Gull-billed Terns in north-western Australia: subspecies identification, moults and behavioural notesDanny I. Rogers A F , Peter Collins B , Rosalind E. Jessop C , Clive D. T. Minton D and Chris J. Hassell E
A Johnstone Centre, Charles Sturt University, Albury, NSW 2640, Australia. Present address: 340 Ninks Road, St Andrews, Vic. 3761, Australia.
B RMB 4009, Cowes, Phillip Island, Vic. 3922, Australia.
C Phillip Island Nature Park, PO Box 97, Cowes, Phillip Island, Vic. 3922, Australia.
D 165 Dalgety Rd, Beaumaris, Vic. 3193, Australia.
E Turnstone Nature Discovery, PO Box 3089, Broome, WA 6725, Australia.
F Corresponding author. Email: firstname.lastname@example.org
Emu 105(2) 145-158 https://doi.org/10.1071/MU04045
Submitted: 10 October 2004 Accepted: 4 April 2005 Published: 30 June 2005
Two subspecies of the Gull-billed Tern (Sterna nilotica) occur along the coasts of north-western Australia: the large, pale Australian subspecies macrotarsa, and a smaller, darker migratory subspecies from northern Asia. On the basis of banding data we describe the measurements and moult strategies of both subspecies in north-western Australia and identify the Asian migrants as subspecies affinis. Asian migrants have a predictable plumage cycle including regular alternation between breeding and non-breeding plumage in adults. The moult strategy of Australian macrotarsa is more varied and we argue it is adapted to exploit unpredictable breeding opportunities. Plumage and structural characters described in this paper allow the two subspecies to be distinguished in the field, and field observations demonstrate some broad ecological differences between them. Adult affinis occur in Australia from August to April, with smaller numbers of immatures remaining during the dry season; they are strictly coastal, occurring in highest abundance over intertidal flats near mangrove systems where they pluck prey from the surface of mud while in flight. Subspecies macrotarsa uses the north-western Australian coast as a non-breeding area, but it does so mainly during the dry season and also uses grasslands and inland wetlands; unlike affinis, in Roebuck Bay it is regularly kleptoparasitic, stealing large crabs (Macropthalmus sp.) from Whimbrels (Numenius phaeopus).
We thank the Australasian Wader Studies Group for access to their banding data, Broome Bird Observatory for its role as a base of banding operations, and both for data on migratory departures. John Stoate of Anna Plains Station provided essential access and support during fieldwork on Eighty Mile Beach. Data on intertidal distributions were collected by D. I. Rogers as part of a shorebird study supported by Environment Australia and a post-graduate scholarship from Charles Sturt University. Capture and banding of Gull-billed Terns in north-western Australia was under licence from the Western Australian Department of Conservation and Land Management and numbered metal bands were supplied by the Australian Bird and Bat Banding Scheme. Adrian Boyle was instrumental in making Gull-billed Tern catches at crucial times and provided much helpful discussion. For access to the museum specimens in their care, we thank Rene Dekker (Rijksmuseum van Natuurlijke Historie, Leiden), Rory O’Brien (Museum of Victoria) and Ron Johnstone (Western Australian Museum). Invaluable discussion and comment were provided by Ken Rogers, Mike Carter, Chris Corben, Jill Dening, Dion Hobcroft, David James, Jutta Leyrer, Ian Nisbet, Peter Pyle, Kees Roselaar, Iain Taylor, Tony Tree and two anonymous referees.
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