Abundance, diet and Salmonella contamination of gulls feeding at sewage outfalls

The abundance and activity of gulls at sewage outfalls in South Wales and southern England was studied between 1972 and 1999. In winter, the black-headed gull was the most abundant species, followed by herring, common and lesser black-backed gulls. The abundance of black-headed gulls and herring gulls was significantly correlated with the volume of sewage discharged. Sewers supported only a small portion of the gull populations present in both areas, and a higher proportion of juvenile birds were present than at other feeding sites. Sewage comminution increased the numbers of black-headed gulls feeding, but infrequent intermittent discharge reduced them by over a half. Gulls selectively consumed waste foodstuffs from sewage and thus helped recycle some of its organic constituents. However, some individuals carried bacterial pathogens, and could thus have contaminated nearby bodies of freshwater, or grassland, by washing and roosting there. Full sewage treatment, as planned for most of the discharges in both study areas is likely to provide the most effective means of gull control.