Distance to feeding area: Shorebirds tend to roost in areas close to their feeding habitat (Zharikov & Milton, 2009) as this reduces expenditure of energy on travelling. For example, for Spotted Greenshank Tringa guttifer , roosts in man-made ponds were around 1 km away from foraging sites (Yu et al. , 2019). For Eurasian Oystercatchers Haematopus ostralegus in the Wadden Sea, roosting sites were around 2 – 4 km from foraging sites (Bakker et al ., 2021). Accessibility: Islands that are close to the mainland and/or in shallow water may be more accessible to terrestrial organisms, especially at low tide (Landin & Soots, 1978). This includes wild animals such as foxes and rats, domestic cats and dogs, and humans – which may prey upon and/or disturb shorebirds. Competition and predation: Larger shorebird species can reduce the value of artificial islands for target shorebirds. Gulls, for instance, may establish breeding colonies earlier each year than terns, and consequently discourage terns from using the same island (Quinn et al ., 1996). Gulls may also prey upon eggs and chicks of smaller shorebirds (Quinn et al ., 1996). Various actions can be used to exclude problematic birds or protect nests (Williams et al ., 2013). For example, in Canada, covering islands with plastic sheeting early in the breeding season discouraged gull nesting and maintained habitat availability for terns (Quinn & Sirdevan 1998). Mammalian predators can also be a problem for shorebirds. On dredge islands in Maryland, USA, fox predation of terns was mitigated by trapping, along with an education programme to address public concerns (Erwin et al. , 2007). Existing habitat: In areas where there is already plenty of suitable shorebird habitat, there may be little to gain from creating new islands. Researchers in the USA suggested that dredge islands are used extensively only where alternative sites are not available (Landin & Soots, 1978). Use of attractants: Shorebirds may be hesitant to use newly created sites. Decoys and/or vocalisations can be used as attractants (Williams et al ., 2013). For example, a study in the USA reported that Forster’s Terns Sterna forsteri only started nesting on artificial structures once decoys and vocalisations were added (Ward et al ., 2011).
4. Implementation
Sediment is available from dredging practices for maintaining human infrastructure and transport corridors, such as ports and waterways (Sheehan & Harrington, 2012). Islands can be created by confining the dredged sediment in one place by using, for example, rubble, wooden cribs, or by planting vegetation around it (Yozzo et al ., 2004; Scarton et al. , 2013). Careful planning is needed to make sure sediments do not get washed away within a few years (Chi-Yeung Choi, pers. comm.). It is generally sensible to invest in protection on the windward side of islands, for example in the form of dikes or breakwaters. Consider whether an absence of existing islands is an indicator of high energy levels that would lead to rapid erosion (Golder et al ., 2008). Finer sediments (clay, silt or fine-grained sand) are more susceptible to wind erosion and will take longer to stabilise than coarser sediments (Golder et al ., 2008). Fine sediments can be stabilised by mixing in coarser sediments or sand, or by depositing coarse material on top (Landin & Engler 1986; Golder et al ., 2008). To maintain their size, it is
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