Britain's estuaries have an international importance for wildlife. Those apparently empty bleak windswept places are actually larders packed with food used by overwintering birds. Dig into the mud and you'll find loads of worms and shellfish which only give away their presence by a small hole or cast on the surface.

Ask any birdwatcher where the best place is to be on a cold winter morning and they'll not say curled by a cosy fire but instead they'd much rather be out with a telescope on one of our many estuaries.

Saltmarsh Copyrighted image Credit: Used with permission

We'll come back to those exciting winter months later, first take a look at your local estuary in summer. Don't just zoom past it on the way to the sandy beach, stop and have a look at the plants and animals living there and ask how they can survive in a place that often gets covered in salty water.

Why have the plants got thick fleshy leaves like desert succulents instead of thin flimsy leaves that you might expect to see on plants from other damp places. Well there is plenty of water around but it generally contains a high concentration of salt. Water flows from areas with a low concentration of salts (the plant cells) to the more concentrated sea water so 'normal' plants in these circumstances would actually loose too much water from their cells and die.

One way specialised saltmarsh plants overcome this problem is by building up the levels of salts in their roots, once the concentration inside the roots is higher than in the surrounding mud then water can flow into the plant. The leaves are tough and waxy to reduce water loss and prevent physical damage by wave action. Some of the plants also excrete salt through glands on their leaves and you can sometimes see - and taste - a fine crusting of salt all over them.

The other problem saltmarsh plants face is a lack of oxygen, plants need oxygen to 'breathe' (respire) just as much as humans do but down in the thick black mud of the estuary there is often very little oxygen so the roots are in danger of dying. To counteract this lack of air many of the plants have hollow tubes going down from the leaves into the roots.

Glasswort Copyrighted image Credit: Used with permission Light green Glasswort plants with barnacles on a rock in the background.

If you look across the estuary you'll immediately see that the vegetation is arranged in zones. Lowest down towards the waterline there is Glasswort - its soft fleshy bright green round stems looking like miniature cacti. It is a good edible vegetable which can be prepared in the same way as asparagus and has a very long tradition of being enjoyed as a good nutritious food.

If you are thinking of eating anything from estuaries - be it plants, shellfish or fish - then you do need to bear in mind that many of Britain's estuaries have been polluted in the past and may still contain toxic chemicals or harmful micro organisms.

Another very common plant on the marsh and one that is good at holding the mud together is cord-grass.

Moving up the shore where the next vegetation zone may look rather grey and is not necessarily inundated with every tide, it often contains large stands of sea purslane.

Further up again the shore can be ablaze with purple sea lavender and late in the year sea aster comes into flower looking rather like a garden plant sticking up above the grey surrounding vegetation.

The plants are living in a very rich mud, nutrients having been brought down by the river and deposited as flow rate slows before entering the sea. This muddy soup is also ideal for a wide range of filter feeding shellfish such as oysters, mussels and cockles, various worms such as ragworms, lugworms and fish such as juvenile sea bass, dabs and flounder.

Water flow patterns through the estuary are often rather complex and can vary with different states of the tide and at different times of year. The speed that the water is moving can be very important in determining the size of particles that are deposited, lighter sediments only fall out of suspension when the water is still. If the pattern of flow is disrupted - for example by building a new marina - then the distribution of sediments will change and this may have adverse effects such as depositing large amounts of material on an oyster-bed and so destroying it.

 
Oystercatchers and knot [Image: Mike Dodd Photography] Copyrighted image Credit: Mike Dodd

Return to the estuary in winter - wrapped up warm with a thick coat, gloves and hat, perhaps with that new telescope or binoculars and identification guide you have just got for Christmas.

The frozen Thames estuary Copyrighted image Credit: Used with permission Thames estuary showing frozen 'sea'.

Estuaries are less saline than the open ocean as they contain a mixture of fresh water (from the river) and seawater so they are more likely to freeze. East coast estuaries are also more prone to ice and snow, depriving the over-wintering birds of their food. This means that estuaries in the west can be very important in hard winters, when birds from our east coast and the Low Countries come here.

Birds using Britain's estuaries are often long distance migrants that have bred far to the north in areas that are snowbound during the winter. They rely on our mild climate and rich food supply to survive until next year. But they are often on the limit of their endurance after a long, non-stop crossing of the rough northern seas.

So when you see a flock of waders feeding on the shore don't let your dog go and chase them. They may be recovering from a long crossing or trying to build up their reserves for the next leg of the journey, and so even a small amount of disturbance can mean the difference between life and death.

Watch the birds carefully as they can look rather similar in their winter plumage, check your identification guide for hints on the best way to tell similar species apart – is it by overall size, colour of legs, curviness of the beak or aspects of the plumage and don't forget to listen out for the bird's call.

Avocet Copyrighted image Credit: Used with permission Avocet – a success story, they recolonised the UK as a breeding species in the 1940's and now overwinter on estuaries where their thin upturned beak is ideal for feeding on the fine mud.

Looking at the birds you'll see that they have a wide range of different types of mouthparts. They vary in length by nearly a factor of ten from the curlew's very long curved beak to the short stubby beak of the turnstones and sanderlings via the strong chisel like red bill of the oystercatcher.

Each species specialises in a different type of prey, birds such as curlews and godwits can reach lugworms deep in the mud whereas sanderlings rush along just in front of the incoming tide picking food items off the surface. Turnstones, as the name suggests, turn over stones looking for invertebrates hiding underneath and oystercatchers either sneak up on mussels and stab them before they have time to close or hammer at a weak point on the side of the shell to smash them open.

If you watch the birds feeding on a estuary then you'll often see them with very muddy faces where they've stuck their beak right up to the hilt in the silt trying to find food.

Conflict in the estuary: birds and boats Copyrighted image Credit: Used with permission Potential conflict between birds (curlews and oystercatchers) on a nature reserve and boat users in this very popular tourist resort in the south west of England.

What happens to the birds if an estuary is lost to development? This is a very difficult question to answer but also a very important one as most estuaries face considerable pressure from tidal barrage schemes, marinas and other forms of development. In a very careful study a group of researchers looked at the Cardiff Bay barrage where 200 hectares (just under 500 acres) of estuary were lost.

They found that the survival rates of redshank fell by 44%. Britain's mild estuaries hold overwintering bird populations from very large areas of the frozen north so by damaging our estuaries we are negatively affecting the bird populations in many other countries. We have an international duty to protect these areas as well as having the opportunity to go out and enjoy them ourselves.

All Images for this article supplied by Mike Dodd Photography. This article was originally published September 2006.