Local movements, home ranges and body condition ofCommon Eiders Somateria mollissima wintering in

Southwest Greenland

Merkel F.R., Mosbech A., Sonne C., Flagstad A., Falk K. & Jamieson S.E.2006. Local movements, home ranges and body condition of CommonEiders Somateria mollissima wintering in Southwest Greenland. Ardea94(3): 639–650.

We examined local movements, home ranges and body condition ofwintering Northern Common Eiders Somateria mollissima borealis inSouthwest Greenland from late winter until spring migration in 2000and 2001. At key marine habitats at coastal areas and in the inner fjordsystem of Nuuk, we implanted 33 Eiders with satellite transmitters andcollected Eider carcasses for body condition analyses. Most Eiders exhib-ited strong site fidelity during the study period with a mean 95% homerange size of 67.8 km2 and a mean core area (50%) of 8.1 km2. Diurnalmovements peaked at dawn and dusk when birds presumably movedbetween feeding areas and roosting sites. Roosting occurred near day-time activity centres, on average 1.7 km apart. Among birds marked atcoastal habitats only between 8% and 29% also used the inner fjordhabitats, despite high levels of hunting at the coastal area. Birds that didmove to the inner fjord system did not return to the coastal area. Thesefindings accentuate the need for managing wintering Common Eiders inSouthwest Greenland also at a local scale, taking site fidelity intoaccount. The body condition of adult fjord birds was either equal orsuperior to that of coastal birds. However, within-years and between-years-variation in body condition were larger for fjord birds, suggestingthat they were challenged by a higher unpredictability at the local scale.Observations indicate that differences in habitat characteristics and thebehaviour of avian predators may limit the exchange of Eiders betweenthe coastal area and the inner fjord system. Our study suggests thatoften Eiders build up body reserves for the breeding season elsewhere;in three of four cases adult body condition declined from late winter toearly spring.

Key words: satellite telemetry, home range, movements, body condition,Northern Common Eider, site fidelity, Southwest Greenland, winter

1Greenland Institute of Natural Resources, P.O. Box 570, 3900, Nuuk,Greenland; 2National Environmental Research Institute,Frederiksborgvej 399, P.O. Box 358, 4000 Roskilde, Denmark;3The Royal Veterinary and Agricultural University, Bülowsvej 17,1870 Frederiksberg C, Denmark; 4c/o Greenland Institute of NaturalResources, P.O. Box 570, 3900, Nuuk, Greenland; 5University of NewBrunswick, ACWERN, Bag 45111, Fredericton, N.B, E3B 6E1, Canada; *corresponding author (fme@dmu.dk)

Flemming R. Merkel1,2,*, Anders Mosbech2, Christian Sonne2,Annette Flagstad3, Knud Falk4 & Sarah E. Jamieson4

INTRODUCTION

Migratory arctic birds spend most of their annualcycle away from breeding areas; consequently, theavailability and suitability of non-breeding areasand their management strongly influence theirpopulation dynamics (Scott 1998). During thenon-breeding period, birds must restore bodyreserves lost during the breeding season and mustmaintain sufficient reserves to buffer againstsevere weather and short photoperiods (King &Murphy 1985, Blem 1990). Among some largemigratory arctic birds, body condition at the end ofthe wintering season has been shown to positivelycorrelate with aspects of breeding performance(Ankney & MacInnes 1978, Ebbinge & Spaans1995, Bêty et al. 2003). To optimise individualwinter survival, birds should select the habitat thatprovides the highest net gain of energy per timeunit (Werner et al. 1981, Abrahams & Dill 1989).However, factors other than food availability andquality may influence habitat selection in winter,such as predation risk, human activity, site fidelityand social interactions (Greenwood 1980,Guillemette et al. 1993, Madsen & Fox 1995). Ahigh degree of site fidelity, for whatever reason,will make birds more sensitive to local environ-mental conditions, including human activities.

We sampled body condition and used satellitetelemetry to examine local movements ofNorthern Common Eiders Somateria mollissimaborealis wintering in Southwest Greenland. TheseEiders breed in eastern Arctic Canada and westernGreenland and migrate to the Southwest Green-land open water area when Baffin Bay and northand west parts of Davis Strait become ice-covered(Merkel et al. 2002, Lyngs 2003). We focused onthe degree of site fidelity from late winter untilspring migration when Eiders have traditionallybeen most intensively harvested in SouthwestGreenland. Winter habitats were utilized forapproximately five months before the studyperiod, making food depletion a possibility duringlate winter. Thus, we assumed that the risk ofbirds encountering poor feeding conditions andhunting disturbances would be highest during late

winter and spring, making the Eiders more proneto search for alternative habitats. We aimed to (1)determine the home ranges of Common Eidersand validate the temporal importance of winteringareas previously identified as high density areasthrough aerial surveys (Merkel et al. 2002), (2)quantify the degree of interchange between coastaland fjord habitats, and (3) to clarify if body condi-tion of Eiders differed significantly between thesetwo distinct habitats. Coastal and fjord habitats areless than 100 km apart, however, hunting intensityand habitat characteristics differ distinctivelybetween these two areas. We predicted that theless disturbed inner fjord system would act as abuffer region, to which Eiders would resort whenconditions became unfavourable at coastal habi-tats. Common Eiders are considered to be particu-larly sensitive to disturbance (Bell & Owen 1990).Results are discussed in relation to available infor-mation about Common Eider distribution andbehaviour in the study area and in relation to man-agement perspectives.

METHODS

Study area The study area comprised the coastal waters andfjords surrounding Nuuk, the capital city ofGreenland (Fig. 1). This is part of the GreenlandOpen Water Area, which is internationally impor-tant for wintering seabirds (Boertmann et al.2004) and a key wintering area for CommonEiders, with c. 25 000 birds in the coastal area and32 000 in adjoining fjord systems based on 1999aerial surveys (Merkel et al. 2002). The coastalarea is an extensive archipelago including exten-sive areas of shallow waters, whereas the fjords tothe east are lined with steep cliffs with little shal-low water. AVHRR satellite images showed theinnermost fjord system was ice covered throughoutFebruary, March, and part of April. At peak, inmid-March 2000, c. 53 km (c. 14%) of the inmostfjord sections was frozen. In general, less iceoccurred in 2001 when in mid-March only c. 25km of the innermost sections was frozen. Mean

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temperatures in Nuuk in February, March andApril were –9.6°, –8.4° and –0.3°C in 2000 and–7.8°, –5.6° and –1.9°C in 2001, respectively. Fromautumn to spring, commercial and recreationalhunters in Nuuk shot on average 11 579 ± 1071Eiders (1993–2001, Dept. of Fishing and Hunting,Greenland Home Rule), of which estimated 70%were Common Eiders; the remainder King EidersSomateria spectabilis (Frich & Falk 1997, Merkel2004). Typically, Eider hunting occurs in thecoastal zone within 30 km of Nuuk, so huntingdeep in the fjords is rare (Merkel 2004). Up until2002 the hunting season was open from 1 Octoberuntil 31 May.

Capture and markingEiders were captured in mist nets, 21 m long and3.2 m high (D235/70, mesh size 70x70 mm,Ecotone) supported on aluminium stakes fixedinto wooden floats. Three or four mist nets wereconnected end to end, extending 63–84 m from

anchorage points on shore and deployed with plas-tic Eider decoys. Eiders were also captured at nightby means of dazzling. A large and highly elasticnet was set up at the stem of the research vessel,and at the base a horizontal mist net was placed tocatch and entangle birds sliding down from thevertical frontline net. When approached duringlow visibility (snowfall or dense fog) Eiders werelikely to orientate towards the vessel searchlight.

We implanted birds with Satellite PlatformTransmitting Terminals (50 g Microwave Teleme-try Inc. PTT 100). Transmitters were programmedto operate at various duty cycles: 4 PTTs transmit-ted 3 h every 7 h, 4 PTTs 4 h every 9 h, 6 PTTs 4 hevery 14 h, 13 PTTs 4 h every 34 h, and 6 PTTstransmitted 6 h every 66 h. We marked 21 femalesand 12 males between 23 February and 6 March in2000 and 2001 in the Nuuk study area (Fig. 1), 25adults, 7 subadults (2nd or 3rd winter), and onejuvenile (Baker 1993). External aging of subadultCommon Eiders is, however, not accurate (F.R.Merkel, K. Falk, and S.E. Jamieson, unpubl.), andit is likely that some of these subadults wereadults.

We implanted 28 of the 33 transmitters in theabdomen (Korschgen et al. 1996a) and 5 transmit-ters were subcutaneously deployed (Korschgen etal. 1996b). We avoided heat loss on re-exposure toseawater by avoiding removal of feathers from theabdominal surgery site by aligning feathers to eachside of the midline and by making the incisionalong the linea alba (Mosbech et al., unpubl.).Inert eye ointment (80% Vaselin, 20% paraffin oil)was used for cornea protection during anaesthesia.Postoperatively, an injection of enrofloxacin(15–25 mg intramuscularly) was given (Baytril, 50mg ml–1, Bayer) and 0.02 mg of butorphanol(Torbugesic, 10 mg ml–1, Forte Dodge) for analge-sia. Birds were kept in boxes and released twohours after surgery.

Movements and home range analysesData were received via Argos (DIAG format, ArgosUser’s Manual, http://www.cls.fr/manuel). Weretained all location quality codes of classes 3, 2,and 1, rejecting positions greater than 5 km from

Merkel et al.: WINTER MOVEMENTS OF EIDERS 641

50 km

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GREENLAND

CANADANuuk

Figure 1. The Nuuk study area in Southwest Greenland.Numbers show locations at which Common Eider carcas-ses were collected (1–9) and satellite transmittersdeployed (1–8).

other locations obtained over short time intervals,using the PC-SAS Argos Filter (Douglas 2003).Altogether, we rejected 38% of locations and fromthe time of deployment until the start of springmigration 1977 locations were used in the analy-sis. The onset of spring migration was determinedas the date when birds started to cross the DavisStrait between Greenland and Canada. For individ-ual birds this date ranged from 18 April to 23 May(median date 30 April, n = 5).

To delineate the area used by each individual,we used a probabilistic home range technique toestimate the 50% home range (or core areas) and95% home ranges based on the fixed kernel homerange estimator (Worton 1989), and applied leastsquares cross-validation to determine the smooth-ing parameters. This method is less biased com-pared to other home range estimators (Seaman &Powell 1996, Hooge et al. 2004). We used theArcView GIS extension ‘Animal Movements’ forhome range calculation (Hooge et al. 1999). Forsome Eiders, we calculated the seasonal homerange area (from late winter until spring migra-tion) as the sum of multiple local area estimates,corresponding to the sum of all wintering areasused, each separated in time and space. Homeranges were split into multiple wintering areaswhenever a bird moved more than 5 km from themean activity centre (arithmetic mean point) on apermanent basis (i.e. the bird did not returnwithin 5 km of the former activity centre).Locations received en route between such succes-sive wintering areas were excluded from kernelhome range estimation. Birds with <20 locationswere excluded from the analysis.

Home range sizes were log-transformed tomeet the assumptions of both normality andhomogeneity of variances. Home range sizes werecompared using t-tests. We analysed movements inrelation to the time of day, using mean distancesbetween consecutive locations separated in timeby less than one hour. We tested the distributionsof these distances between consecutive locationsusing a non-parametric Mann-Whitney test. Testsfor serial autocorrelation among consecutive loca-tions were not conducted. Studies have shown that

aims to create independent data, by including onlylocations with a certain minimum time interval,are likely to produce a greater bias than thatcaused by autocorrelation (Andersen & Rongstad1989, Reynolds & Laundre 1990).

Body condition analysisCommon Eider carcasses from the study area wereobtained from the legal subsistence harvest. Birdswere either shot or retrieved (unintentionallydrowned) from fishing nets. We analysed 141adult birds from 2000 and 2001 during the track-ing period from late February until 18 April, whenmigration was first initiated. Nine birds wereexcluded from the final analysis because of dam-age sustained during collection that preventedmeasurements from being taken. Juvenile andimmature carcasses were not analysed because oflack of these age classes during some periods.

Eiders were dissected, sexed and aged after(Merkel et al. 2006). We estimated body conditionas total carcass lipids (TCL) from an equationwhich explained 93% of variation in total carcasslipids of Common Eiders in our study area(Jamieson et al. 2006). We analysed the influenceof sex, habitat (coast or fjord) and period (latewinter = 19 February – 18 March; early spring =19 March – 18 April) on body condition usinganalyses of covariance (ANCOVA). For significantinteractions between independent variables, weused least square means for multiple comparisonswith the Tukey-Kramer test, which accounts forunequal sample size (Day & Quinn 1989, Zar1999). We analysed 2000 and 2001 separatelybecause of a significant three-way interactionbetween year, period, and habitat. PC1, from aprinciple component analysis on the length ofhead-bill, tarsus bone, and keel, was used in allANCOVAs as a covariate to adjust for body size.The variables had similar factor loadings(0.56–0.60) on the first principal component(PC1) and explained 72% of the total variance. Weincluded the covariate because of a weak, buthighly significant, correlation between PC1 andTCL (Pearson, r = 0.24, P = 0.005). The ANCOVAassumption of homogenous slopes was met, i.e.

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there were no significant interaction terms thatincluded the covariate and any of the independentvariables. We used a significance level of 0.05 forall statistical tests. Throughout the paper wereport values as means ± SE.

RESULTS

Location period and mortalityFor 32 of 33 Common Eiders we received locationsfor 0.2 to 13.9 months (one transmitter failedbefore data collection began), compared to anexpected 1.9–11.2 months battery life (dependingon duty cycles). Of these 32 PTTs, 5 birds werecaught accidentally in gillnets and 2 were shot,within 0.5–1.9 months after instrumentation.Seven birds apparently died 0.2 to 1.3 monthsafter release according to PTT information, whichindicated a decrease in body temperature. Five ofthese seven PTTs continued to transmit locationsafter the temperature had dropped. These Eidersmay have been predated by White-tailed EaglesHaliaeetus albicilla groenlandicus and carried toexposed mainland or island positions. One markedbirds was caught in gillnets four years after PTTdeployment.

Home rangesFifteen Eiders provided more than the minimum20 locations for winter home range estimationsaround Nuuk. Mean 95% home range size was67.8 ± 8.3 km2 and the mean core area (50%)

was 8.1 ± 1.3 km2 (Table 1). Among all birds (n =32), 72% used only one wintering area, the remain-der used on average 2.1 sites. Among birds trackedfrom late February until spring migration initiation,two of seven birds (29%) used more than one win-tering area. Eiders tracked for the full period hadsimilar mean core areas (50%, t = 0.58, df = 13, P= 0.57) and 95% home ranges (t = 0.32, df = 13,P = 0.76) compared to birds that were tracked for ashorter period (Table 1). Birds using multiple win-tering areas moved a median distance of 11.6 km(range: 5.1–71.9 km) between two areas and onlytwo birds moved more than 30 km.

Diurnal movements and habitat useExchange between coastal and fjord habitats wasseasonal and permanent, regular exchange wasnot observed. Two females marked in coastal habi-tats (Maluutu and Nepisat Sound, Fig. 1, location3 and 4) moved to the inner Nuuk fjord system inearly March and did not return to their originalcoastal habitat. One left the fjord system in lateApril to migrate towards Canada. The other onewas caught in gillnets in early April, while still inthe fjord. Remaining Eiders either stayed withinthe coastal area or within the inner fjord system,where initially marked, until spring migration oruntil we lost track of them. Exchange of birds fromthe coast to the inner fjord was 8%, based on allcoast-marked Eiders (2/26). Assuming some birdsmay have moved into the fjord after we lost trackof them, a less conservative estimate of exchangeis 29% (2 of 7 birds from the coastal area, Fig. 2).

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Period Tracking Birds Locations 50% HR (km2) 95% HR (km2) Max distance covered period from AC (km)

days n n mean ± SE median range mean ± SE median range range

< Fulla 18 (15–25) 8 93 (42–178) 7.8 ±2.0 6.1 3.2–20.9 60.5 ±7.3 34.0 37.9–99.0 6.2 – 21.9= Fullb 64 (30–89) 7 158 (26–445) 8.6 ±1.9 7.9 4.0–19.3 76.1 ±16.0 76.3 25.1–155.7 7.4 – 32.2Pooled 39 (15–89) 15 123 (26–445) 8.1 ±1.3 7.1 3.2–20.9 67.8 ±8.3 69.4 25.1–155.7 6.2 – 32.2

aTransmissions stopped before spring migration.bFollowed throughout the study period from late January or early February until spring migration.

Table 1. Kernel Home Range (HR) sizes and maximum distances moved from the mean activity centre (AC) of a givenHR for Common Eiders wintering in the Nuuk study area, Southwest Greenland.

A small fjord south of Nuuk (sampling site 5,Fig. 1), disconnected from the main fjord system,was the only site where birds used both coastaland fjord areas on a daily basis. For two females,daytime activity was centred at the mouth of thefjord, while night time activity centres werelocated 1.7 km and 3.1 km into the fjord.

The mean linear distance moved between con-secutive locations of all marked birds was 3.5 ±0.1 km (range 0.1–72.3 km, 96% less than 10km). The median distance moved did not differ

between males and females (Z = 0.36; n1, n2 =484, 1493; P = 0.72). Within each home range orwintering area (when home range consisted ofmultiple wintering area), the maximum distancemoved from the mean centre of activity rangedfrom 6.2 to 32.2 km (Table 1). On a daily basis,mean linear distance between consecutive loca-tions (<1 h time intervals) peaked in the hourafter sunrise and two hours around sunset (Fig. 3).The median linear distance moved within thesethree hours was significantly larger than for the

644 ARDEA 94(3), 2006

20 km

Figure 2. Winter home range areas (95% and 50% utilization distribution) for individual Common Eiders (distinguis-hed by different shading) tracked by satellite telemetry in Southwest Greenland from late February until spring migra-tion (2000 and 2001). Only birds marked at the coastal area off Nuuk are shown, and among these only birds forwhich it is known if they moved to the inner fjord system within the full study period (n = 7).

remaining hours (Fig. 3, Z = 2.66; n1, n2 = 122,1343; P = 0.008). For individual Eiders, the meancentre of daytime activity was located near themean centre of night time activity: on average1.7 ± 0.3 km, ranging from 0.6 to 3.1 km (n = 8,all from coastal habitats). The mean centre ofactivity at night was closer to the coast than byday, often within sheltered bay areas.

Body conditionIn both years, sex and interactions between sexand other variables did not significantly contributeto any the variation in Eider body condition(ANCOVA, all: P > 0.38), so sex was excludedfrom the final model. Adjusted for body size(PC1), we found a significant interaction betweenhabitat and period in 2000 and 2001 (ANCOVA:F1,56 = 6.23, P = 0.02 and F1,66 = 8.77, P =0.004, respectively). Post hoc comparisons showedthat the body condition of fjord birds decreasedsignificantly from late winter to early spring in2000, while the opposite was the case in 2001(2000: T = –4.7, P = 0.0001, 2001: T = 3.4, P= 0.006, Fig. 4). Body condition of coastal birds

varied less between periods and changed in thesame direction in both years (Fig. 4). Pooled overyears, there was a significant fall in body conditionfrom late winter to early spring (F1,57 = 5.77, P =0.02). In two of four periods, fjord birds weresuperior in body condition compared to coastalbirds (2000, late winter: T = 3.9, P = 0.002;2001, early spring: T = 2.6, P = 0.06), but atother times fjord and coastal birds did equally well(2000, early spring: T = 1.3, P = 0.60; 2001, latewinter: T = –1.8, P = 0.27) (Fig. 4).

DISCUSSION

Home rangesThe Eiders tracked in the present study weremarked at locations previously identified as high-density wintering areas for Common Eiders(Merkel et al. 2002, Merkel 2004). From late win-ter until spring migration, wintering CommonEiders around Nuuk showed strong site fidelity tothese areas, with mean core areas of c. 8 km2 and95% home ranges around 68 km2. These areas

Merkel et al.: WINTER MOVEMENTS OF EIDERS 645m

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Figure 3. Diurnal movement pattern of Common Eiderswintering in Nuuk, Southwest Greenland, as determinedby satellite tracking from late February until springmigration (2000 and 2001). For each hour, the distancemoved (± SE) represents the mean linear distance bet-ween consecutive locations. Only consecutive locationsseparated in time with less that one hour are included(n = 1465).

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Figure 4. Body condition (total carcass lipids, g ± SE) ofCommon Eiders wintering in Nuuk, Southwest Green-land, in relation to the habitat, year, and period of samp-ling. Late winter: 19 February – 18 March; early spring:19 March – 18 April.

(sampling sites 1–5, Fig. 1) constitute the majorhunting areas for Common Eiders, King Eiders S.spectabilis and Brünnich’s Guillemots Uria lomvia(Merkel 2004). Despite this, most Common Eiders(71%) remained within a single small winteringarea throughout our study period. Recent satellitetelemetry on King Eiders indicates that small win-ter home range areas, like those detected in ourstudy, are not always the case for that species. InSouthwest Greenland large numbers of King Eiderswinter over offshore shallow banks (Mosbech &Johnson 1999, Mosbech et al. 2001), where indi-vidual kernel home ranges range from c. 50 km2

(50%) to 600 km2 (95%) (Mosbech et al.,unpubl.).

Diurnal movementsNuuk hunters consider that Eiders use coastalhabitats for feeding during daylight hours and theinner fjord system to roost at night, but our studydid not support this theory. Distances travelledbetween roosting and feeding areas were generallya few kilometres and there was no general regularpattern of Eiders movement towards the innerfjord at night. Visual observations from the coastalarea have since confirmed the proximity of feedingareas and roosting areas. At Nepiset Sound (sam-pling site 3, Fig. 1) birds fed during daylight hoursand swam to and from the roost in a small shel-tered bay, c. 3 km away (Merkel and Mosbech,unpubl.), consistent with the satellite tracking(Fig. 3). Contrary to our findings, Goudie et al.(2000) reported that Common Eiders wintering inNewfoundland, Canada, moved offshore at nightto drift in compact flocks on ocean currents andflew 50–100 km inshore in early morning to feed.Clearly, daily movement patterns of CommonEiders are highly site specific.

Habitat selectionMost coastal Common Eiders showed high sitefidelity despite intensive hunting and gillnet activ-ity (Merkel 2004). There was little support for thesuggestion that the inner fjord system acted as abuffer, to which Eiders resorted when conditionsbecame unfavourable in coastal habitats. Our

results indicate that exchange of Eiders betweencoastal and fjord habitats was not common. Thismay suggest that disturbance levels were not criti-cally high within the coastal area, that less dis-turbed alternative habitats were poorer in quality,or that some other factor favoured site fidelity(e.g. social behaviour). Birds using coastal habitatsfed mainly by day, while birds wintering in theinner fjord fed only at night; presumably an anti-predator strategy resulting from White-tailed Eagleactivity (Merkel and Mosbech unpubl.). Lack ofshallow waters constrains Eiders to feed very closeto land in the fjord, exposing them to Eagle preda-tion during the day. Coastal habitats support largershallow water areas, where Eiders can feed at safedistances from land. Familiarity of specific areas(for breeding, staging or wintering) is believed tobe one of the most important factors responsiblefor establishing site fidelity (Robertson & Cooke1999). The juxtaposition of feeding areas androosting areas is also advantageous in terms ofcosts incurred by travelling. Flight is energeticallyvery expensive (King 1974), and those birds thatcan survive the non-breeding period without regu-lar relocation flights presumably can allocate moreenergy to daily maintenance and stores.

Body conditionOnly the fjord Eiders accumulated body fat (in2001) in our study area prior to the migrationperiod (Fig. 4). This agrees with information onbody condition of Common Eiders collected ineastern Canada in late spring (Jamieson 2003)and information from satellite telemetry onmigrating birds from Southwest Greenland(Mosbech et al. 2006), which indicate that mostaccumulation of stores for the breeding seasontakes place subsequent to arrival in EasternCanada.

In both 2000 and 2001 Eiders from the coastalarea lost body fat from late winter to early spring(Fig. 4) as predicted if body reserves were influ-enced by day length and weather. Fat levels oftenincrease in relation to decreasing daylength (Nolan& Ketterson 1983, Blem 1990), increasing latitude(Blem 1976, Rogers et al. 1993), and decreasing

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ambient temperature (King 1972, Lima 1986,Gaston 1991), and reflects the need to store addi-tional reserves for periods when foraging condi-tions are sub-optimal. For fjord birds, body condi-tion reflected ambient temperature. High fat levelscarried by the fjord birds in late winter 2000 (Fig.4) coincided with below average February andMarch temperatures when the inmost fjord systemwas ice covered. The strategy to carry more fatduring sub-optimal wintering conditions does,however, presume that birds are capable of in-creasing foraging efficiency. It is unknown whetherthis was the case in our study area. It is clear, how-ever, that variation in body condition, both withinand between years, was relatively large for Eidersamongst fjord compared to coastal birds (Fig. 4).This may have been caused by greater unpre-dictability in the local fjord environment: perhapsbecause feeding accessibility varied according toice conditions, and/or was perhaps related to thepresence/absence of White-tailed Eagles. Fjordhabitats may therefore represent a more profitableforaging area in winter, but unpredictabilityand/or predation pressure may favour winteringon the coast.

Management implicationsHunting mortality and drowning in gillnets wasfrequent in the present study (22%) and confirmsexisting knowledge that the harvest level was highin the Nuuk area. Recent surveys of embeddedshot loads among wintering Eiders in SouthwestGreenland (Falk et al. 2006) substantiates highhunting levels along the Nuuk coast, with up to35.0% of adults inflicted. The high site fidelitydetected among Common Eiders in late winter,combined with the local hunting pressure requiresgreater vigilance regarding management and con-servation of this population. Our results accentu-ate the need for a flexible conservation protocol,which will enable adaptive management of sub-unit winter populations according to local environ-mental conditions. Presently, wintering CommonEiders are managed in the same way throughout

the open water area in Southwest Greenland, byopen and closed seasons. This may, however, putexcessive and unsustainable pressure on local frag-ments of the population. Unsustainable use, as aresult of site fidelity and poor management of win-tering grounds has been reported for goosespecies. In Ireland and Britain such factors lead tolocal extinctions of Greenland White-fronted GeeseAnser albifrons flavirostris (Ruttledge & Ogilvie1979, Norriss & Wilson 1988). For Common Eiderswintering in Southwest Greenland, we recommendthe use of non-hunting disturbance-free reserves asa supplemental conservation tool. Experimentswith such a reserve network for migratory andwintering waterfowl in Denmark showed that birdnumbers and diversity increased after establish-ment of reserves (Madsen et al. 1998, Madsen1998). The experiment showed that reserves notonly attract more birds to the core area, but also toadjacent areas. Hunters thereby gain some com-pensation for lost hunting areas through improvedhunting opportunities in surrounding areas(Madsen et al. 1998). In our study area, samplingsites 1, 2, and 3 (Fig. 1) constitute good reservecandidates, based on human activities and Eiderdensities (Merkel et al. 2002, Merkel 2004). Wesuggest, that future reserves are designated at thecentre of high-density wintering areas. The presentstudy indicates that strong site fidelity constrainsEiders from finding alternative wintering sites.Common Eiders are generally site-faithful betweenyears (Spurr and Milne 1976, Goudie et al. 2000),making such local management even more impor-tant. Our study also supports this assumption: twoof the three Eiders bearing transmitters, whichcontinued to transmit during the next autumn,returned from Canada to their original catchingsites in Nuuk. The third bird arrived in Greenlandin mid-October c. 150 km south of Nuuk and hadnot returned to Nuuk when transmissions stoppedin early November. A fourth Eider was recoveredat the original sampling site (8, Fig. 1) four yearsafter implantation (Mosbech et al., unpubl.).

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ACKNOWLEDGEMENTS

This study is part of an Eider winter ecology research pro-gramme in Southwest Greenland, funded by theGreenland Institute of Natural Resources, the NationalEnvironmental Research Institute of Denmark, and theDanish Environmental Protection Agency as part of theDancea (Danish Cooperation for Environment in theArctic) environmental support programme. TheDepartment of Nature and Environment, GreenlandHome Rule, granted us the necessary permits for the fiel-dwork. AVHRR satellite images were purchased atDundee Satellite Receiving Station, UK. We thank thecrew of the Adolf Jensen research vessel for invaluablesupport and great skills in handling nets and catchingbirds, and we thank the Captain, Flemming Heinrich, forsafe and excellent navigation in challenging conditions.Also, thanks to Margaret Petersen and Daniel Mulcahy,Alaska Science Center, for technical advice and assistancein surgical matters.

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SAMENVATTING

In dit onderzoek worden de zwerfbewegingen, de groottevan het leefgebied en de lichaamsconditie van overwinte-rende Noordelijke Eiders Somateria mollissima borealisonderzocht bij Nuuk op Zuidwest-Groenland. Het onder-zoek vond plaats aan het einde van de winters in 2000 en2001. Hiertoe werden 33 Eiders voorzien van een satel-lietzender en werden dode Eiders verzameld. De meesteEiders waren plaatstrouw met een gemiddelde ‘homerange’ van 67,8 km2, en een kerngebied van 8,1 km2. Dedagelijkse trekbewegingen piekten tijdens de ochtend- enavondschemering, wanneer de vogels kennelijk tussenvoedsel- en rustgebied pendelden. Er werd op gemiddeld1,7 km afstand van de foerageerplek geslapen. Van devogels die langs de kust waren gevangen, gebruikte

650 ARDEA 94(3), 2006

slechts 8–29% de binnenlandse fjorden, ondanks de hogemate van verstoring langs de kust. De vogels die naar hetbinnenland waren getrokken, keerden echter niet naar dekust terug. De waarnemingen onderstrepen het belangom bij het beheer van overwinterende Eiders inZuidwest-Groenland rekening te houden met de sterkeplaatstrouw van de soort. De lichaamsconditie van vol-wassen vogels in de fjorden was gelijk aan, of beter dan,die van vogels langs de kust. De lichaamsgewichten vari-eerden in de fjorden echter meer dan aan de kust, het-

geen erop duidt dat de voedselbronnen in de fjordenminder voorspelbaar zijn. Ondiep water, waar de Eidersvoedsel zochten, was in de fjorden beperkt tot een strookvlak onder de wal waardoor ze bovendien gevoelig ble-ken voor predatie door Zeearenden Haliaeetus albicilla.De lichaamsconditie nam aan het eind van de winter af(n = 4), hetgeen erop duidt dat lichaamsreserves voor debroedperiode elders worden verzameld.

Received 1 March 2005; accepted 30 April 2006