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Breeding Behaviour of the Swallow-Tailed Gull in the Galapagos

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image of Behaviour

The calls and displays of the swallow-tailed gull (Creagrus furcatus) are described and fitted into the pattern of larid displays; some ecological aspects of reproductive behaviour are briefly described. 1. The striking vocabulary includes a scream, snore, rattle and whistle. There is a 'long call' apparently homologous with the generalised gull long call and a special call associated with downward choking. 2. Swallow-tailed gulls breed in relatively small groups, the reproductive activities of the population of any island being highly unsynchronised, though those of the group significantly more so. The 'local' synchronisation is probably socially facilitated. 3. Fighting is rare, but includes a twisting movement reminiscent of the kittiwake's (Rissa tridactyla) cliff-adapted method. 4. Threat gaping is shown. 5. The oblique postures may be separated into (a) the retracted oblique (probably not equivalent to the generalised forward oblique of some other gulls) (b) a low oblique (probably homologous with the generalised oblique of other gulls). 6. A downward jerking movement may be a form of long calling and swallow-tails show a backward head jerk in closequarter territorial encounters. 7. An aggressive upright may be distinguished from an anxiety upright. 8. Upward choking (as in the kittiwake, but not in other gulls) and downward choking occur; both forms are hostile displays on the nest site. 9. The butterfly flight is probably strictly an intra-pair display and is associated with snore-scream calls. It may be related to the soar-and-swoop of the black-headed gull (Larus ridibundus), which is probably ritualised pursuit flight. 10. In the swallow-tail foot-looking is commonly associated with agonistic behaviour. 11. Courtship feeding of female by male is a regular part of pre-laying pair behaviour as in other larids. 12. Pre-copulatory behaviour is accompanied by head tossing. 13. Nest building is accompanied by long bouts of nest site calling. The nest is architecturally functionless. 14. The single egg (incubation period 35 days and incubation stint about 24 hrs) produces a highly cryptic chick. 15. Egg shells are not systematically removed. 16. The chick may accompany its parents some distance from the site in the first 48 hours after hatching. 17. Squids form a large part of its diet. The position of the swallow-tailed gull with respect to other larids is tabularised and discussed.

Affiliations: 1: Zoology Dept., University of Oxford, England


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