Daily Activity Patterns of South Polar and Brown Skuas Near Palmer Station, Antarctica

-I conducted a behavioral study of sympatrically nesting South Polar (Catharacta maccormicki) and Brown skuas (C. lonnbergi) near Palmer Station, Antarctica. A total of 4,058 bird-hours of observations was made on eight South Polar, three Brown, and one mixed-species pair during the 1979-1980 and 1980-1981 austral summers. I used subsets of these data to analyze various aspects of skua activity patterns. South Polar Skuas exhibited maximum resting and minimum foraging activity during the twilight period around 2400. Brown Skuas appeared to rest and forage more randomly. Members of the mixed pair exhibited patterns similar to those of their respective species. Several hypotheses are suggested to account for species differences in activity patterns; most of these relate dietary differences and differential foraging abilities at low light levels. In both species, agonistic and preening activities occurred at all hours. South Polar Skuas, however, were most often observed bathing in early afternoon and preening in afternoon and twilight. Pairing activity showed no discernible relationship with time. Foraging bouts averaged 13 min for Brown Skuas with feeding territories and about 1 h for those without. South Polar Skuas averaged feeding trips of 2-3 h when the sea was open and over 7 h during heavy ice cover. The timing of an individual's activity was correlated negatively with that of its mate during incubation, brooding, and postbrooding. A nonbreeding pair and breeders that had suffered nest failure showed more positive correlations. Thus, individual activity patterns were shaped by reproductive priorities as well as by feeding ecology and light levels. Received 13 March 1985, accepted 18 March 1986. AN extensive literature exists on the relationship between daily activity patterns of birds and various light regimes (see reviews by Aschoff 1967, Daan and Aschoff 1975, Gwinner 1975, Rusak 1981). Nevertheless, little research has been done on activity patterns of birds under natural conditions of continuous light, as occurs in polar regions in summer. Three decades ago, Armstrong (1954) noted that such information from arctic regions was meagre, while that from the Antarctic was too scarce to warrant discussion. Since then, antarctic researchers have examined activity patterns of Adelie Penguins (Pygoscelis adeliae; MullerSchwarze 1968), Blue-eyed Shags (Phalacrocorax atriceps; Bernstein and Maxson 1984), and Southern Black-backed Gulls (Larus dominicanus; Maxson and Bernstein 1984, Fraser pers. comm.). Literature on skuas in Antarctica contains only an incidental comment on timing of activity (Spellerberg 1969). At Palmer Station (64?46'S, 64?03'W) on Anvers Island, Antarctica, I had an opportunity to observe South Polar (Catharacta maccormicki) and Brown skuas (C. lonnbergi)l where they nest sympatrically and occasionally form mixed pairs. Investigations of allopatric populations suggest that the activity patterns of these species are very different. Spellerberg (1969) noted that South Polar Skuas on Ross Island (77?33'S, 166?09'E), the southern extreme of their breeding range, showed a 24-h cycle of activity even during continuous daylight: "Between 01:00 and 05:00 ... all breeding pairs are roosting, incubating, or brooding and little or no foraging takes place." On the other hand, Young (1978) documented Brown Skuas feeding regularly during hours of darkness on the Chatham Islands, New Zealand (44?22'S, 176?11'W). STUDY AREA AND METHODS This study was conducted during the 1979-1980 and 1980-1981 austral summers on several small is1 Pending resolution of their taxonomic status, I refer to these as separate species. For discussions of skua systematics and nomenclature, see Pietz (1984, 1985). 726 The Auk 103: 726-736. October 1986 This content downloaded from 207.46.13.131 on Sat, 15 Oct 2016 04:28:52 UTC All use subject to http://about.jstor.org/terms October 1986] Skua Daily Activity Patterns 727 lands and peninsulas within 5 km of Palmer Station, Antarctica. The ice-free areas provided nesting sites for nearly a dozen bird species (Parmelee et al. 1977), including South Polar Skuas (approx. 250 pairs) and Brown Skuas (up to 12 pairs). From late November to late January there is enough light at Palmer to conduct fieldwork 24 hours a day, despite the fact that the sun dips below the horizon for a short period around 2400. Skuas were color-banded to allow recognition of individuals at a distance. Most of these birds had been banded in previous years and thus had documented breeding histories. Sexes of birds were determined by copulatory position and courtship feed-