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

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Daily Activity Patterns of South Polar and Brown Skuas Near Palmer Station, Antarctica DAILY ACTIVITY PATTERNS OF SOUTH POLAR AND BROWN SKUAS NEAR PALMER STATION, ANTARCTICA PAMELA J. PIETZ Departmentof Ecologyand BehavioralBiology, University of Minnesota, Minneapolis,Minnesota 55455 USA ABSTRACT.--Iconducted a behavioral study of sympatricallynesting South Polar (Cathar- acta maccormicki)and Brown skuas(C. lonnbergi)near Palmer Station, Antarctica. A total of 4,058 bird-hours of observationswas made on eight South Polar, three Brown, and one mixed-speciespair during the 1979-1980 and 1980-1981 austral summers.I used subsetsof these data to analyze various aspectsof skua activity patterns. SouthPolar Skuas exhibited maximum resting and minimum foragingactivity during the twilight period around 2400. Brown Skuasappeared to rest and forage more randomly. Membersof the mixed pair exhibited patternssimilar to those of their respectivespecies. Severalhypotheses are suggestedto accountfor speciesdifferences in activitypatterns; most of theserelate dietary differencesand differential foraging abilitiesat low light levels. In both species,agonistic and preening activitiesoccurred at all hours. South Polar Skuas, however, were most often observedbathing in early afternoonand preening in afternoon and twilight. Pairing activity showed no discerniblerelationship with time. Foragingbouts averaged 13 min for Brown Skuaswith feeding territoriesand about 1 h for thosewithout. SouthPolar Skuasaveraged feeding trips of 2-3 h when the seawas open and over 7 h during heavy ice cover. The timing of an individual's activity was correlatednegatively with that of its mate during incubation, brooding, and postbrooding.A nonbreeding pair and breedersthat had sufferednest failure showed more positive correlations.Thus, individual activity patterns were shapedby reproductivepriorities as well as by feeding ecologyand light levels. Re- ceived13 March 1985, accepted18 March 1986. AN extensive literature exists on the relation- Brown skuas (C. lonnbergi)• where they nest ship between daily activity patterns of birds sympatrically and occasionally form mixed and various light regimes (see reviews by As- pairs. Investigations of allopatric populations choff 1967, Daan and Aschoff 1975, Gwinner suggestthat the activitypatterns of thesespecies 1975, Rusak 1981). Nevertheless, little research are very different. Spellerberg(1969) noted that hasbeen done on activity patternsof birds un- South Polar Skuas on Ross Island (77ø33'S, der natural conditions of continuous light, as 166ø09'E),the southern extreme of their breed- occursin polar regions in summer. Three de- ing range, showeda 24-h cycle of activity even cades ago, Armstrong (1954) noted that such during continuous daylight: "Between 01:00 information from arctic regions was meagre, and 05:00 ... all breeding pairs are roosting, while that from the Antarctic was too scarce to incubating, or brooding and little or no forag- warrant discussion. Since then, antarctic re- ing takes place." On the other hand, Young searchershave examined activity patterns of (1978) documented Brown Skuas feeding reg- Ad•lie Penguins (Pygoscelisadeliae; Muller- ularly during hours of darkness on the Chat- Schwarze1968), Blue-eyed Shags (Phalacrocorax ham Islands, New Zealand (44ø22'S,176ø11'W). atriceps; Bernstein and Maxson 1984), and STUDY AREA AND METHODS Southern Black-backed Gulls (Larus dominican- us; Maxson and Bernstein 1984, Fraser pets. This study was conductedduring the 1979-1980 comm.). Literature on skuas in Antarctica con- and 1980-1981 austral summers on several small is- tains only an incidental comment on timing of activity (Spellerberg 1969). •Pending resolution of their taxonomic status, I At Palmer Station (64ø46'S,64ø03'W) on An- refer to these as separatespecies. For discussionsof vers Island, Antarctica,I had an opportunity to skua systematicsand nomenclature,see Pietz (1984, observeSouth Polar (Catharactamaccormicki) and 1985). 726 The Auk 103: 726-736. October 1986 October1986] SkuaDaily Activity Patterns 727 lands and peninsulaswithin 5 km of Palmer Station, that had been pooledover the 24 h. Finally, weighted Antarctica.The ice-freeareas provided nesting sites hourly meanswere used to calculateangular-linear for nearlya dozenbird species(Parmelee et al. 1977), correlations(Johnson and Wehrly 1977), in a test for includingSouth Polar Skuas(approx. 250 pairs)and 24-h patternsrelated to either sine or cosinefunc- Brown Skuas(up to 12 pairs). From late November tions. to late January there is enough light at Palmer to Agonistic,preening, bathing, and pairing behav- conductfieldwork 24 hours a day, despitethe fact iors occurredtoo rarely to examine variation among that the sun dips below the horizon for a short pe- individuals or pairs. For each of the first three of riod around 2400. thesebehavioral groups,data on all individuals from Skuaswere color-bandedto allow recognitionof all observationdays were combinedby hour within individuals at a distance. Most of these birds had species.For pairing behaviors,only datafor the male been banded in previousyears and thus had docu- of each pair were combined. mented breeding histories. Sexesof birds were de- Dawn-to-dusk observations from 1980 (1,406 bird- terminedby copulatoryposition and courtshipfeed- hours)and the shorterobservation periods of 1981 ing. (805 bird-hours) were included only in calculating Skua feeding habitsalso had been documentedat lengthsof foragingtrips (or time away from the nest Palmer in previousyears (Parmeleeet al. 1978, Neil- territory) and in evaluating correlations of activity son 1983). Brown Skuashere, as in many other areas between pair members. (e.g. Stonehouse1956, Moors 1980, Sinclair 1980, Tri- Activity correlations between pair members were velpieceet al. 1980),concentrated on penguin eggs assessedfor 3,860 bird-hours of observations on 8 and chicksand sometimesdefended penguin colo- South Polar, 3 Brown, and I mixed skua pairs over niesnear their nestsas feeding territories. Some South two seasons.Behavioral data were sampledfrom the PolarSkuas feed on penguinsin areaswhere Brown time budgetsonce every 30 min, at the end of each Skuasare few (Parmeleepets. comm.)or absent(e.g. 30-min interval. For each pair at each stage of the Eklund 1961, Le Motvan et al. 1967,Trillmich 1978). breeding cycle, I constructeda 2 x 2 contingency At Palmer, however, most South Polars fed at sea on tableand tabulatedpresence or absenceof a specified fishand krill, a commonhabit in muchof their range behavior by the male and the female. Product mo- (Eklund 1961,Young 1963,Trivelpiece and Volkman ment correlation coefficients and their standard errors 1982). were calculatedfor each table (Bishopet al. 1975); Between8 December1979 and 22 January1980, we the weighted means of these coefficients,with their documentedthe behavior of 4 SouthPolar Skua pairs, associatederror terms, were calculatedacross pairs 2 BrownSkua pairs, and I mixed-speciespair. A total for each species. of 50 bird-daysof observationswas obtainedfor South For the purposesof these analyses,the breeding Polar Skuas, 14 for Brown Skuas, and 8 for the mixed seasonwas divided into (1) pre-egg-laying,(2) incu- pair. All were observedfor 24-h periods. bation (from the time the first egg was laid), (3) Between27 Januaryand 9 March 1980, when in- brooding (from the hatching of the first chick until creasingdarkness prevented 24-h observations,we it wasno longer regularlybrooded), (4) postbrooding watchedeach of the 7 pairs for 6 dawn-to-duskpe- (until the time the first chick flew well enough to riods. From 7 Januaryto 6 March 1981, I made ad- avoid capture),and (5) fledging. A sixth category ditional observationsusing sampleperiods of 3-14 (failed-nest)contained observations on pairs follow- h. EightSouth Polar Skua pairs, 2 BrownSkua pairs, ing lossof eggs or chicks. and I mixed pair constitutedthe samplein the sec- Of the 4 South Polar Skua pairs observed in ond season.The mixed pair, 1 of the Brown, and 4 1979-1980, 1 courted and defended a territory but of the SouthPolar pairs were observedin both years. failed to produceeggs. For purposesof comparison During all observationperiods, the behaviorof each with the three breedingpairs, this pair'sseason was bird wasnoted every 30 s usinga metronome(Wiens dividedinto sectionsreflecting the averagetiming of et al. 1970).From these observations hourly and daily incubationand brooding observedin the breeders. time budgetswere constructedfor each individual. In several cases, related behaviors were combined Only data collectedduring 24-h observationpe- beforeanalysis. Grouped behaviors included resting riodsof the 1979-1980season (1,691 bird-hours) were (i.e. sitting, incubating,brooding), foraging (i.e. pro- usedfor assessingrelationships between activities and curing, transportingand eating prey, activity out of time of day. For restingand foraging behaviors,the view off the nestterritory), pairing (i.e. courtshipand meanand standarderror were calculatedby hour for courtship feeding, copulation, attempted copula- eachindividual and for eachpair. Weightedhourly tion), agonisficdisplays (i.e. long call, alarm call, bent meanswere determinedfor both speciesusing pair neck, wing raising), and agonistic encounters(i.e. data,and for eachsex of both speciesusing individ- running or flying displacement,swooping, aerial ual data. Mean hourly standarderror estimateswere chasing,fighting). weighted acrossindividual or pair standarderrors Assessmentof foragingbehavior presented special 728 P^MELAJ. PIETZ [Auk, Vol. 103 SOUTH POLAR
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