dubius (Aulert et al. , 2012), Snowy Plover Charadrius nivosus (Powell & Collier, 2000), Great Black-backed Gulls Larus marinus (Aulert et al ., 2012), and Caspian Terns Hydroprogne caspia (Martin & Randall, 1987; Quinn & Sirdevan, 1998). A study in France (Aulert et al ., 2012) reported that although shorebirds used a constructed island and other constructed roosting areas these structures did not fully compensate (in terms of shorebird numbers) for the loss of an open-sea resting area due to development of a port. At Mai Po Marshes, Hong Kong, low-lying islands were constructed in a shallow lagoon and provided a high tide roost site throughout the summer wet season. This resulted in first-year (subadult) Terek Sandpipers Xenus cinereus over-summering in the area. Previously, all aquaculture ponds around Deep Bay were maintained with deep water over summer and so although tidal flats were available for foraging there was no place to roost at high tide and the area held no over-summering waders (David Melville, pers. comm.).
3. Factors that can affect outcomes
Area: The optimum island size may vary depending on the species. Larger islands can support more birds, but smaller islands may act as refuges for solitary birds. Therefore, having a variety of sizes is ideal if creating multiple islands. Dredge islands in the Atlantic and Gulf Coast estuaries of the USA range in size from 1 – 80 ha (Yozzo et al ., 2004). Elevation : If islands are intended to be used as roosting sites, they should be high enough so that they are not frequently inundated by tides. If islands are intended to be used as breeding sites, they should be high enough that they are never inundated by tides – at least during the breeding season. Consider the likely influence of future sea level rise and climate change (and related storms). Profile: Shallow slopes provide opportunities for the creation or development of tidal flats or salt marshes. Open, flat-topped, gently sloping islands may be preferable for species that roost in large, tight flocks such as oystercatchers and knots (recommendation in Burton et al. , 1996). Steep-sided islands provide more shelter for species that roost in small or loose flocks, such as Common Redshank Tringa totanus (recommendation in Burton et al ., 1996). Sediment: The grain size of the sediment may influence if and how shorebirds use islands. Evidence from Least Terns Sterna antillarum , Gull-billed Terns Sterna nilotica and Black Skimmers Rhyncops niger nesting on dredged material suggests shell material within the substrate is beneficial as it may play a role in egg camouflage and vegetation control (Mallach & Leberg, 1999). Finer substrates may be invaded by weedy plants while coarser substrates may better resist plant invasion (Powell & Collier, 2000). Vegetation: The amount of vegetation at a site will affect how birds use it, although the preferred amount varies between species and whether the site is used for nesting or roosting. One study on the use of islands as breeding habitat found that shorebirds were more likely to nest on sparsely vegetated islands than heavily vegetated ones (Burgess & Hirons, 1992). At a site in the UK with created gravel islands, the decline in their use by nesting shorebirds (gulls and terns) was thought to be due to increasing dominance of woody vegetation over 25 years (Akers & Allcorn, 2006).
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