Hans Christian Pedersen, Norwegian Institute for Nature Research, Tungasletta 2, N-7004 Trondheim, Norway
BACKGROUND AND OBJECTIVES
Most South Polar Skua Catharacta maccormicki populations studied are found at the coast of the Antarctic continent, of ten feeding on krill and fish or scavenging on refuse or penguins (Furness 1987). According to Furness (1987) breeding has been
documented only in one inland population found in the Theron Mountains, whereas breeding is probable but not weU documented in Dronning Maud Land and Byrd Land. However, in 1985 about 50 breeding pairs of South Polar Skua was found at Svarthamaren (71°53'S, 05°IQ'E), in Mtihlig-Hofmannfjella, Dronning Maud Land (Mehlum et al. 1985). At Svarthamaren the South Polar Skua was found breeding in association with colonies of Antarctic Petrel Thalassoica antaretiea and Snow Petrel Pagodroma nivea, and the skuas were totally dependent on the petrels as food.
We stayed at the breeding grounds at Svarthamaren between 12 January and 18 February 1990, with one main objective being to study different aspects of
reproductive behaviour in this South Polar Skua population. The questions asked were the following:
l) Do territorial behaviour limit breeding numbers?
2) To what extent do possible replacement birds adopt chicks of widowed birds?
3) Is breeding success correlated with territory size?
In addition severai behavioural observations were carried out throughout the whole period.
44 MATERIAL AND ME THODS
Territorial behaviour and breeding success was studied in the South Polar Skua population from 12 January to 18 February 1990. Mapping of 18 territories was carried out through direct observation of territorial behaviour of both birds in a pair towards their neighbours and non-territorial birds. To identify the different territorial birds 14 of them were ringed, 12 with different combinations of colour-rings.
During the period 22 January to 3 February the males were removed from eight different territories. At the time of removal the birds were incubating addled eggs on two territories, had one-two weeks old chicks on three territories, and four weeks or older chicks on three. During the period 3-17 February four males were successively removed from one of the territories. All removed males were weighed, and the length of wing, beak and tarsus measured. The situation in the vaeant territories were
registered by severai daily inspections both with respect to possible replacement males and with respect to survival of chicks.
PRELIMINARY RESULTS
Territorial behaviour and breeding numbers
Altogether 82 breeding pairs of South Polar Skua were found in a narrow belt at the base of the Antarctic Petrel colony. In addition about 80 non-breeding skuas were observed. A great number of these birds were of ten observed at one club site on the glaeier about 300 m from the breeding skuas.
Non-breeding birds were frequently observed searching for vacancies on the occupied area, and were equally frequently observed being attacked and chased away by
territorial birds. After the males were removed from eight territories non-breeding birds were observed circling over the territories for long periods. Eventually replacement males settled on five of the territories. The time between removal and replacement varied considerably. The three territories where the widowed birds remained single had all one to two weeks old chicks at the time of removal. In one of the territories with addled eggs, replacement was very rapid. Here four males were removed successively and the third and fourth replacement bird was found to be ringed at Svarthamaren in 1985.
Adoption by replacement males
It was possible to get data on this behaviour on ly from two territories. The males at the se territories behaved quite differently with respect to investment in the chicks of their new mate. One of the males was not observed off the territory at all. Nor was it observed that the female on this territory flew up in the petrel colony to get food for the chicks. Eight days after the replacement male had settled on the territory the youngest chick was found dead and eaten on the territory. Five days later the oldest chick was found dead and eaten, also this one within the territory. It is, however, impossible to say whether the chicks starved to death or were killed by the
replacement male. Shortly after the chicks were dead, both the male and the female were observed eating freshly killed Antarctic Petrel chicks on the territory.
On the other territory remains of freshly killed petre l chicks were found continuously.
The replacement male was observed bringing petrel chicks to the territory, and imrnediately after landing he was chased away from the petrel chick by the female.
The female then tore the petrel chick into smaller pieces and finally flew to another part of the territory where she fed the chicks.
Territory size and breeding success
Breeding success measured as number of chicks fledged per territory was ca1culated for 26 territories at the end of the fieid season (17-18 February). On these territories the mean number of chicks fledged was 1.54.
On the mapped territories the results indicate that breeding success was Iower on small than on medium and big territories. It is not dear whether this is caused by a greater proportion of territories with only one egg on small territories compared with medium and big territories, or because small territories have a higher risk of being visited by neighbouring skuas which eat eggs and chicks.
REFERENCES
Fumess, R. W. 198 7: The Skuas. T & AD Poyser, Ltd. Calton, England.
Mehlum, F., Bech, C. & Haftom, S. 1985: Ornithological investigations in Miihlig
Hofmannfjella, Dronning Maud Land. Nor. Polarinst. Rapportserie 22,27-34.