Knot Calidris canutus on the artificial lagoon;high tide wader roost at Freiston Shore RSPB reserve, The Wash, Lincolnshire, England

Artificial islands

The RSPB manages more than 300 man-made islands at its wetland reserves – providing nesting, roosting and loafing sites for waders and other wild birds.

What are they?

Islands provide nesting, roosting and loafing sites secure from land-based predators, and many bird species will choose them in preference to mainland locations with similar habitat features.

Islands are also created to concentrate birds in front of hides to provide visitors to reserves with good views without disturbing the birds.

The species found on islands will vary depending on the habitat and location of the water body. Avocets and little terns are mainly found in the south, and they prefer sparsely vegetated islands, while red-throated divers, goosanders and red-breasted mergansers will be found mainly in the north and choose tall and dense vegetation.

Several small islands in a water body are more valuable to a breeding colony than one large one.

These pages provide information on the design, siting, construction and maintenance of man-made islands.

Design and location

Shape

The shape of an island must be always determined by practical considerations. However, teardrop-shaped islands with the narrower and lower portion towards viewing hides, and the wider and higher end pointing away from the hides would provide optimal viewing conditions for the public.

At sites where erosion from a particular direction is a problem, the shape and alignment of the islands may have to be determined by the prevailing current.

Making the island margins crenulated provides sheltered bays for young birds, increases the length of edge for feeding birds, and allows direct access to the water for island-nesting birds.

The surface

Once the island has been created, two or three overlapping layers of polythene bags must be laid flat on the spoil base to produce a constant and smooth appearance.

Their purpose is to prevent coarse, deep-rooted plants from taking hold. Take care not to leave depressions that will fill with water, sand or any other materials that could damage the plastic. The bags can then be covered with one of two coverings:

  • A shingle surface
    Spreading shingle and small stones placed on the plastic to a depth of at least 10–15 cm (approximately 3 tonnes per 10 m2).

  • A mud surface
    Although much more unstable and more likely to disappear, 5–15 cm of mud can be added over the plastic and flattened into shape.

    For both of these options the polythene is held down with large rocks, pre-cast concrete or mud boulders. You can also use branches of old willow or any other species well adapted to water, sticking them all around the borders as stakes.

    This option will give the island a more natural appearance. However, perforating the polythene with these stacks can affect its durability if the island is placed in a highly erosive environment. 
     

Note for waterfowl islands

The exact profile of islands for breeding ducks, geese, swans and other waterfowl is not as critical as for terns and waders. It is more important that the islands are allowed to vegetate until they support a tall (0.5–1.9 m) vegetation cover, because this provides ideal waterfowl nesting habitat and reduces predation of their nests (in areas where nests are easily accessible).


Location

The distribution of islands in water bodies will influence their use by breeding birds. For example, islands further from the shoreline support a higher density than those close to mainland.

Island distribution will also determine the visibility of birds on islands to the public, which needs to be considered at visitor-oriented reserves or leisure areas. Provision of islands with different amounts of vegetation cover attract different breeding birds.

Black-throated diver, nesting on artificial island

Construction

Islands can be constructed in two basic ways:

  • By dumping material into shallow water (deposit islands)
  • By excavating around areas of land (remnant islands) and flooding the lowered area.

Some of the materials used to construct these islands are:

Deposit islands

  • Coal slag
  • Heaped peat
  • Heaped estuarine clays
  • Heaped rubble
  • Heaped riverine soils
  • Concrete frame with sand and single on top
  • Shingle

Remnant islands

  • Estuarine clay
  • Freshwater clays

The main materials should typically be local, so costs are minimised and the possible impact of non-native materials is reduced. Heap up the soil with a bulldozer and leave it to settle for a year. Once settled, profile and smooth the islands with a tractor.

Black-throated diver, nesting on artificial island

Management

Resurfacing the islands is required if they are to remain attractive for birds every year. However, this task should not take longer than 15 minutes per island. Hand weeding, raking or spraying should be enough.

Duck and goose droppings can be a problem on densely populated islands, since they will accumulate to a thick layer with time and be almost impossible to decompose since the plastic layers underneath will limit the action of microorganisms.

It is vital to remove this excess every year, preferably during late winter or early spring before the birds start to nest. Too many droppings may attract unwanted parasites that could deter some species. Moving the droppings from the islands into the surrounding water may cause a problem with increased nutrients. It may be better to remove the droppings and use them as fertilizer elsewhere, rather than throwing them into the water.

Islands with mud surfaces over plastic require more management because the mud provides a better substrate for the establishment of vegetation. On these islands, management generally involves the removal of plenty of vegetation in the early spring and in the autumn, and where possible flooding over winter.

Finally, islands without a plastic lining tend to be rapidly overgrown. However, these can still be kept attractive for terns and some waders by annually removing all the vegetation in the early spring and in the autumn, again combined with flooding over winter if possible.

Minsmere RSPB Reserve, general view of Boomacre Mere