Seasonal movements and urbanisation of Silver Gulls
The movements of birds, whether true migrants, dispersal, nomadic or irruptive movements are adaptations to enhance the chances of survival of those individuals that make them. How and when movements are achieved, and where individuals go, can be complex for some species.
Past studies of Silver Gulls at coastal colonies demonstrate partial migration, especially by younger birds, between breeding colonies and winter refuge areas (Carrick et al. 1957; Murray and Carrick 1964). Dispersal from colonies varies widely in proportion of birds involved, possibly according to the winter food resources of the breeding area, and in the direction and distance the location and survival areas are located. Return by young at age 3 or more and breeding adults to their own colony is also common at coastal colonies. However, knowledge of inland colonies (those west of the Great Diving Range), their movements and habitat preferences are less well known. Permanent Silver Gull colonies west of the Great Diving Range are now common place and are often centred on human populations and their rubbish. None of these colonies have been well studied in an aspect of ecology.
Silver Gulls were first reported breeding on Lake Burley Griffin in 2003 on moored boats (Holland 2004a,b) however, anecdotal evidence suggested that gulls had been breeding for at least 10 years prior in the same location (Davey and Fullagar 2012). There is no mention of gulls breeding in the ACT which might indicate that it’s a recent phenomenon (Higgins and Davies 1996). In 2008, Silver Gulls were reported breeding on Spinnaker Island and Davey and Fullagar (2010, 2011) have reported subsequent breeding seasons. In October 2011, a maximum of 400 adults were reported on Spinnaker Island and by December an additional estimated 145 fledged young were observed free flying close to the breeding colony. Based on two years of data, Davey and Fullagar (2011) have suggested that the Silver Gull colony has not increased since 2010 and might be resource limited not by nesting space but by food availability. This assumption has yet to be tested adequately and raises the question of what is the population size of Silver Gulls in the ACT, what their trend is and where do juvenile Silver Gulls disperse to?
Increased availability of human-derived food is one factor that may lead to avian population changes and some gull populations worldwide have undergone dramatic increases over the past 50 years (del Hoyo et al 1992). These species exploit novel feeding opportunities at garbage depots, fishing enterprises, food outlets and agricultural production areas around human populations. Like their congeners in the northern hemisphere, past increases in the Silver Gull population in Australia have also generally been attributed to urbanisation and the resulting reliable, abundant food sources. Large populations of gulls can cause serious concerns among the public from both a health and aesthetic viewpoint.
An alternative reason why the ACT Silver Gull colony may have reached a plateau as Davey and Fullagar (2011) have suggested is through juvenile and adult dispersal or delayed breeding by some individual birds. Availability of food resources may play a key factor in the size of the Silver Gull population in the ACT but requires investigation. Winter food resources could be critical in influencing the survival and movements of Silver Gulls in the ACT.
This study will investigate the seasonal movements of Silver Gulls in the ACT and identify key habitats that maintain their survival. Mark-resight analyses will allow an estimate of the Silver Gull population size and trend in the ACT. This study will further contribute to our knowledge of urbanised gulls and help identify if Silver Gulls are likely to become super abundant in the ACT as they have done in other large urbanised centres.
This project aims to determine the seasonal movements and habitat utilisation of Silver Gulls in the ACT. Population size and trend data will be inferred from mark-resighting and will help identify if the Silver Gull colony in the ACT is increasing.
Knowledge generated from this project will further contribute to our understanding of anthropogenic effects to wildlife, including super abundant species like some gulls.
Key questions to be investigated:
• Do Silver Gulls in the ACT show a distinctive pattern of seasonal movement?
• If so, where are these birds going and are they returning to the ACT?
• What is the population size of Silver Gull in the ACT?
• Is there a trend in the population of Silver Gulls in the ACT?
• What are the key habitat requirements for Silver Gulls in the ACT?
Carrick, R., Wheeler, WR. and Murray, MD (1957) Seasonal dispersal and mortality in the Silver Gull, Larus novae-hollandiae Stephens, and Crested Tern, Sterna bergii Lichtenstein, in Australia. Wildlife Research 2: 116-144.
Davey, C. and P. Fullagar. (2011). Silver Gulls breeding on Spinnaker Island, Lake Burley Griffin, Spring 2010. Canberra Bird Notes 36: 81-83.
Davey, C. and P. Fullagar. (2012). Silver Gulls breeding on Spinnaker Island, Lake Burley Griffin, Spring 2011. Canberra Bird Notes 37: 180-184.
Holland, J. (2004a). Silver Gulls breeding on moored boats on Lake Burley Griffin. Canberra Bird Notes 29: 9-15.
Holland, J. (2004b). Confirmation of the start of the Silver Gull breeding season on moored boats on Lake Burley Griffin. Canberra Bird Notes 29: 108.
Murray, MD. and Carrick, R. (1964) Seasonal movements and habitats of the Silver Gull, Larus novaehollandiae Stephens, in south-eastern Australia. Wildlife Research 9: 160-188.