The Significance of Rubbish Tips as an Additional Food Source for the Kelp Gull and the Pacific Gull in Tasmania by e.­ e--~'" (& G. M. Coulson, B.A. (Hans.), Dip.Ed. (Melb.) and .J~ R.I. Coulson, B .Sc., Dip.Ed. (Melb.) Being a thesis submitted in part fulfilment of the requirements for the degree of Master of Environmental Studies Centre for Environmental Studies University of Tasmania August, 1982 nl\, (l,_ '/ Cl-- \'\~'.2. ACKNOWLEDGEMENTS We are grateful to our supervisors, Dr. A.M.M. Richardson and Dr. J.J. Todd, and our external advisors, Mr. A.W.J. Fletcher and Mr. J.G.K. Harris, for their guidance in the preparation and planning of this thesis. Mrs. L. Ramsay typed the thesis. Mr. D. Barker (Tasmanian Museum, Hobart) and Mr. R.H. Green (Queen Victoria Museum, Launceston) gave us access to their gull collections. Mr. G. Davis assisted with the identification of chitons. Officers of the Tasmanian National Parks and Wildlife Service provided information, assistance, equipment and specimens, and officers of the Tasmanian Department of the Environment gave us information on tips. The cities and municipalities in south-east Tasmania granted permission to study gulls at tips under their control, and we are particularly grateful for the co-operation shown by council staff at Clarence, Hobart and Kingborough tips. Mr. R. Clark allowed us to observe gulls at Richardson's Meat Works, Lutana. CONTENTS ABSTRACT 1 1. INTRODUcriON 3 2. GULL POPULATIONS IN THE NORTHERN HEMISPHERE 7 2.1 Population Increases 8 2.1.1 Population size and rates·of change 10 2.1.2 Growth of breeding colonies 13 2.2 Reasons for the Population Increase 18 2.2.1 Protection 18 2.2.2 Greater food availability 19 2.3 Effects of Population Increase 22 2.3.1 Competition with other species 22 2.3.2 Agricultural pests 23 2.3.3 Public health risks 24 2.3.4 Urban nesting 25 2.3.5 Aircraft bird-strikes 25 2.4 Future Trends 26 3. GULLS IN AUSTRALIA 32 3.1 The Silver Gull 34 3.1.1 Description and distribution 34 3.1.2 Feeding ecology 37 3.1.3 Status 38 3.2 The Pacific Gull 40 3.2.1 Classification 40 3.2.2 Description 41 3.2.3 Distribution 43 3.2.4 Behaviour 47 3.2.5 Diet 54 3.2.6 Reproduction 57 3.2.7 Population 59 3.2.8 Status 61 3.3 The Kelp Gull 63 3.3.1 Classification 63 3.3.2 Description 64 3.3.3 Distribution 69 3.3.4 Behaviour 76 3.3.5 Diet 83 3.3.6 Reproduction 84 3.3.7 Population 87 3.3.8 Status 90 4 0 BEHAVIOUR OF GULLS IN TASMANIA 92 4.1 Distribution of Gulls at Rubbish Tips 93 4.1.1 General survey method 93 4.1. 2 General survey results 96 4.2 Detailed Surveys of Large Tips and Shoreline Sites 104 in South-East Tasmania 4.2.1 Description of study sites 104 4.2.2 Method of gull survey 112 4. 2. 3 Temporal changes in gull numbers 113 4.2.4 Comparison of gull numbers at different 128 feeding sites 4.3 Behaviour at Tips and Shoreline Sites 132 4.3.1 General behaviour patterns 132 4.3.2 Intraspecific and interspecific dominance at tips 141 4.3.3 Feeding strategies at tips 145 50 MANAGEMENT OF LARGE GULLS IN TASMANIA 154 5.1 Ecological Impact of the Kelp Gull in Tasmania 155 5.1.1 The course of colonisation by the Kelp Gull 155 5.1.2 General implications of an increased Kelp 158 Gull population 5.1.3 Competition with Pacific Gulls 161 5.2 Possible Control Measures 163 5.2.1 Direct control 163 5.2.2 Habitat modification 165 5.3 Conclusions 169 6o REFERENCES 171 6.1 Publications 172 6.2 Personal Communications 190 APPENDIX 192 1 ABSTRACT r·lany 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 ne\.; breeding colonies. This gra..;th 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 novaehoZZandiae) 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, \'lith particular reference to the significance of rubbish tips as a food source for the t\.;o 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 \.;ere 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 t.~e tips from water. Numbers of gulls at tips were highest in June and July then generally declined, but exhibited wide fluctuations •..;hich \•rere not strongly correlated with any of nine meteorological and tidal variables. Pacific Gulls of all ages \·rere 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, 2 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. 3 1 Introduction 4 1. INTRODUCTION Human activity has had a profound effect on many wildlife species. There has been a dramatic acceleration in the rate of extinction in historic times, and many more species have suffered a reduction in numbers and range. These changes rarely have a single cause, but the most common pressures created by human activity are loss of habitat, hunting, pollution and competition with introduced species (Fisher, 1971). Less often, human activities have the opposite effect and a species undergoes an increase in population size and distribution. One of the most spectacular changes of this type has been achieved by gulls. Gulls as a group are adaptable and opportunistic scavengers. They are thus well suited to exploit the food provided by humans in such forms as garbage and offal, and it is generally agreed that the enormous volumes of these types of food have enabled gulls to build up to very high population levels, often with serious environmental consequences (Graham, 1975a) • These changes have been most marked and best documented in the Northern Hemisphere, particularly in Great Britain and the United States. In Australia the small Silver Gull (Larus novaehollandiae) has followed this general pattern (e.g. Sharland, 1956). However, the only other species, the large endemic Pacific Gull (Larus pacificus), is unusual in that it has suffered a reduction in its historic range and is common only in parts of its present distribution (Serventy et al. 3 1971). During this century a similar species of large gull, the Kelp Gull (Larus dominicanus) , has reached Australia and established a breeding population. It is a Southern Hemisphere species which occurs in South Africa, South America, the sub-antarctic islands, the Antarctic continent, New Zealand and now Australia (Serventy et al., 1971). It is closely related to the Herring Gull (Larus argentatus) and the Lesser Black-backed Gull (Larus fuscus) which have been so successful in the Northern Hemisphere (Moynihan, 1959) and it has shown the same propensity for population growth. Although it is known to breed in three areas of Australia, the Kelp Gull has displayed by far its most rapid growth in south-east Tasmania (Thomas, 1969) • The Pacific Gull and Kelp Gull are similar in size and morphology, so it could be predicted that they would compete for resources such as food or nest sites.