands of miles to spend the winter here from all around the arctic, but if the food is covered up here they'll shift further south," he added.
To find out more about birdwatching events at
The RSPB: Stour Estuary Reserve
visit the website.