The significance of rubbish tips as an additional food source for the Kelp Gull and the Pacific Gull in Tasmania

Many species of gull have exhibited dramatic population increases, particularly in the Northern Hemisphere, in response to protection and to an increased supply of food provided by human activities. Population increases have been manifested by higher population densities as well as increases in range and the formation of new breeding colonies. This growth has had a number of adverse environmental effects: gulls have disadvantaged other bird species and have become agricultural pests, public health risks, urban nuisances and aviation hazards. In Australia, the small Silver Gull (Larus novaehollandiae) has displayed a similar pattern of population growth. By contrast, the large endemic Pacific Gull (Larus pacificus) has experienced a reduction in range. A second large species, the Kelp Gull (Larus dominicanus) has a circumaustral distribution; it has recently become established in Australia and is most numerous in south-east Tasmania. A review of the biology of the Pacific Gull and the Kelp Gull indicates that the two species have similar requirements and could be expected to compete for resources. This study examined the nature and extent of competition for food, with particular reference to the significance of rubbish tips as a food source for the two species. Gull numbers were monitored at 11 tips in northern Tasmania and 17 tips in south-east Tasmania during winter of 1981. Regular monitoring and detailed behavioural observations were conducted at three large tips and a number of representative shoreline feeding sites in the Hobart area. Tips were found to be an important food source for Kelp Gulls in Tasmania, and have probably contributed to their population growth. Pacific Gulls also utilize tips but to a lesser extent. Numbers of Pacific and Kelp Gulls were highly correlated with the human population served by the tips, but no relationship was detected between gull numbers and the distance of the tips from water. Numbers of gulls at tips were highest in June and July then generally declined, but exhibited wide fluctuations which were not strongly correlated with any of nine meteorological and tidal variables. Pacific Gulls of all ages were dominant over Kelp Gulls in overt competition for food. Pacific Gulls utilized a predominantly kleptoparasitic strategy at tips while Kelp Gulls tended to forage steadily, but overall the two species had equivalent feeding efficiencies. In general, Kelp Gulls showed a preference to feed at rubbish tips, whereas Pacific Gulls preferred shoreline sites. At some shoreline feeding sites adult Pacific Gulls defended winter feeding territories singly or in pairs against Kelp Gulls and immature Pacific Gulls. There was also no clear evidence that the Pacific Gull has suffered a population decline since the arrival of the Kelp Gull in south-east Tasmania, and the degree of resource partitioning shown by the two species indicates that they are not competing closely for food. However, competition for nest sites on the breeding islands has not been fully studied. Continued growth of the Kelp Gull population in Tasmania is likely, and potential environmental problems are apparent. A range of control measures is available, but control does not appear to be necessary at present. Management of the Kelp Gull and the Pacific Gull in the future will require periodic population monitoring and a comprehensive breeding study to examine the relationships between the two species in mixed colonies.