Feeding and Nesting Ecology of Sympatric South Polar and Brown Skuas

--In the 1979-1980 and 1980-1981 austral summers, I examined nesting and feeding ecology of South Polar (Catharacta maccormicki) and Brown (C. lonnbergi) skuas near Palmer Station, Antarctica. As earlier studies had suggested, South Polar Skuas fed mostly at sea on fish, and Brown Skuas fed mostly on penguin eggs and chicks. These dietary differences correlated with differences in the skuas' time budgets, activity patterns, territory types, breeding chronologies, nest densities, clutch sizes, and fledging success. With combined data from this and other studies at Palmer, I calculated average productivity for both species over 7 breeding seasons. Brown Skuas fledged higher and more consistent numbers of young per pair, probably because of their more accessible and reliable food source. Received 13 March 1985, accepted 18 January 1987. SKIdAS of the Southern Hemisphere are well known for their opportunistic feeding habits. South Polar Skuas (Catharacta tnaccorrnicki) and Brown Skuas (C. Ionnbergi) • have been recorded as predators, scavengers, and kleptoparasites on a wide variety of organisms (e.g. Stonehouse 1956; Young 1963a, 1978; Burton 1968; Johnston 1973; Moors 1980; Sinclair 1980; Maxson and Bernstein 1982; Osborne 1985; Green 1986). Despite their opportunism, most skuas depend primarily on a few prey types. Where they nest allopatrically on the coasts of the Antarctic Continent, South Polar Skuas generally fish at sea, prey on penguin eggs and chicks, or both (Eklund 1961, Young 1963a, Le Morvan et al. 1967). On subantarctic islands, allopatric Brown Skuas feed primarily on penguin eggs and chicks or on adults of small Procellariiformes (Stonehouse 1956, Young 1978, Jones 1980, Moors 1980, Sinclair 1980, Fraser 1984, Osborne 1985). Brown and South Polar skuas nest sympatrically in a few areas, mostly along the Antarctic Peninsula between 61 o and 65oS. At these sites there appears to be a sharp partitioning of food resources; Brown Skuas monopolize the terrestrial food source of penguin eggs and chicks, and South Polar Skuas forage almost exclusively at sea (Parmelee et al. 1978, Trivelpiece et al. 1980, Hemmings 1984). One might expect to find other differences in • Pending resolution of their taxonomic status, I refer to these as full species. For discussions of skua systematics and nomenclature, see Pietz (1984, 1985). Brown and South Polar skua ecology that correlate with these differences in diet. A few such correlates already have been documented. Nellson (1983), near Palmer Station on Anvers Island (64o46'S, 64o03'W), and Trivelpiece and Volkman (1982), at Point Thomas on King George Island (62o10'S, 58o30'W), noted parallels between breeding chronology and food habits. Nellson also noted differences in average duration of foraging trips, and both Trivelpiece et al. (1980) and Nellson observed differences in fledgling success. My purpose was to examine skua ecology and behavior for correlates of the observed dietary differences. Possible correlates such as time budgets, diurnal activity patterns, and variation in egg and clutch size received little or no attention in earlier studies. Aspects of skua ecology studied previously merit further attention because skuas are long-lived birds (probably a 30-40-yr life span; Ainley 1981) and experience tremendous variability in feeding conditions within (Parmelee et al. 1978) and between (e.g. Trillreich 1978, Trivelpiece et al. 1980) nesting grounds. STUDY AREA AND METHODS This study was conducted during the 1979-1980 and 1980-1981 austral summers on several small islands and peninsulas within 5 km of Palmer Station. The ice-free areas around Palmer provide nesting sites for nearly a dozen bird species (Parmelee et al. 1977), including over 200 pairs of South Polar Skuas and up to 12 pairs of Brown Skuas. Each year there is also at least one mixed-species pair, usually consisting of a 617 The Auk 104: 617-627. October 1987 618 P^MEL^ J. Pmrz [Auk, Vol. 104 South Polar Skua male and Brown Skua female. These mixed pairs produce viable hybrid offspring, capable of producing viable F2 offspring (Parmelee and Rimmet pets. comm.). Most skuas involved in this study were color-banded to allow individual recognition at a distance. Birds were captured with long-handled hand nets at the nest sites, or with rocket nets at bait sites. Most had been banded in previous years and thus had known breeding histories. Sexes were determined from observations of their copulatory positions, their roles in courtship feedings, or both. Diet.--Neilson studied skua feeding habits at Palmer in the mid-1970's; he noted (Nellson 1983) that many Brown Skua pairs defended Ad•lie Penguin (Pygoscelis adeliae) colonies near their nest sites as feeding territories. Like Neilson, I gathered data on skua diets by recording foods regurgitated when adults fed chicks or mates and when adults or chicks were captured. Additional information came from direct observations of adult foraging activity. Time budgets.--During observation periods at several skua nest sites, the behavior of each pair member was noted every 30 s using a metronome (Wiens et al. 1970). From these records hourly and daily time budgets were constructed for each individual and pair. A total of 4,058 bird-hours of observations was made in the two seasons. Between 8 December 1979 and 22 January 1980, a field assistant and I documented the behavior of 4 South Polar Skua pairs, 2 Brown Skua pairs, and 1 mixed-species pair using 24-h sample periods. We obtained 50 bird-days of observations for South Polar Skuas, 14 for Brown Skuas, and 8 for the mixed pair. Between 27 January and 9 March 1980, when increasing darkness prevented 24-h observations, we watched each of the 7 pairs for 6 dawn-todusk periods. From 7 January to 6 March 1981 I made additional observations, using sample periods of 314 h. Eight South Polar Skua pairs, 2 Brown Skua pairs, and 1 mixed pair were sampled in the second season. The mixed pair, 1 Brown pair, and 4 South Polar pairs were observed in both years. Some related behaviors were combined before time budget analysis. Grouped behaviors included resting (sitting, incubating, and brooding), foraging (procuring, transporting, and eating prey, and out of view off the nest territory), pairing (courtship and courtship feeding, copulation and attempted copulation), agonistic displays (long call, alarm call, bent neck, and wing raising), and agonistic encounters (running or flying displacement, swooping, aerial chasing, and fighting). Skuas were assumed to be resting during hours of darkness. Assessment of foraging behavior presented special problems related to differences in observability of terrestrial and marine feeding. Total foraging time included periods when birds were off their territories and out of view as well as when they were feeding visibly (see Pietz 1986). Breeding biology.--I divided the breeding season into (1) pre-egg laying, (2) incubation (after the first egg was laid), (3) brooding (after the first chick hatched), (4) postbrooding (after the first chick was no longer brooded regularly), and (5) fledging (when the first chick flew well enough to avoid capture). A sixth category (failed-nest) contained observations on pairs following loss of eggs or chicks. Of the 4 South Polar Skua pairs observed in 19791980, 1 courted and defended a territory but failed to produce eggs. For comparison with the 3 breeding pairs, this pair's eggless season was divided into sections based on the average timing of incubation and brooding of the breeders. In each season, breeding chronologies, egg sizes, chick growth rates, and reproductive success were documented for over 20 South Polar Skua pairs and for all Brown Skua and mixed-species pairs in the study area. Egg volumes were calculated from measurements of length and breadth, using the formula V = (0.00048)LB 2(Furness 1983). Chicks were weighed and their tarsi and seventh and tenth primaries measured every 2-4 days. Nest counts, fledgling counts, and notes on breeding chronology were recorded for the Ad•lie Penguin colonies. Approximately 14,000 Ad•lie pairs nested on islands in the study area.