The importance of dredge islands for breeding waterbirds. A three-year study in the Venice Lagoon (Italy)

Since 1989, intertidal dredge islands have been constructed in the Venice Lagoon using sediments originating from regular dredging of lagoon channels and inlets. Between 2005 and 2007, 75 dredge islands were surveyed in each year and the number of breeding pairs of seabirds and shorebirds estimated. The results showed that, of the 13 species that nested at least once, eight represented more than 1% of their total Italian population, sometimes even higher than 10%. Our results indicated that the majority of birds prefer site dimensions of between 10 and 30 ha, even if some species use small or very small (<1 ha) sites particularly heavily. Most of the other environmental variables we measured concurred in explaining species’ occurrence and abundance. Redshank and Shelduck selected sites with high vegetation coverage, whereas sites with lower vegetation were preferred by Kentish Plover and Little Tern. More pairs than expected were observed at sites between 25 and 30 ha. These sites have a considerable wealth of habitat types, becoming suitable for species with contrasting nesting habitat requirements. Density of breeding pairs ranged between one and four pairs/10 ha; these values compare well with those observed in natural habitats existing in the Venice Lagoon, and support the opinion that dredge islands are a good alternative to natural sites. Along coastal sites where human pressure on beaches is particularly heavy, man-made habitats such as dredge islands may become a valuable alternative breeding site for those seabirds and waders of conservation concern. The results presented allow an assessment of the importance of dredge islands for breeding waterbirds over a short to medium period. They may also be used to estimate the expected richness and abundance of breeding birds that will use intertidal man-made sites, when these are built in a temperate coastal marsh.