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Feeding Territories of Brown Skuas (Catharacta

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Feeding Territories of Brown Skuas (Catharacta FEEDING TERRITORIES OF BROWN SKUAS (CATHARA CTA LONNBERGI) WAYNE TRIVELPIECE,1 RONALD G. BUTLER,1 AND NICHOLASJ. VOLKMAN2 Collegeof Environmental Scienceand Forestry, State University of New York, Syracuse, New York 13210 USA ABSTRACT.--Themaintenance of feedingterritories by Brown Skuas (Catharactalonnbergi) a was observedin a pygoscelidpenguin rookery on King GeorgeIsland during the 1977-1978 austral summer. Brown Skuas fed exclusivelyon penguin eggsand chicks oncethey becameavailable in late October, and skua pairs holding feeding territories in the penguin rookery defended these areas against intruders that attempted to obtain food items from these territories. These intruders included conspecifics,Southern Black-backed Gulls (Larus dominicanus), and American Sheath- bills (Chionis alba). Although territorial Brown Skuas attacked and chased overflying intruder conspecificsand gulls indiscriminately,only intruding skuaselicited the long call display from the territorial pair. Male skua energy investment in hunting and territorial defensewas significantly greater than that of the female. Brown Skua territories were defined as optimal or suboptimal based on their available food resources.Analysis of skua breeding data suggeststhat an adaptive advantage in maintaining an optimal feeding territory appears to be increasedreproductive success. Although there were no statistical differences in egg production, Brown Skuas defending optimal territories fledged sig- nificantly more chicks per pair than all other skuas [i.e. suboptimal territorial Brown Skua, nonterritorial Brown Skua, mixed Brown Skua-South Polar Skua (C. maccormicki), and South Polar Skua pairs]. This increasedreproductive success of optimal territorial pairs may be linked to the proximity of an abundant food source.Received 27 September1979, accepted6 March 1980. THE Brown Skua (Catharacta lonnbergi) is an opportunistic feeder that obtains its food through scavenging, kleptoparasitism, and predation (Burton 1968a, b; Johnston 1973). Brown Skuas have also been reported to defend feeding-breeding territories in rookeriesof pygoscelidpenguins (Stonehouse 1956, Sladen 1958). These territories are defended against conspecificsand contain coloniesof penguinsthat produceeggs and chicksthat are exploitedas a food resourceby the territorial pair. To date, the maintenance of feeding territories by Brown Skuas has not been de- scribed as completely as has been done for the congenericSouth Polar Skua (C. maccormicki) (Young 1963a, b, 1972; Wood 1971; Trillmich 1978). The objectives of the presentstudy were (1) to describequantitatively the defenseof feeding-breed- ing territories by Brown Skuas in a pygoscelidpenguin rookery, and (2) to determine whether Brown Skuas with penguin nestsin their territories bred more successfully than those without. STUDY SITE AND METHODS This study was conductedat Point Thomas, King GeorgeIsland, South ShetlandIslands (62ø10'S, 58ø30'W) from 1 November 1977 through 21 February 1978. All three pygoscelidpenguins breed in two rookeries(designated East and West) at Point Thomas. The rookeriesare approximately3 km apart and ' Presentaddress: Seabird ResearchGroup, The Mount Desert Island BiologicalLaboratory, Salsbury Cove, Maine 04672 USA. u Present address:Box 1507, Sedona, Arizona 86336 USA. a The two skuas in this paper are referred to binomially pending resolutionof their biologicaland systematicstatus (Watson 1975). 669 The Auk 97: 669-676. October1980 670 TRIVF.LPIECE, BUTLER, AND VOLKMAN [Auk, Vol. 97 ß Brown Skua Nest Site "Feeding Territory LandingBeach o S. Polar Skua Nest Site g B.-S.R Skua Mixed Pair ß Sheathbill Nest Site ß Gull Nest Site •:i*Adelie Colony ß ChinstrapColony m Gentoo Colony ß LandingBeach o o o o ß LandingBeach I / I / '?--'"...... Fig. 1. Map of the West rookery at Point Thomas showing locationsof feeding territories, penguin colonies,and nest sites of other seabirds. are separatedby a glacial tongue. An estimated 18,000 Adelie (Pygoscelisadeliae), 1,040 Chinstrap (P. antarctica), and 2,600 Gentoo pairs (P. papua) breed in the Point Thomas rookeries. In addition, 28 Brown Skua, 13 SouthPolar Skua, 4 mixed Brown-South Polar Skua, 12 American Sheathbill(Chionis alba), 13 SouthernBlack-backed Gull (Larusdominicanus), and 120Southern Giant Fulmar (Macronectes giganteus)pairs nestedat Point Thomas in 1977-78. The majority of data (includingall behavioraldata) were collectedin the West rookery, which contained16 Brown, 8 South Polar, and 4 mixed skua breeding pairs. All of the gulls, nine sheathbills,and 40 of the SouthernGiant Fulmar pairs bred there also. The Polish Academy of Science'sresearch station, Henryk Arctowski, is located I km south of the West rookery. Twelve pairs of territorial skuasin the West rookerywere selectedfor behavioralobservations from 1 November 1977 through 5 January 1978. At least one memberof each pair was capturedand banded with USFWS bands and patagial tags (constructedof Herculite) to facilitate individual identification. Skuaswere then sexedon the basisof courtshipbehavior and copulations.Behavioral observations (127 territory-observationh) of the hunting activities (e.g. attempts to obtain food items, successfulattempts, etc.) and estimated overflight altitudes of territorial skuas and all intruding birds were recorded using binoculars,digital stopwatches,and portablecassette recorders. Behavioral data were collectedfrom a vantage point that permitted an unobstructedview of the territory from a distanceof approximately 100 m, and all responsesof territorial birds to intruderswere recorded.The mapping of feedingterritories was basedupon the agonisticinteractions between territorial pairs and intruding birds over the course of the breeding season(Fig. 1). The reproductive successof 36 skua nest sites in both rookerieswas monitored at weekly intervals until late February, when skua chicks began to fledge. All nestingskuas and gullsdefended breeding territories (Type B accordingto Hinde 1956).To facilitate discussion,however, only skuasdefending feeding-breeding territories (Type A accordingto Hinde 1956) have been designatedas "territorial" in this paper. Definitionsof the terms rookery and colonyare those used by Penney (1968). October1980] BrownSkua Feeding Territories 671 SOUTH POLAR SKUA j BROWN SKUA ' • EGGS CHICKS 30 7 15 23 1 9 17 25 2 10 OCT NOV DEC JAN Fig.2. Thebreeding chronology of the Adelie Penguin (88% of all penguinpairs) and the Brown and South Polar skuas at the Point Thomas West rookery in 1977-1978. Statisticalanalysis was performed using the Kruskall-Wallisand Dunn's tests for rankeddata (H values).Data werefurther analyzed with contingencytables (X 2 values)and using the Wilcoxontwo- sampletest. The latter test was performed asdescribed bySokal and Rohlf (1969: 392) for large samples and tied values;the resultsappear as t valuesin the text. RESULTS Characteristicsof feeding territories.--Brown Skuas began arriving at Point Thomasin late October,and feedingterritories (in whichthe skuasalso nested) were establishedin both the East and West rookeriesby early November. The 12 feeding territoriesselected for studyencompassed all of the pygoscelidcolonies in the West rookery.The breedingchronology of the AdeliePenguins (88% of all penguinpairs) and the Brown and SouthPolar Skuasis presentedfor this rookeryduring the 1977- 1978 seasonin Fig. 2. Territoriescontained from 90 to 2,011 penguinnests (œ = 1,028),and 2 territoriescontained 2 or morepenguin species. Breeding pairs that defendedfeeding territories preyed and/or scavengedexclusively on penguineggs and chicks. Territorial boundaries remained constant over the course of the repro- ductive cycle. Brown Skuashunted by flying over a colony,searching for a potentialfood item, hovering,and droppingto seizethe food item. Most successfulhunting attempts were initiated from an overflightaltitude of 1-3 m. The skua generallyflew back to the nestsite with the eggor chick. Althoughboth "active predation"and "scav- enging"(i.e. seizureof desertedeggs or deadchicks) were recorded, it wasgenerally not possibleto distinguishbetween them when a bird acquireda food item. The mean (m SE) durationof 50 aerial "huntingbouts" for territorialskuas was 68.8 (m 8.0) s. Intruder skuasand gullsused a similarmethod, except that they flew directlyout of the territorywith a fooditem. Intruderswith a fooditem that were sightedby a territorialskua were chased. Occasionally, the intruderwas forced to drop the egg or chick, which the territorialanimal then recovered.Hovers and 672 TRIVELPIECE,BUTLER, AND VOLKMAN [Auk, Vol. 97 TABLE 1. Comparisonsof rates of overflights, attempts, and egg-chickstaken by territorial skuas and intruder species(mean +- S.E.) during 127 h of territory observation.a Intruders Terri- torial South Southern Brown Brown Sheath Polar Giant Skuas Skuas Gulls bills Skuas Fulmars Overflights/h 2.1 +- 0.3 11.1 +- 0.6** 4.5 +- 0.6** 0.6 +- 0.2** 0.1 +- 0.1'* 0.2 +- 0.1'* Attempts to obtain eggsor chicks/h 0.7 +- 0.1 0.5 +- 0.1 0.2 +- 0.1' __b 0* 0* Eggs and chicks taken/h 0.6 +- 0.1 0.4 +- 0.1 0.2 +- 0.1' 0.1 +- 0.1' 0* 0* a Comparisonsanalysed with Kruskall-Wallis and Dunn's tests;* - significanceat the 0.01 level; ** = significanceat the 0.005 level. b Sheathbillswere never observedmaking an aerial attempt to obtain a penguin egg. They usually entered the colony on foot to obtain broken or abandoned eggs. attempts to seize prey items by intruders that were sighted also resulted in a chase and/or an attack by the territorial bird.
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