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

Abstract

Abstract 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.