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


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Coulson, GM and Coulson, RI 1983 , 'The significance of rubbish tips as an additional food source for the Kelp Gull and the Pacific Gull in Tasmania', Other Degree thesis, University of Tasmania.

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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
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.

Item Type: Thesis - Other Degree
Authors/Creators:Coulson, GM and Coulson, RI
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