The west coast habitats of the Uists are internationally important for their populations of breeding waders. Recent changes in numbers have been mainly linked with impacts of predation by hedgehogs which were introduced to South Uist in 1974. However, changes in agriculture, other predation pressures and other factors may also contribute to the changing status of these birds. This project analyzed two independently-collected data sets, one on breeding waders (sampled in the 1980s, 1990s and 2000s) and one on vegetation (sampled in 1976 and 2009) from four machair sites in North Uist and Benbecula, sites where hedgehogs have either been absent (North Uist) or present at low densities (Benbecula). Relationships between the abundance of breeding waders and machair vegetation characteristics (in the relative absence of hedgehogs), and how both have changed over the past three decades, were examined and presented in the context of changes in land use as determined from interviews with the grazing clerks for the study areas.
Common relationships amongst the breeding waders were tendencies to avoid areas of young dunes and to be more concentrated into areas with soils that were wetter or more basic. Ringed plovers were the exception, as they did not show a relationship with soil moisture. Important species specific relationships were demonstrated with measures of salinity, fertility, moisture, vegetation structure (density and height), and arable cultivation. Associations between locally measured changes in habitat and breeding wader abundance were relatively few and sometimes contradicted the general habitat – bird relationships that were otherwise apparent. The greatest measured changes in habitat were confounded with large changes in agricultural management, which otherwise reduced the habitat quality for some breeding waders (e.g. through reduced invertebrate availability).
Calladine, J., Pakeman, R., Humphreys, L., Huband, S & Fuller, R. (2011). Population changes in breeding waders on machair in North Uist and Benbecula and their associations with vegetation and land use. SNH commissioned report 411.