Shorebird habitat use and macroinvertebrate composition in Robbins Passage/Boullanger Bay wetlands, NW Tasmania

Virtually all species of shorebirds are decreasing on a global scale, due primarily to habitat loss and/or modification, through wetland reclamation, increased coastal erosion, decreased water quality and rising sea levels due to climate change. Many shorebirds are migratory and travel thousands of kilometres between their breeding grounds in Siberia and Alaska, to their winter feeding grounds in Australia, with over 2 million shorebirds arriving each year. These winter feeding grounds are crucial for the shorebirds' survival, supplying an abundant and predictable source of food for the birds, and places where they can roost undisturbed. The Robbins Passage/Boullanger Bay wetlands in northwest Tasmania are the most important shorebird site in Tasmania, supporting over 25,000 shorebirds each summer. As a primary site and the end point of the migratory shorebird network, and for its intrinsic values, it is important to investigate the ecology of the wetland and the habitat requirements of the shorebirds, before the area is modified or affected by increasing agriculture and coastal development within its catchments. The overall aim of this study therefore was to investigate how shorebirds use the resources of coastal wetlands at a local regional scale within the Robbins Passage/Boullanger Bay wetlands, to allow for the effective and sustainable management and conservation of the shorebirds and the wetlands as a whole. The present study investigated the habitats used by feeding and roosting shorebirds within the wetlands, and the relationships among physical environmental and biological variables, in addition to developing the first roost choice model for shorebirds in temperate Australia. In order to investigate shorebird feeding habitat use, the spatial variation of intertidal macroinvertebrates were determined. In general, the mid-intertidal stratum had the greatest invertebrate density and diversity, while the low intertidal stratum had the greatest biomass. Seagrass biomass, i.e. dry mass of seagrass leaves and roots, partly explained the differences in invertebrate composition and abundance among and within sites, with sites with seagrass having increased invertebrate abundance and diversity. The investigation of the low tide foraging distribution of shorebirds over the tidal flats within the Robbins Passage/Boullanger Bay wetlands showed that shorebird distribution within and among sites was non-random. The greatest I densities and numbers of shorebirds were found at Shipwreck Point and East Inlet, the sites with the greatest invertebrate densities, and the greatest invertebrate biomass and species diversity, respectively. Palaearctic shorebirds were only found at Shipwreck Point and East Inlet. Within each site, the greatest shorebird densities were observed along the waters edge and low intertidal stratum, where invertebrate biomass was greatest, although shorebird distribution varied among species. Generally, on a small spatial scale, invertebrate diversity was positively correlated, and seagrass leaf mass negatively correlated, with shorebird feeding density, while on a larger spatial scale, invertebrate biomass and seagrass root mass were positively correlated ,with shorebird feeding density. Seagrass may inhibit the feeding method of some shorebird species, such as pied oystercatchers, as they tended to feed in areas where seagrass biomass was low. The larger spatial scale produced a stronger relationship between shorebird distribution and environmental variables. Shorebird habitat use during the ebbing tide concurred with low tide habitat use, with the greatest densities and numbers of shorebirds occurring at both Shipwreck Point and East Inlet. Shorebird abundance was only significantly different at East Inlet and Robbins Passage, with shorebirds observed in greatest numbers two hours before low tide at East Inlet and four and zero hours before low tide' at Robbins Passage. During the ebbing tidal cycle, the feeding distribution of pied oystercatchers was generally greater along the water's edge and the low intertidal stratum, while red-necked stints were observed along the water's edge and low intertidal stratum in greater numbers at East Inlet, and mid-intertidal stratum at Shipwreck Point. Shorebirds used traditional roost sites throughout the wetlands, and while all roosts were used consistently over the 18-month period, total shorebird abundance and species richness fluctuated significantly over the seasons. The greatest numbers of roosting shorebirds occurred during the summer months, December to February, when the Palaearctic species (e.g. Pacific golden plover, red-necked stint and ruddy turnstone) were present in greater numbers, due to their arrival from their breeding grounds in the northern hemisphere. Resident shorebird species were generally observed roosting in greater numbers during autumn and winter when they had completed their breeding season, as compared to summer and spring. The number and species of shorebirds at each roost site also varied, with six times the mean number of birds found at East Shipwreck Point as compared to the other sites. Shorebird roost choice appeared to be driven by the distance of the roost from the feeding grounds and the width of the site. These factors allow the birds to reduce their energy expenditure by roosting near feeding areas and decreasing the flying distance, and minimise the risk of predation, as a wider site provides greater distance to cover for potential predators. The roost choice model had an overall classification success rate of 87.5%. Further work is required in the Robbins Passage/Boullanger Bay wetlands, but these results can be used to assist in the development of management plans for the wetlands and the conservation of important shorebird areas, as well as contributing to the growing body of information on shorebird habitat use and providing a roost choice model for temperate coastal Australia