Chaulk.indd 121Chaulk et al. 2004: Polar Research 23(2), 121–130 Understanding regional and annual variation in the breeding ecology of organisms is impor- tant for conservation and management purposes. Variable expression of ecological characteristics often occurs in response to change in the envi- ronment where organisms live. Breeding ecology can be infl uenced by biophysical factors at differ- ing temporal and spatial scales (Scott et al. 2002). Aspects of avian ecology such as nest density, clutch size, and egg volume can be infl uenced by population dynamics, population genetics, habi- tat quality, and/or food availability (Lack 1967; Ryder 1970; Robertson et al. 2001; Hario & Selin 2002). Furthermore some components of breed- ing ecology may be prone or resistant to annual and/or regional fl uctuations in the biophysical environment (Avise & Hamrick 1996; Erikstad, Tveraa et al. 1998; Bregnballe 2002; Hario & Selin 2002). Common eiders (Somateria mollissima) are an important species for many northern peoples, as a source of meat, eggs and down. These birds exhib- it substantial variation in the timing of nesting, nesting density, clutch size and other aspects of their breeding ecology (Goudie et al. 2000; Rob- ertson et al. 2001; Chaulk et al. in press). Under- Regional and annual variability in common eider nesting ecology in Labrador Keith G. Chaulk, Gregory J. Robertson, & William A. Montevecchi Nesting densities are often used to estimate breeding population size and with other measures of reproductive performance can be useful indica- tors of population status. These aspects of breeding biology often show considerable spatial and temporal variation. Between 2000 and 2003, we surveyed nesting common eiders (Somateria mollissima) on 172 islands in three archipelagos (Nain, Hopedale, Rigolet) on the Labrador coast. Rigolet was the largest archipelago (2834 km2) followed by Nain then Hopedale, and island density varied inversely with archipelago size. Over- all means were: nest density 52.0 ± 141.9 (SD) nests/ha; nest initiation 12 June ± 12 days; clutch size 3.7 ± 1.2 eggs/nest; egg volume 98.8 ± 10.4 cm3; and clutch volume 392.3 ± 135.0 cm3. Rigolet had the highest average egg volumes and nest densities, the highest single island nest density of 1053 nests/ha, and the earliest average nest initiation date. We found signifi - cant differences in nest densities among archipelagos and across years; signifi cant archipelago and year interactions were detected for nest ini- tiation date and clutch size. Signifi cant differences were found among individual islands for all response variables except egg volume. For egg volume, within-archipelago island differences were not signifi cant, but between-archipelago differences were signifi cant. Thus egg volume may be a useful diagnostic to identify population affi liation. K. G. Chaulk, Labrador Inuit Association, Box 382 Station “C”, Goose Bay, Labrador, NL, Canada A0P 1C0, kchaulk@nunatsiavut.com; G. J. Robertson, Canadian Wildlife Service, 6 Bruce Street, Mount Pearl, New- foundland, NL, Canada A1N 4T3; W. A. Montevecchi, Department of Psychology, Memorial University of Newfoundland, St. John’s, Newfoundland, NL, Canada A1B 3X9. 122 Regional and annual variability in eider nesting ecology standing patterns of annual and regional varia- tion in eider breeding performance can be very important for management and conservation pur- poses. Early nest initiation dates and large clutch sizes are generally indicative of favourable breed- ing conditions, while nesting densities are often used to estimate breeding population size. Unfor- tunately most eider research is limited in spatial and/or temporal scope, and the rare papers that contain multi-year and/or multi-site comparisons, are generally from temperate climes in northern Europe (Milne 1974; Swennen 1983; Coulson 1984, 1999; Hario Selin 1988; Bregnballe 2002; Hanssen et al. 2003). The goal of this paper was to examine annual and regional variation of common eider nesting ecology at three distinct sites on the mid-Lab- rador coast, a subarctic region, over a four-year period (2000 to 2003). In this paper, we inves- tigate nest density, nest initiation, clutch size, egg and clutch volume. In general we expected both regional and annual variation in most nest- ing characters, as these traits have been shown to vary in eiders. Of greater interest was wheth- er annual patterns of variation were maintained across regions, or whether traits varied independ- ently within regions across the years. Study area Archipelagos near the communities of Nain, Hopedale and Rigolet were surveyed from 2000 to 2003 (Fig. 1). The extent of each archipela- go was determined by calculating a minimum convex polygon (Mohr 1947) containing all islands that were completely searched. The total geographic area of the three archipelagos was estimated to be 4785 km2, and contained approxi- mately of 1600 islands (Table 1). The archipelago adjacent to Rigolet covered the largest geograph- ic area while the archipelago adjacent to Hope- dale was the smallest (Table 1). Overall the aver- age size of islands within the three archipelagos was 30.1 ha ± 288.8 (1 SD); on average the larg- est islands were found in the Rigolet archipelago (Table 1). The greatest island density occurred in the Hopedale archipelago (0.90 islands/km2) fol- Table 1. Location, island characteristics, sampling intensity and sampling dates of archi- pelagos in Labrador where data were collected on nesting ecology of common eiders (Somateria mollissima), 2000–2003. Nain Hopedale Rigolet All Study area location Mean latitude (ºN) 56.36 55.33 54.18 Mean longitude (ºW) 61.06 59.81 57.41 Study area size (km2) 1151 800 2834 4785 Study area perimeter (km) 145 128 278 551 N (islands) in study area 497 720 335 1552 Island density (islands/km2) 0.43 0.90 0.12 0.32 Archipelago island size mean ± 1 SD (ha) 33 ± 318 20 ± 194 47 ± 394 30 ± 289 Range (ha) 0.01 - 6903 0.02 - 3875 0.02 - 5204 0.01 - 6903 Size sampled islands Mean ± 1 SD (ha) 1.3 ± 1.5 1.6 ± 2.6 4.1 ± 10.2 2.3 ± 6.0 Island sampling scheme No. one year only 13 33 17 63 No. two years only 11 19 15 45 No. three years only 10 14 12 36 No. all four years 8 13 7 28 Survey dates 2000 3 - 9 July 28 - 30 June 20 - 26 June 20 June - 9 July Survey dates 2001 5 - 19 July 4 - 17 July 18 - 27 June 18 June - 19 July Survey dates 2002 13 - 22 July 3 - 17 July 17 - 22 June 17 June - 22 July Survey dates 2003 11 - 13 July 3 - 7 July 14 - 20 June 14 June - 13 July 123Chaulk et al. 2004: Polar Research 23(2), 121–130 lowed by Nain then Rigolet (Table 1). All archipelagos shared similar environmental characteristics such as a northern maritime cli- mate, vegetation composed primarily of mosses, lichens, forbes, grasses, and sedges. The archipel- agos were typically comprised of barren islands with sparse vegetation and very limited nesting cover. Islands in the Rigolet area had denser and taller ground vegetation, and on some islands woodier cover, including stunted black spruce (Picea mariana). All three archipelagos are clas- sifi ed as coastal barrens (Lopoukhine et al. 1978), are considered to have a High Boreal ecoclimate (Meades 1990) and a Low Arctic oceanographic regime (Nettleship & Evans 1985). On the cen- tral and northern Labrador coast the concept of discrete archipelagos is somewhat misleading, as the island complex along this coast is for the most part continuous. The island complexes, hereafter referred to as archipelagos, selected for study were typical of this portion of the Labrador coast. Methods From 2000 to 2003 we surveyed three archi- pelagos (Nain, Hopedale, Rigolet) for nesting eiders. Islands were selected for study on the basis of random sampling. In all cases we limit- ed our searches to islands that were estimated to be smaller than 30 ha. Analysis of the spatial dis- tribution of our data set show that our samples were spatially random within the subset of islands that were less than 30 ha within each archipela- go (K. G. Chaulk unpublished data). Ground sur- veys were conducted using standard search meth- ods employed by the Canadian Wildlife Service (Nettleship 1980) and other researchers (Falardeau et al. 2003; Merkel 2004; Chaulk et al. in press); these consisted of two to four people walking lin- early over the islands searching for signs of eider nesting. Islands in the three archipelagos were for the most part barren with limited nest-cover; hens and unattended nests were easily detected. Archi- pelagos were searched at approximately the same time each of the four years (Table 1). For the most part the senior author conducted all surveys, the exception being Nain and Hopedale in 2001 and 2002, where the senior author initiated the surveys but the fi eld crew completed them. For each common eider nest observed, infor- mation was recorded on apparent clutch size, nest age, and nest status. Nests were classed as fol- lows: incubating – current season nest containing Fig. 1.: General location of archi- pelagos surveyed between 2000 and 2003 on the Labrador coast. 124 Regional and annual variability in eider nesting ecology eggs; hatching – at least one chick was breaking its shell; hatched – at least one chick was com- pletely out of its shell; depredated – broken and bloody eggs were present in or immediately adja- cent to the nest; and unknown – nest was in disre- pair with no eggs or signs of depredation. The incubation period was assumed to be 24 to 26 days (Goudie et al. 2000), with incubation gen- erally commencing after the laying of the penul- timate egg (Hanssen et al. 2002). Candling was used to age the eggs (Weller 1956) and to calcu- late nest initiation we added the number of eggs to the egg age and we subtracted this number plus one from the survey date. Nests with more than six eggs were not aged. Apparent clutch sizes were calculated using nests classifi ed as incubat- ing; nests with more than six eggs are generally considered dump nests produced by two or more females (Swennen 1983; Robertson 1995) and were omitted from the analysis. Due to lack of data, nest initiation dates were not calculated for the 2001 breeding year. With respect to egg size measurements we ran- domly selected a subset of previously surveyed islands, and on these we randomly selected nests and eggs for further investigation. Each random- ly selected egg was then measured using Verni- er calipers: egg length was measured from pole to pole, and width was measured at the widest part of the egg; all measurements were record- ed in mm. Egg volume was calculated based on the formula presented by Guild (1974) and Rob- ertson et al. (2001); clutch volume was estimat- ed only for nests for which we had data on egg volume and clutch size. Clutch volume was esti- mated as clutch size multiplied by the estimated egg volume for a given nest. Island nest densities were calculated using islands that were completely searched. Island sizes were derived from digital 1:50000 base maps for the Labrador coast. Geodetic coordinates are reported as latitude and longitude, decimal degrees, North American Datum 1983. Nest density, nest initia- tion, and clutch size were analysed using a gener- al linear model, with the year, archipelago and its interaction as fi xed factors. Island, nested within archipelago, was also included as a random factor to control for multiple measurements of islands across years. Egg volumes and clutch volumes were analysed with one-way ANOVA, with archi- pelago as a fi xed factor and island, nested within archipelago, as a random factor. Critical alpha was set at 0.05 for all tests, which were all two-tailed. Results We sampled 172 islands in the three archipelagos with the greatest island sampling effort expended in Hopedale (Table 1). The average size of sam- pled islands was 2.3 ± 6.0 ha (Table 1). On the islands that were completely searched we count- ed 10 962 eider nests and these contained 35 401 eggs. Overall, on average there were 52 nests/ha or 172 eggs/ha (Table 2). The highest nest counts were observed in Rigolet; one island had 654 nests, and another island, 0.18 ha in size, had the equivalent of 1053 nests/ha. For island nest-den- sity both archipelago and year were signifi cant factors, although they did not show statistically signifi cant interaction (Table 3). There appeared to be greater between-archipelago variation than between-year variation in nest density (Fig. 2). The overall average nest initiation date was 12 June, the earliest average nest initiation dates occurred in the south at Rigolet (3 June) and the latest in the north at Nain (23 June) (Fig. 3). However, we detected a signifi cant interac- tion between archipelago and year for nest ini- tiation date (Table 3, Fig. 4). The overall aver- age clutch size was 3.7 ± 1.2 eggs/nest (Table 2). The largest average clutch size was observed in Hopedale at 3.8 ± 1.2 eggs, followed by Rigolet at 3.7 ± 1.2 eggs, and Nain at 3.6 ± 1.2 eggs; howev- Table 2. Mean ( ± 1 SD) and ranges of nesting traits of common eiders breeding in Labrador, 2000–2003. Period N Mean Nest density (nests/ha) 2000–03 331 52.0 ± 141.9 (range = 0 - 1053) Nest initiation 2000, 02–03 272 12 June ± 12 d (range = 21 May to 9 July) Clutch size 2000–03 10 137 3.7 ± 1.2 (range = 0 - 6) Egg volume (cm3) 2003 415 98.8 ± 10.4 (range = 61.0 - 160.0) Clutch volume (cm3) 2003 415 392.3 ± 135 (range = 77.2 - 1008.4) 125Chaulk et al. 2004: Polar Research 23(2), 121–130 Fig. 2.: Interval plot of mean nest density (nests/ha) by archi- pelago and year with 95 % CI. Horizontal line is equal to the overall mean. Fig. 3.: Interaction plot of mean nest initiation (day of year) by archipelago and year. Horizontal line is equal to the overall mean. Fig. 4.: Interaction plot of mean clutch size (eggs/nest) by archi- pelago and year. Horizontal line is equal to the overall mean. 126 Regional and annual variability in eider nesting ecology er, we detected a signifi cant interaction between archipelago and year for clutch size (Table 3, Fig. 4). Egg volume varied signifi cantly among archi- pelagoes but not across individual islands (Table 3). The largest egg volume was observed in Rig- olet and the smallest in Nain (Fig. 5). The largest clutch volume was observed in Hopedale (Table 2, Fig. 5) although the differences among archi- pelagos were not signifi cant (Table 3). Discussion The highest nest density that we observed occurred in Rigolet (1053 nest/ha). This observation was made on a small island approximately 0.18 ha, and is high relative to nest densities reported else- where. For example, S. m. borealis typically nest at low nest densities (0.8 to 4.5 nests/ha; Cooch 1986), though there are some exceptions, such as East Bay, Southampton Island (Abraham & Finney 1986). Nest densities for S. m. dresseri in the St. Lawrence River averaged 3 nests/ha, but reached as high as 741.5 nests/ha (Chapdelaine et al. 1986). Nesting densities are probably infl u- enced by numerous interacting local factors, including—but not limited to—available nesting islands and brood rearing areas, predators, distur- bance, and overall population size. Previously, we also observed signifi cant differ- ences in nest densities among archipelagos across a larger geographic range in Labrador, but were not able to examine annual variation (Chaulk et al. in press). The present analysis suggests that annual variation is important, and suggests increasing nest densities over the 2000–2003 study period. Nest initiation Spring ice break-up was late in both the Hope- dale and Nain archipelagos during 2002. We found a signifi cant interaction between archipel- ago and year with respect to nest initiation date, with Rigolet in the south showing relatively con- sistent timing, and the two northern archipelagos showing greater annual variation. Earlier, we documented that nest initiation dates in Labrador were positively related to lat- Table 3. Summary of general linear model analysis by variable for nesting characteristics of common eiders breeding in Labrador, 2000–2003. Variable Model fi tR2-adjusted ( % ) Factors df F p Nest density 90.3 Year 3 4.2 < 0.01 Archipelago 2 3.9 0.02 Year–archipelago interaction 6 1.1 0.36 Island nested in archipelago 165 17.9 < 0.01 Nest initiation 72.6 Year 2 42.7 < 0.01 Archipelago 2 62.1 < 0.01 Year–archipelago interaction 4 7.1 < 0.01 Island nested in archipelago 63 2.4 < 0.01 Clutch size 6.6 Year 3 3.5 < 0.01 Archipelago 2 9.4 0.01 Year–archipelago interaction 6 10.1 < 0.01 Island nested in archipelago 116 4.7 < 0.01 Egg volume 6.1 Archipelago 3 5.5 < 0.01 Island nested in archipelago 43 1.1 0.27 Clutch volume 11.5 Archipelago 3 0.8 0.44 Island nested in archipelago 43 2.0 < 0.01 127Chaulk et al. 2004: Polar Research 23(2), 121–130 itude, but we were not able to test year effects (Chaulk et al. in press). This latitudinal pattern still holds over multiple years, but there can be substantial between-year variation in the timing of nest initiation. This highlights the importance of multi-year studies, especially for those aspects of breeding ecology that can be easily infl uenced by biophysical factors, such as sea ice break-up in the spring (Laurila & Hario 1988; Goudie et al. 2000). Given that we found an interaction between archipelago and year in nest initiation date, it is likely that nest initiation date is infl u- enced by ice conditions at the local level. It is important to recognize that common eider nest initiation can be infl uenced by annual variation in the timing of spring break-up. This could mean that common eiders are a potentially good spe- cies for monitoring the effects of climate change in this region of North America, but our results show that more than one site would need to be monitored. Clutch size Clutch size is often used as a comparative meas- ure between different populations (Lewis 1939; Milne 1974; Swennen 1983; Coulson 1984; Rob- ertson et al. 2001; Bregnballe 2002; Chaulk et al. in press). Common eider clutch size is infl u- enced by female body condition, food availabil- ity, disease, body parasites, severity of winter, timing of spring, predation and nest parasitism (Rohwer 1992, Erikstad, Bustnes et al. 1993). Some researchers have suggested that aver- age clutch size may increase during population growth (Hario & Selin 1988), whereas other have found no such trend (Swennen 2002). Our analysis revealed a signifi cant interaction of archipelago and year on clutch size, indicat- ing that any annual patterns were not matched across regions. However, our model had a low R2 value; therefore a signifi cant amount of var- iability remains unexplained by either archi- pelago, year or their interaction. Interestingly, Bregn balle (2002) did fi nd that clutch size varied across years in a similar way across fi ve colonies in Denmark. At a larger spatial scale, Coulson (1999) found that clutch size varied independent- ly across years between Scottish and Dutch col- onies. Clearly, the geographic scale of the analy- sis appears to be important. Previously, we found signifi cant archipelago differences in clutch size but this fi nding was based on analysis of one year of data (Chaulk et al. in press). Annual and regional variation in clutch size is not surpris- ing. Our new fi ndings suggest that average clutch size varies by archipelago and year at the scale of coastal Labrador, and we do not recommend the use of single measures (i.e., one archipelago and/ or year) of clutch size as a basis to assess popu- lation productivity. Instead long-term measure- ments over several archipelagos are needed for robust comparisons between different popula- tions. Fig. 5.: Interval plot of mean egg and clutch volume (cm3) by archipelago with 95 % CI. Horizontal lines are equal to the overall means. 128 Regional and annual variability in eider nesting ecology Egg and clutch volume Some studies have shown that annual variation in eider egg volume is limited (Swennen & van der Meer 1992; Robertson 1995; Laurila & Hario 1998; Hanssen et al. 2002). As such, egg volume may hold value as a comparative measure between populations at large geographic scales (Robertson et al. 2001). In 2003, egg volume differed signif- icantly by archipelago, but not by island, where- as clutch volume differed by island but not by archipelago. Clutch volume is largely infl uenced by clutch size, and likely has limited value as a comparative measure between archipelagos (see above). Our data support the idea that average egg volume has merit to assess population structure at moderate geographic scales (i.e., 100s of km). But with only one year of data and low R2 values for our egg and clutch volume models addition- al data are required to assess spatial and tempo- ral interactions. Using values presented in Goudie et al. (2000) we calculated average egg volumes for two sub- species of common eiders in North America (bore- alis = 96.4 ± 4.4 cm3, range = 93.8 - 102.8; dresseri = 108.0 ± 5.5, range = 100.6 - 115.8). Based on an examination of these values it appears that egg volumes from Nain were most similar to those of borealis, while the values for Hopedale and Rigo- let were on the high end for borealis and on the low end for dresseri. This pattern is likely the result of intergradation between borealis and dresseri in the zone of overlap that is considered to occur in both Hopedale and Rigolet (Mendall 1980; Chaulk et al. in press). Egg volume followed a lat- itudinal pattern with eiders from Rigolet in the south having the largest egg volumes and eiders from Nain in the north having the smallest. Finally, we feel the need to comment on our sampling scheme, as the average size of sam- pled islands was signifi cantly lower than the average size of islands in each archipelago. We actively excluded islands larger than 30 ha from our surveys. We did this for logistical reasons. Large islands require signifi cant effort to search, so instead we focused on smaller islands that could be easily censused by small fi eld crews over restricted time periods. Goudie et al. (2000) reported that common eiders preferred nesting on islands < 75 - 100 ha. Other researchers have used island size thresholds to help identify islands for investigation during eider breeding research (Nakashima & Murray 1988; Robertson & Gil- christ 1998; Merkel 2004) or focused on small islands during breeding surveys (Korschgen 1977; Götmark & Åhlund 1984). It is possible that omitting islands > 30 ha has impacted our results, most likely with respect to our estimates of nest density and percentage of occupied islands. In addition, larger islands might have greater veg- etative coverage, and cover has been shown to increase nest success (Choate 1967; Milne 1974; Schmutz et al. 1983), and nest success could in turn impact some of the breeding characteristics that we discuss in this paper. Since we lack data from islands > 30 ha we have no way of knowing the magnitude or direction of these differences. However, our personal observations and discus- sions with local aboriginal people support the idea that eiders breeding on the mid-Labrador coast rarely use large islands (> 30 ha) for nesting. We recognize that the spatial structuring and the bio- physical characteristics of eider breeding islands are important, but these variables are beyond the scope of this paper. We hope to investigate habitat requirements of common eiders breeding in Lab- rador in future research. Conclusions To summarize our fi ndings, when annual effects were detected, they often interacted with region- al effects. Based on the inter-annual and inter- regional variation, as well as their interactions, long-term surveys over wide geographic regions are needed for comprehensive understanding of population dynamics and responses to environ- mental changes. Such understanding is needed for sound management decisions. For example, large clutch sizes and early breeding could lead to a liberalization of hunting regulations given the expected large number of young birds in the fall fl ight. Conversely, reduced clutches and late breeding might be used as rationale to reduce harvest quotas and limit hunting seasons. How- ever, if only one site is assessed, our results sug- gest that these indicators of breeding conditions would not be representative for an entire breed- ing range. Finally, egg volume does not appear to vary between islands at small geographic scales (i.e., within an archipelago), but does vary at moderate geographic scales (i.e., 100s of km) and could hold promise as an indicator of population genetic differences (Robertson et al. 2001). 129Chaulk et al. 2004: Polar Research 23(2), 121–130 Acknowledgements.—Special thanks to B. Turner (Canadi- an Wildlife Service); N. Anderson, K. Dicker, D. Pottle, W. Wolfrey (Fisheries and Oceans Canada); J. Rowell, B. Ander- son, W. Hunter, I. Winters, E. Merkuratsuk, D. Wolfrey (Lab- rador Inuit Association). Special thanks also go to Shawn Broomfi eld for help in preparing the data for analysis. The Labrador Inuit Association, the Canadian Wildlife Service, the Northern Ecosystem Initiative, the Northern Scientifi c Training Program, Memorial University of Newfoundland, and INCO provided funding for this study. We also thank A. Mosbech and S. Hanssen for their very helpful comments. References Abraham, K. 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