Are Kelp Gulls Larus Dominicanus Replacing Pacific Gulls L. Pacificus

Australian Field Ornithology 2019, 36, 47–55 http://dx.doi.org/10.20938/afo36047055 Are Kelp Gulls Larus dominicanus replacing Pacific Gulls L. pacificus in Tasmania? William C. Wakefield1, 2, Els Wakefield2 and David A. Ratkowsky3* 1Deceased 212 Alt-na-Craig Avenue, Mount Stuart TAS 7000, Australia 3Tasmanian Institute of Agriculture, University of Tasmania, Private Bag 98, Hobart TAS 7001, Australia *Corresponding author. Email: [email protected] Abstract. The nominate subspecies of the Pacific Gull Larus pacificus, widespread along the coast of southern Australia, may be under threat from the slightly smaller, but opportunistically competitive, self-introduced Kelp Gull L. dominicanus. To assess this threat to the Pacific Gull in Tasmania, we documented colony size of large gulls across many Tasmanian islands over a period of 24 breeding seasons (1985–2009). The most northerly Kelp Gull nests on the Tasmanian mainland were located at Paddys Island, St Helens. There were no reports of Kelp Gulls along any part of the northern coast of Tasmania abutting Bass Strait, although there were sporadic sightings on islands of the Furneaux Group. The stronghold of the Kelp Gull in Tasmania is the Estuary of the Derwent River and its surrounding bays and channels, where this species is present in much larger numbers than the Pacific Gull, but nevertheless co-exists with that species. We found no evidence for dramatic changes in numbers since 1985. All Pacific Gull nests were on small islands, and there were none at Orielton Lagoon, which became the third biggest Kelp Gull colony studied in the south-east of Tasmania. At the largest Kelp Gull colony (Green Island), the number of Pacific Gull nests declined while numbers of Kelp Gull nests increased, but no such inverse correlation was observed elsewhere, including Curlew and Hog Islands, where numbers of Kelp Gull nests increased. We conclude that numbers of Pacific Gulls have remained reasonably stable, despite increases in Kelp Gull nests at certain sites. Background to the study (Serventy et al. 1971), occurring mostly along the coasts of southern Australia (Higgins & Davies 1996; Menkhorst et The late Dr William Charles (Bill) Wakefield (WCW), a al. 2017). The Kelp Gull is believed to have self-introduced medical practitioner with a lifelong interest in avifauna, (i.e. colonised) (Serventy et al. 1971), the first published had more than 50 years of experience in research on sightings in Australia having been made at Botany Bay, New seabirds and land birds, and handled and banded at least South Wales, in January and February 1943 (McGill 1943), 100 000 birds in Australia and the United Kingdom. In though two earlier records were recognised subsequently the Australian Bird and Bat Banding Scheme, he was an (Higgins & Davies 1996): at Claremont, Western Australia, A-class bander with experience in all forms of capture of in August 1924 (Ford 1965); and Port Stephens, New birds from a broad range of species. With his wife Cherry, South Wales, in January 1938 (D’Ombrain 1973). After an he moved to Tasmania from the United Kingdom in 1972 expansion in its range in the 1960s, it established breeding and investigated gulls there from c. 1974. His wife and populations in New South Wales, Victoria and Tasmania, three sons accompanied him on visits to refuse tips to and its range expanded into South Australia and the count gulls and checked the beaches where Pacific Gulls southern coast of Western Australia (Close 1981; Warneke Larus pacificus fed during winter. After Bill and Cherry & Dann 2013; Menkhorst et al. 2017). It became numerous divorced in 1985, Bill met and married Margaret (Maggie), in south-eastern Tasmania but remains scarce elsewhere who, until she became ill in 1999, often accompanied him and, like the Pacific Gull, is largely absent from northern to the islands and assisted with the banding of chicks Australia (Higgins & Davies 1996; Menkhorst et al. 2017). and counting of nests. Beginning in January 1985, in In addition to having a similar geographical distribution in addition to the bird-banding activities, Bill undertook Australia, the two species are similar in morphology and several scientific projects concurrently, two of them being there is a considerable overlap in their diets (Coulson & ‘Trends in breeding bird population sizes’ and ‘Interactions Coulson 1993). However, there is a major difference in between populations of gulls and terns’. During Maggie’s their world-wide distribution. Whereas the Pacific Gull is illness, Bill had to abandon his banding and counting confined to the coasts of southern Australia, the Kelp Gull activities between the breeding seasons 1999–2000 and is widely distributed in the Southern Hemisphere, with 2002–2003. Counting resumed in October 2003 with the large breeding populations in South Africa (Whittington et assistance of Els Hayward (later Els Wakefield, EW) and al. 2016), South America (Yorio et al. 1998), New Zealand continued until the end of breeding in 2009–2010. He died (Fordham 1967), many sub-Antarctic islands (Sibley & in 2011. Monroe 1990), and the Antarctic Peninsula (Watson 1975; Quintana & Travaini 2000; Branco et al. 2009). Most Introduction populations are coastal but the species ranges far inland in Patagonia and New Zealand (Burger & Gochfeld 1996). Tasmania has two species of large gull, the Pacific The first documented sighting of the Kelp Gull in Gull Larus pacificus Latham, 1801 and the Kelp Gull Tasmania occurred in 1955 at Ralphs Bay (Wall 1956). L. dominicanus Lichtenstein, 1823. Historically, only Less than a decade later, in 1963, the first breeding record the former was present in Australia, where it is endemic was reported at Curlew Island (Wolfe 1969) (see Table 1 48 Australian Field Ornithology W.C. Wakefield et al. for the latitude and longitude of this and other Tasmanian extent that these gulls were preventing adult Pacific Gulls localities), and within a few more years the Kelp Gull from holding territories during winter, possibly influencing had established a considerable stronghold in Pitt Water survival or reproductive success of that species. Coulson (Thomas 1967), where it was reported in all seasons of & Coulson (1998) suggested that control of the Kelp Gull the year. population might be warranted if there were evidence of further detrimental effects. They concluded that more study Before 1985, the only information about nesting was needed to determine whether the population increase and the diets of the Pacific and Kelp Gulls in Tasmania of the Kelp Gull might influence the survival or reproductive was obtained from a study between 6 November and success of the Pacific Gull. 5 December 1981 on Green Island, where both species were known to breed. Examination of 43 regurgitated pellets Here we report on colony size (a likely correlate of from 27 Pacific Gull nests and 44 regurgitated pellets from reproductive output) of these two species of large gull 275 Kelp Gull nests revealed that both species utilised a resident in Tasmania obtained over a period of 24 breeding wide range of food sources (Coulson & Coulson 1993). seasons from 1985 to 2009, for 22 Tasmanian off-shore For the Pacific Gull, the most frequent food categories islands and one land-based vantage point. We use these were crabs (Crustaceae: Decapoda), fish, and chitons data to address the question of whether the population of (Mollusca), and many pellets contained food remains one of these species is increasing to the detriment of the from more than one of these sources. Refuse was rare in other species. these pellets. In contrast, the most frequent component in the Kelp Gull pellets was refuse (including glass, string, Methods plastic, paper, aluminium foil and chop bones), although remains of chitons and fish were also quite common. From Study site localities surveys of large refuse tips and specific shoreline sites during winter in 1981 and 1992, recording numbers and The various surveys did not cover the whole of Tasmania, age classes of each species, Coulson & Coulson (1983, and no attempt was made to cover the major coastal regions 1998) found that Kelp Gulls of all ages were far more of the state. Because of close proximity, the Derwent abundant at refuse-disposal sites, whereas adult Pacific River and its Estuary, and many of the numerous islands Gulls fed preferentially at more natural shoreline sites. in its various bays and channels, were a particular focus. During the 11-year gap between the two surveys, the These included, but were not limited to, Arch Rock, Barren, Kelp Gull in south-eastern Tasmania had increased to the Betsey, Curlew, Fulham, Green and Hog Islands, Iron Pot, Figure 1. Map of Tasmania showing the locations that were visited in this survey. Names of closely overlapping sites are not shown. The full list follows: E – Governor I., Lachlan I., Visscher I.; NE – Paddys I.; NW – Howie I.; SE – Derwent River Estuary: Betsey I., Iron Pot; D’Entrecasteaux Channel: Arch Rock, Charity I., Curlew I., Garden I., Green I., Huon I.; Frederick Henry Bay: Hog I., Isle of Caves, Slopen I., Spectacle I.; Pitt Water: Barren I., Orielton Lagoon, Susie Islet; Norfolk Bay: Fulham I., King George I., Smooth I. See Table 1 for the latitudes and longitudes of these localities. I. = Island; E = east, NE = north-east, NW = north-west, SE = south-west. Are Kelp Gulls replacing Pacific Gulls in Tasmania? 49 King George, Smooth and Spectacle Islands, and Susie islands where both species bred, the nests were sometimes Islet (see Table 1). Many of the islands were already known intermingled, and sometimes quite close together. In to be important breeding places for one or both species. general, Pacific Gull nests were tidier, higher-walled and There were no visits to the southern coast, the south-west, larger than Kelp Gull nests, which were messier and lower- or the north-western coast south of Cape Grim.

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