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Polar Biol DOI 10.1007/s00300-016-2072-1

ORIGINAL PAPER

Breeding biology of Arctic terns (Sterna paradisaea) in the Canadian High Arctic

Mark L. Mallory1  · Kelly A. Boadway2 · S. E. Davis3 · M. Maftei3 · Antony W. Diamond2 

Received: 2 May 2016 / Revised: 28 December 2016 / Accepted: 28 December 2016 © Springer-Verlag Berlin Heidelberg 2017

Keywords Arctic · Clutch size · Polynya · Predation · Seabird

Introduction

The Arctic tern (Sterna paradisaea) is an iconic Arctic species with a circumpolar breeding distribution (Hatch 2002). It is also a trans-equatorial migrant that winters around Antarctica, and, consequently, this species travels farther each year than any other organism (Egevang et  al. 2010), potentially in excess of 90,000 km (Fijn et al. 2013). The breeding range of the Arctic tern extends from 84°N in coastal Greenland south to 42°N on the eastern coast of North America (Hatch 2002), with the majority of its breeding population within the Subarctic through High Arctic marine zones (Salomonsen 1965). Despite this, most research on this bird has focused on colonies in the south-ernmost, temperate regions of the Boreal marine zone (e.g., Hawksley 1957; Lemmetyinen 1973b; Gaston et al. 2009a). To date, few studies of Arctic tern breeding biology have been conducted in High Arctic Europe (e.g., Bengtson 1971; Węsławski et  al. 1994, 2006; Egevang 2010; Ege-vang et al. 2010) and even fewer, often partial seasons with a very specific focus, have been conducted in the North American Arctic (e.g., Drury 1960; Boekelheide 1980). Recently, concern has been expressed by management agencies over a perceived decline in international Arc-tic tern populations in some parts of their range (CBIRD 2014), which has been supported by some survey work (e.g., Gilchrist and Robertson 1999; Barrett et  al. 2006), although tern colonies are notoriously difficult to census (e.g., Egevang and Frederiksen 2011).

Seabirds are generally recognized as sensitive indi-cators of marine ecosystem conditions (Cairns 1987;

Abstract The Arctic tern (Sterna paradisaea) is a well-known polar seabird which breeds around the circumpolar Arctic, and which undertakes the longest known annual migration of any organism. Despite its familiarity, there is little information on its breeding biology in the High Arctic, an important baseline against which future stud-ies of climate change impacts on northern wildlife can be compared. We studied the breeding biology of Arctic terns in the Canadian High Arctic during five field sea-sons, and compared this to breeding biology of terns from more southern parts of its range. Because our field site was beside a productive polynya, we expected that repro-ductive metrics for terns nesting there would be relatively high. However, mean clutch size (1.7 eggs), mean egg size (40.2  mm × 29.0  mm), mean nest initiation dates (6 July) were similar to Arctic terns breeding elsewhere. With our data, we could not assess the independent effects of preda-tion pressure, poor weather or low food supplies, but two years with low tern reproduction were also years with low adult body mass and low clutch size (indicating poor food supplies), as well as low hatching success and high nest abandonment (possibly due to high predation pressure).

* Mark L. Mallory mark.mallory@acadiau.ca

1 Department of Biology, Acadia University, 33 Westwood Avenue, Wolfville, NS B4P 2R6, Canada

2 Atlantic Laboratory for Avian Research, University of New Brunswick, P.O. Box 45111, Fredericton, NB E3B 6E1, Canada

3 High Arctic Gull Research Group, Bamfield, BC B0R 1V0, Canada

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Montevecchi 1993). Aside from their tissues serving as key biomonitors of levels of environmental contamina-tion (Mallory and Braune 2012), their annual reproduc-tive success and longer-term population trajectories have been used to track effects of environmental stressors like climate variation for some species in Arctic Canada (e.g., Gaston et  al. 2009a). In this region, however, we currently lack knowledge about the breeding ecology of many seabird species (Gaston et  al. 2012; Mallory and Braune 2012), including terns, limiting their utility as indicators. This is unfortunate, because Arctic terns for-age at the water surface relatively high in the marine food web and have limited phenotypic flexibility (Durant et al. 2009; Grémillet and Charmantier 2010) and, thus, should prove particularly responsive to annual fluctuations in food supplies. Certainly farther south in their breeding range, Arctic tern breeding effort and success have exhib-ited marked variation and declines in the Bay of Fundy, Canada, due in part to changing food availability (Dia-mond and Devlin 2003). Furthermore, marine bird spe-cies may differ in their breeding ecology in different parts of their range (Mallory et al. 2008), and there may also be marked natural variability in annual reproductive metrics for seabird colonies (Grémillet and Charmantier 2010); thus, it is important to evaluate whether knowledge gath-ered from short-term studies or studies from various parts of a species’ range can be extrapolated elsewhere, such as the Canadian Arctic. Basic knowledge about the ecology of Arctic-breeding seabirds is especially urgent at a time when a variety of natural and anthropogenic stressors are changing and perhaps increasing in intensity in the Arctic marine environment (ACIA 2005; Arctic Council 2009).

We spent five field seasons studying the reproduc-tive biology of Arctic terns at a colony on a small island situated beside a recurrent polynya in the High Arctic of Nunavut, Canada, with the goal of establishing baseline values and annual variability in reproductive metrics of terns at these latitudes, as well as examining environ-mental factors that may have influenced annual variation. Polynyas are hotspots for biodiversity and productivity in Arctic marine waters (e.g., Stirling 1997; Hannah et  al. 2009; Black et al. 2012; Maftei et al. 2015) and, thus, we predicted that reproductive metrics would be high and rela-tively consistent across years at our site, compared to areas considered more environmentally unpredictable (e.g., sites along shorelines affected by fisheries changes, temperature or current changes). However, both high levels of preda-tion pressure and inclement weather are associated with reduced reproductive success by terns (e.g., Power 1964; Becker and Specht 1991; Levermann and Tottrup 2007), so we expected that years with bad weather or higher preda-tor counts would have lower tern reproductive effort and success.

Methods

Study site

We studied the breeding biology of Arctic terns at a large colony on Nasaruvaalik Island in Penny Strait, Nunavut (75° 49′N, 96° 18′W; Fig.  1), between 15 June–9 August 2007, 16 June–17 August 2008, 16 June–29 July 2009, 15 June–7 August 2010, and 3 June–30 July 2011. There are several other tern colonies within 100  km (Maftei et  al. 2015), but Nasaruuvalik is the largest colony in the region. The island is approximately 3 km × 1 km in size (Mallory and Gilchrist 2003), is crescent-shaped, and is made of alluvial gravel that has risen from the surrounding sea due to isostatic rebound from historic glaciation. Terns nested in two colonies among old beach ridges on the low, south-western and northeastern ends of the island (separated by 1.5 km), typically at elevations <5 m above sea level (asl). Our base camp was located on a central plateau of the island approximately 30 m asl, and out of view of nesting terns; this was approximately 1 km north of the main south-western colony where we conducted our studies (hereafter “colony”). This colony supported about 300 pairs of nest-ing terns within 0.17 km2, with nests 1–15 m from the next closest nest. We walked from the camp to the colony several times daily (except in periods of rain or heavy fog), arriv-ing at blinds (below) that were >50 m from the main nest-ing concentration of birds. From these blinds, several nests could be observed using binoculars and scopes, though few nest contents could be recorded. Although many research-ers elevate their blinds to help increase observation effec-tiveness, this was not possible on this island, as stochas-tic visits by polar bears (Ursus maritimus) resulted in the destruction of anything erected vertically. Generally, our arrival did not cause an obvious change in behavior of the birds during nesting. There are also common eiders (Soma-teria mollissima borealis), long-tailed ducks (Clangula hyemalis) and Sabine’s gulls (Xema sabini) nesting among the terns (Mallory et al. 2012; Maftei et al. 2015). Each day in 2010 and 2011, one or two observers counted all adult Arctic terns within view on the ground (i.e., not the entire colony, but those visible from that vantage point) from the main blind in the morning and evening (weather-permit-ting), and we used the maximum number observed as our number of terns on the colony that day.

Measuring reproductive effort and success

Upon our arrival at the field camp, we began watching for terns settled on the ground, indicating nesting locations, which we checked immediately. If a nest was discovered, we recorded geographic coordinates with a GPS, erected a nest marker (numbered wooden stick) 1  m from the nest,

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and revisited the nest daily until a second egg was laid (or five successive days had passed; Hatch 2002) to establish egg-laying dates and sequence. When eggs were found, they were marked “A” or “B”, weighed (±0.2  g) using a 50-g Pesola spring scale, and length and breadth were measured (±0.01  mm) using digital calipers. Starting 20 days after the first egg was laid, we made daily visits to each nest to determine date of hatching of each egg where possible. Note that in 2007 we found nests with 1 or 2 eggs and followed the nest fate without knowing egg laying order. In 2009, we established a study plot containing ~100 nests, encompassing most habitat types found within the colony including the periphery and the centre to ensure no bias with regard to nest location in the colony (Coul-son 1968; Anotolos et al. 2006). This plot was the only sec-tion of the colony searched for new nests in 2009–2011, but

otherwise the methods from 2007 to 2011 were unchanged. We defined nests as successful if >1 egg hatched.

Any time after the first week of incubation, we trapped adult terns on their nest using a bownet (Salyer 1962). We then weighed terns with a 300-g Pesola spring scale, took a variety of morphometric measures of body size (see Dev-lin et al. 2004), and then attached a stainless steel, uniquely numbered band to the tarsus and released the birds. We conducted several preliminary analyses to correct body mass for structural size (e.g., Peig and Green 2009), but most structural measures had low correlation or explained minimal variation in body mass. Therefore, we used first-capture body mass itself as a proxy of body condition of the terns (Monaghan et al. 1992).

Each day, we walked to the colony and elsewhere on the small island to work on marine birds (e.g., Mallory et  al.

Fig. 1 Location of the study site at Nasaruvaalik Island in Penny Strait, Nunavut (75° 49′N, 96° 18′W), in the Cana-dian High Arctic

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2012), and at the end of each day, field teams pooled infor-mation on how many potential predators were observed on the island, irrespective of their behavior (polar bear, Ursus maritimus; arctic fox, Vulpes lagopus; common raven, Cor-vus corax; glaucous gull, Larus hyperboreus; long-tailed jaeger, Stercorarius longicaudus; parasitic jaeger, Sterc-orarius parasitica; pomarine jaeger, Stercorarius pomari-nus; peregrine falcon, Falco peregrinus; gyrfalcon, Falco rusticolus), adjusting counts for likely duplication of indi-viduals based on times of observations. We used total avian predators observed daily as our index of predation levels on the colony, divided into pre-breeding (before mean nest ini-tiation date) and incubation periods. The mammalian pred-ators occurred infrequently, so we did not include them in our predator index but we comment on specific instances of mammalian predation on the terns.

We recorded maximum daily temperatures and total pre-cipitation using a Davis Vantage Pro2® weather station, set to record hourly measurements.

Statistical analysis

Data were tested for approximation of normal distribu-tions using Kolmogorov–Smirnov tests, and subsequent comparisons of annual means were conducted using t tests, Mann–Whitney tests, analysis of variance (ANOVA) and Tukey–Kramer post hoc tests (when data approximated normality) or Kruskal–Wallis nonparametric ANOVA with Dunn’s multiple comparison tests (when data did not approximate normality, even after transformation). Propor-tions of nesting success were compared using Fisher exact tests, and Spearman rank correlations were used to test for trends where appropriate. A generalized linear model (GLM; binomial logit link) was used to test whether nest-ing success was predicted from nest initiation date, con-trolling for the effect of year. All analyses were performed using R (R Development Core Team 2011), InStat (Graph-Pad Software Inc. 2009) or Statistica (Statsoft Inc. 2015). Means are reported ± SD unless otherwise noted.

Results

Colony attendance

Daily maximum counts of Arctic terns in 2010 and 2011 exhibited considerable variation through the breeding season (Fig. 2). Terns arrived at the colony in early June, but numbers observed on the colony increased rapidly in late June, then fluctuated throughout July, and slowly declined through August. Other than 2010, when terns abandoned the colony near the end of the incubation period, the research teams always departed (latest 2 Sep)

prior to the terns departing. For the period 29 Jun – 29 Jul, which covered nest initiation and hatching each year, we counted a daily average of 235 ± 47 (n = 30 d) terns in 2010 which was similar to that in 2011 (223 ± 48, n = 25 d; t53 = 1.0, p = 0.3). However, daily maximum counts ranged by >100% (141–339 terns). During 2010 and 2011, we had 35 days where we conducted counts in the morning (0830–1030) and evening (1600–1900), and mean counts in the morning (232 ± 45) were significantly higher than in the evening (180 ± 47; paired t34 = 5.7, p < 0.001).

Weather conditions

Across the five seasons, the maximum daily tem-peratures prior to nest initiation differed significantly (Table  1; ANOVA; F4,50 = 12.6, p < 0.001), with tem-peratures warmer in 2010 than in 2007, 2009 and 2011 (Tukey–Kramer multiple comparison tests, all p < 0.01), and 2008 warmer than 2009 (p < 0.05; see Fig.  3). How-ever, during the incubation period, temperatures were simi-lar in all five years (F4,115 = 1.3, p = 0.28). The colony is situated beside a recurrent polynya and thus open water was available upon arrival from northward migration in each year.

Predators

The number of avian predators observed daily varied across years, both before nest initiation (Table  1; ANOVA on log-transformed data, F4,50=29.7, p < 0.001), and during incubation (F4,104=132.5, p < 0.001). Prior to incubation, more predators were seen daily in 2010 than in any other

Fig. 2 Timing of adult Arctic tern attendance at the colony during the breeding season in 2010 and 2011 (160 = 9 June). Counts are for all individuals on the ground visible from a blind, but do not repre-sent the entire colony

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year except 2011 (Tukey–Kramer tests, all p < 0.001), and 2007 and 2009 had similar daily numbers of predators, which were lower than other years (all p < 0.001). Dur-ing incubation, more predators were spotted daily in 2010 than in any other year (all p < 0.01), while fewer predators were observed in 2007 than in any other year except 2008 (all p < 0.001), and fewer were seen in 2008 than in 2009

(p < 0.001). In 2010, we observed nine adult tern carcasses, apparently killed by a peregrine falcon which we observed on three days over the island (one fresh kill on a day we saw the falcon). We did not see freshly killed adult terns in any other year.

Annually, polar bears visited the island 3 ± 1 d before nest initiation (i.e., three different days) which was similar to observations during incubation (2 ± 2 d; Mann–Whit-ney test, U’=16.5, p = 0.42). Numbers of visits were more variable across years during incubation, and 2009 and 2010 experienced more incubation days with polar bears than 2007, 2008 and 2011 (Table 1). The only time we saw an arctic fox on the island was prior to tern nest initiation in 2009, but it left the island before the incubation period.

Mean nest initiation and incubation period

Tern nest initiation was relatively fixed across years, with an overall mean of 6 Jul (Table 2), although there was still significant variation among years (KW test; H4,834 = 109.8, p < 0.001). Nests were initiated earlier in 2011 than in all other years (Dunn’s Multiple Comparisons test, p < 0.001). Overall, there was no obvious relationship between pre-nesting mean maximum temperature and median nest ini-tiation date (rs5=0.56, p = 0.35). There was nearly a one month spread in nest initiations: the earliest nest initiation

Table 1 Comparison of annual predator counts and temperatures on Nasaruvaalik Island, Nunavut before nest initiation (n = 11 d) and during typical incubation period across years (n = 24 d)

Year Mean (SD) daily avian predator count

Days with polar bears Mean (SD) maximum tempera-ture (°C)

Before nest initiation

During incubation Before nest initiation

During incubation

Before nest initiation

During incubation

2007 3.6 (0.9) 3.9 (0.9) 4 1 4.3 (2.4) 5.4 (3.2)2008 6.5 (1.5) 4.4 (1.4) 2 0 6.0 (2.8) 4.7 (2.8)2009 3.4 (0.6) 6.4 (1.3) 2 5 2.9 (1.9) 4.2 (2.4)2010 7.1 (1.8) 18.4 (4.9) 3 3 8.5 (3.4) 5.6 (3.6)2011 6.8 (1.5) 5.2 (0.6) 2 1 3.9 (1.5) 5.9 (2.6)

Fig. 3 Daily maximum temperatures (°C) over 5 years of monitoring at Nasaruvaalik Island (170 = 19 June). Vertical lines represent the mean start and end of incubation across all years

Table 2 Reproductive parameters of Arctic terns in 2007–2011 at Nasaruvaalik Island, Nunavut

*Nests were not checked systematically as in later years, so mean nest initiation date could have been earlier

Year Mean (SD), range, n of Arctic tern reproductive parameters

Nest initiation date Adult mass (g) Clutch size Incubation period (d) Nesting success (%), n

2007 6 Jul (1.7), 27 Jun-9 Jul, 250* 111 (6), 111, 55 1.8 (0.4), 2, 2112008 6 Jul (4.0), 28 Jun–21 Jul, 324 111 (7), 111, 26 1.8 (0.4), 2, 314 21.3 (0.1), 21, 90 95, 1042009 6 Jul (5.5), 30 Jun–21 Jul, 93 104 (7), 103, 50 1.5 (0.5), 1, 167 21.2 (0.2), 21.5, 31 78, 452010 5 Jul (1.6), 3–17 Jul, 85 103 (7), 102, 172 1.5 (0.5), 2, 113 22.4 (0.2), 22, 36 28, 1132011 2 Jul (4.6), 23 Jun–15 Jul, 82 110 (7), 110, 105 1.8 (0.4), 2, 108 93, 108Overall means 6 Jul (3.9), 23 Jun–21 Jul, 834 107 (8), 107, 401 1.7 (0.4), 2, 885 21.3 (0.1), 21, 157 72, 370

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was 23 Jun and the latest was 21 Jul. Terns spent about 21 d incubating, although this differed among years (H2,157=22.1, p < 0.001), with incubation approximately 1 d longer in 2010 (p < 0.05).

For those nests where we measured the incubation period, we controlled for mean annual nest initiation and then compared initiation dates among birds that incu-bated for <21, 21–22, and >22 days. Terns that initiated nests later in the season had a shorter incubation period than those that initiated their nests earlier (H3,157=10.2, p = 0.017; <21 vs. >22 days, p < 0.05).

Clutch size

From 2007 to 2011, terns nesting at Nasaruvaalik Island had a mean clutch size of 1.7 ± 0.4 eggs (Table 2), but this differed among years (H4,885 =115.8, p < 0.001). Clutch sizes were similar in 2007, 2008 and 2011, but smaller in 2009 and 2010 (p < 0.001). In 2008, where we had the largest seasonal effort tracking clutch size and correspond-ing nest initiation, the mean nest initiation date for 1-egg clutches (188 ± 4) did not differ from that of 2-egg clutches (189 ± 4; U’=7119, n = 314, p = 0.20). Only one 3-egg clutch (0.1%) was found in five field seasons.

Nesting success

Nesting success was not monitored in 2007, but in the four other years it averaged 72% and differed significantly among years (�2

3 =157.8, p < 0.001). Nesting success was

similar in 2008 and 2011 (Fisher Exact test, p = 0.56), but differed among all other year combinations (all p ≤ 0.014). Using a subset of nests and controlling for year, nests initi-ated earlier in the season were no more likely to be success-ful than those initiated later (GLM; p = 0.11).

Eggs

For all eggs measured, mean egg length was 40.6 ± 1.7 (n = 1354), which was similar across years (F4,1350=1.6, p = 0.16). However, mean egg breadth was 29.4 ± 1.0 (n = 1354) which differed across years (H4,1354=33.7, p < 0.001). Eggs were broadest in 2009, similar to 2007 and 2011 (all p > 0.05), while eggs in 2008 and 2010 were slim-mer (p < 0.001). When we restricted the analysis to only first-laid eggs in 2008–2011 (to eliminate possible influ-ence of pseudoreplication from individual birds), the pat-tern held: egg length was similar in all years (H3,550=0.7, p = 0.87) but eggs were broader in 2009 than 2008 and 2010 (H3,550=13.9, p = 0.003), although egg breadth was similar in 2009 and 2011 (p > 0.05). For 374, 2-egg nests, egg volumes of first laid eggs (34.8 ± 3.5) were not

statistically larger than that of second laid eggs (34.5 ± 3.0, paired t test, t373 = 1.7, p = 0.097).

Adult mass

Adult Arctic tern body mass differed among years (Table 2; ANOVA; F4,410 = 30.1, p < 0.001). Body mass of terns captured during incubation was similar in 2009 and 2010 (p > 0.05), and mass in those years was lower (all p < 0.001) than in 2007, 2008 or 2011 (which did not differ signifi-cantly from each other; p > 0.05).

Discussion

By conducting the first multi-year study of Arctic tern breeding biology in the Canadian Arctic, we established baseline information on breeding parameters of this spe-cies in the core of the tern breeding range in Canada, which allows for broad comparisons to similar baseline studies at other sites (e.g., Pettingill 1939; Hawksley 1957; Bengt-son 1971; Lemmetyinen 1972, 1973a, b; Chapdelaine et al. 1985; Egevang 2010; Vigfusdottir 2012; Table  3). We found that Arctic terns initiated nests in the first week of July, incubated nests for a little more than 21 days and aver-aged 107 g body mass during this time, and approximately three quarters of their nests hatched. However, across years we found marked differences in adult body condi-tion, clutch size, nest initiation dates, incubation periods, hatching success and predator numbers, all valuable data on helping assess the range of the “normal” ebb and flow of tern reproduction at these latitudes (Grémillet and Char-mantier 2010). Collectively, our data suggest that variation in predation pressure, food supplies, or the possible interac-tion of these factors have a strong influence on Arctic tern reproduction at high latitudes.

We predicted that reproductive metrics of terns would be high at our site because it is situated beside a polynya (an area of recurrent open water when other areas remain ice-covered; Hannah et al. 2009), and these sites are known to be highly productive and support greater biodiversity than surrounding areas (reviewed in Stirling 1997). The tern colony is the largest we know of in the Queens Chan-nel region and perhaps the Canadian High Arctic, support-ing ~900 birds (Maftei et  al. 2015). Contrary to our pre-dictions, terns at Nasaruvaalik Island had a similar mean clutch size (1.7 eggs) compared to other colonies located in the Arctic (Table 3), laid eggs of similar size (Hatch 2002; Egevang and Stenhouse 2007), and nest initiation dates and the production of chicks (measured as nesting success) also appeared to be similar to those recorded by Levermann and Tøttrup (2007) in northeast Greenland (another site in the High Arctic with similar environmental conditions).

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Our study and that from other locations suggest that Arc-tic tern clutch size is variable across years, presumably in response to environmental conditions, but generally colony averages are <2 eggs and, consequently, Arctic terns breed-ing at Nasaruvaalik Island were not exceptional compared to reproduction at other Arctic sites, despite being situated beside the polynya.

Arctic marine birds have a narrow window of time in which to pair bond and copulate, lay eggs, rear young and depart for southern waters, the timing of which is largely driven by temperatures and sea ice patterns (e.g., Gaston et  al. 2005; Mallory and Forbes 2007). Terns arrived at Nasaruvaalik Island in mid-June and probably departed in mid to late September each year. Depending on the year, daily maximum temperatures were generally above 0 °C (excluding wind chill) for the entire time we observed terns at the colony. Although the range of nest initiation dates was similar, the mean timing of nest initiation by terns was typically ~1 week later than that of sympatrically nesting Sabine’s gulls (Mallory et  al. 2012), and a few days later than nest initiations by black-legged kittiwakes (Rissa tri-dactyla) and thick-billed murres (Uria lomvia) at Prince

Leopold Island (74ºN, 90ºW), another High Arctic nesting site 270  km to the southeast (Gaston et  al. 2005). Lever-mann and Tøttrup (2007) suggested that annual decisions to breed by gulls and terns in northeast Greenland were made between 1 and 15 July. Thus, across marine bird species, the last week of June and first week of July seems to be a key period for nesting among High Arctic marine birds, at least in areas where sea ice remains into July.

Arctic terns on Nasaruvaalik Island attempted to breed in each year of this study, as they have every year since at least 2002 when visits to the island began (Mallory and Gilchrist 2003), and paleolimnological evidence suggests that Arctic terns have nested on Nasaruvaalik Island for at least 100 years (Michelutti et al. 2010). However, 2009 and 2010 proved to be poor breeding years for the terns, with the lowest clutch sizes and hatching success of all recorded years (Fig. 4). Like most seabirds, Arctic terns experience good and bad years of reproduction. These fluctuations are potentially influenced by many environmental factors, but the most frequently invoked are weather conditions (Power 1964; Becker and Specht 1991; Robinson et al. 2002), food availability (e.g., Lemmetyinen 1972, 1973b; Monaghan

Table 3 Mean clutch sizes of Arctic terns from various locations in the circumpolar Arctic

Site Latitude (ºN) Longitude (ºW) Year Mean clutch size (SD)

n References

Frederickshaab Glacier, Greenland 62.5 50.2 1943 1.7 (0.5) 279 Elkund (1944)Alexander Archipelago, Alaska 56.7 133 1945 2.0 (0) 45 Williams (1947)Bylot Island, Nunavut 73 79.5 1954 1.6 (0.1) 18 Drury (1960)Devon Island, Nunavut 75.6 84.5 1967 1.6 (0.1) 22 Hussell and Holroyd (1974)Svalbard, Norway 78.2 15.2 1967 1.7 (0.5) 152 Bengtson (1971)Cornwallis Island, Nunavut 74.7 95 1969 1.5 (0.6) 4 Geale (1971)Svalbard, Norway 78.2 15.2 1970 1.8 (0.4) 46 Lemmetyinen (1972)Cooper Island, Alaska 71.3 156 1975

19771.9 (0.1)1.5 (0.1)

5161

Boekelheide (1980)

Belcher Islands, Nunavut 56.2 79.5 1997 1.6 (0.5) 270 Gilchrist and Robertson (1999)Nasaruvaalik Island, Nunavut 75.8 96.3 2002

2003200420052006

1.8 (0.1)1.9 (0.1)1.9 (0.1)1.9 (0.1)1.9 (0.1)

2525202515

ML Mallory, unpubl. data

Kitssissunnguit, Greenland 68.8 52 20022003200420052006

1.8 (0.5)1.7 (0.4)1.8 (0.6)1.9 (0.4)2.0 (0.5)

292413446234367

Egevang and Frederiksen (2011)

Sand Island, Greenland 74.7 20.3 20072008

1.4 (0.5)1.6 (0.5)

10960

Egevang (2010)

Snaefellsnes, Iceland 64.9 23.3 2008 2.0 (0.4) 66 Vigfusdottir et al. (2013)2009 1.7 (0.5) 2722010 1.8 (0.4) 2012011 1.4 (0.6) 96

Breinykskjeran, Norway 67.5 12 2013 1.6 (0.1) 99 Barrett et al. 2014

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et al. 1989, 1992; Avery et al. 1992; Suddaby and Ratcliffe 1997; Votier et al. 2008; Vigfusdottir 2012), and the pres-ence of predators (Lemmetyinen 1973a; Levermann and Tottrup 2007; Wojczulanis-Jakubas et al. 2008). Below we consider how each of these factors may have contributed to the reduced reproductive success of the terns in 2009 and 2010.

In 2009, temperatures prior to and early in breeding were the coldest we recorded in this study and, while terns bred, reproductive effort was reduced (smaller clutch sizes), as parental investment should be lower in years when the expectation of reproductive success may be lower, espe-cially for long-lived birds (Montgomerie and Weatherhead 1988). Lower temperatures could be linked to late break-up of sea ice, and consequent delayed or reduced marine food availability (e.g., Gaston et al. 2005). In contrast, 2010 was a warm year but with more precipitation, and stormy weather with heavy rains (or wet snow) early in chick-rearing can cause widespread failure in tern colonies due to exposure and starvation (Power 1964; Becker and Spe-cht 1991; Robinson et al. 2002). However, at Nasaruvaalik, a major storm took place in 2010 before most chicks had hatched, and those that had hatched were already dying, most likely from starvation. Moreover, Levermann and Tottrup (2007) found that even when the terns abandoned breeding in Greenland, they still found food for courtship pairing and had high feeding success, and we also found that terns in poor breeding years were returning to the col-ony with food.

Food availability could not be sampled directly in this study, but inferences can be made. Body weight of adult terns during incubation was approximately 8% lower in 2009 and 2010 compared to other years. During their long migrations, Arctic terns use energy reserves

obtained in the wintering grounds to maintain their body condition (Agius 2008), and must replenish these reserves on arrival at the breeding colony. The lower body weights of terns in 2009 and 2010 likely resulted from low food resources at the breeding colony on arrival (Monaghan et al. 1989; Wendeln 1997). In support of this hypothesis, we observed differences in courtship feeding across years: in 2007 and 2008, ~5 cm polychaete worms were brought to the colony frequently but, in 2009 and 2010, courtship prey was smaller (<2 cm) and, therefore, constituted lesser volumes of food delivered.

We believe that several environmental conditions may have acted synergistically and deleteriously to influence tern reproduction in the poor years, but particularly in 2010. In that year, when we observed complete colony failure (Fig.  4), adults abandoned the colony after sev-eral days of high wind, snow and sleet during the chick-hatching period in the first week of August. This island-wide abandonment was unrelated to human disturbance; although our main (south) colony was visited daily and studied intensively, the north colony was visited only weekly or on alternate weeks in 2009 and 2010, and yet all terns abandoned there as well. During the same stormy period, at least one peregrine falcon visited the colony, as was evident by the carcasses of a few adult terns that stayed on their nests. Moreover, throughout pre-breed-ing and incubation, 2010 saw the most predators at the colony of any year. An increased presence of predators during the pre-breeding portion of the season may cause seabirds to delay the onset of laying, or forgo breeding entirely (Boekelheide 1980; Levermann and Tottrup 2007). Finally, in addition to poor weather and high pred-ator presence¸ adult body mass was lowest in 2009 and 2010, suggesting poor food supplies in the surrounding polynya (Suddaby and Ratcliffe 1997; although sufficient to have terns attempt to breed). Collectively, it is unclear whether one factor dominates decisions by terns regard-ing reproductive effort and success, or whether there are interactions between numerous factors.

Not only were clutch size, hatching success and adult mass all low in 2010, but the mean incubation period was longer that year as well. This was likely a result of high numbers of predators during incubation which kept flushing terns from their nests; the incubation period can be extended when predators are present (Boekelheide 1980). Our qualitative observations of colony-wide flush-ing (‘panic flights’; Hatch 2002) noted that terns were far more likely to flush from their nests even when a preda-tor was not in sight in 2010. However, frequent flushing occurred most often immediately after a falcon came through the colony; this species is a potential predator of nesting adults rather than just nest contents.

Fig. 4 Comparison of breeding metrics of Arctic terns or environ-mental conditions during incubation across years at Nasaruvaalik Island: filled squares mean clutch size (eggs); filled circles mean adult mass during incubation (g); open triangles mean nesting suc-cess (%); open bars mean total number of avian predators daily dur-ing incubation; filled bars days with polar bears during incubation

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High Arctic colony clutch sizes

The comparisons of clutch size with other High Arctic tern colonies (Table  3) suggest that, despite the inclu-sion of two poor breeding years, Nasaruvaalik Island is an average, sometimes good site for breeding Arctic terns. One reason for the quality of the site lies with the proximity of nearby polynyas close to the island, and sev-eral more within 30–35 km (Hannah et al. 2009), within the known foraging distances of some breeding terns (Black 2006). Two smaller polynyas immediately north and south of Nasaruvaalik Island were also open in mid-June in all years of this study. As a consequence, predict-able open water by the polynyas means that nest initia-tion dates of terns at Nasaruvaalik Island may not vary as much as in other Arctic-nesting seabirds, which may experience substantial delays or forgo breeding altogether in late ice years (Boekelheide 1980; Gaston et  al. 2005; Mallory and Forbes 2007).

Seabird breeding phenology has already been docu-mented as shifting in relation to climate change in the last 30 years (e.g., Møller et  al. 2006; Gaston et  al. 2009b; Wanless et  al. 2009), although long-distance migrants appear less likely to adjust arrival dates for breeding than short-distance migrants (Gunnarsson and Tómas-son 2011). Thus, data from this study that will serve as a baseline for future monitoring of Arctic terns may already differ from a baseline that would have been established based on observations collected more than 30 years ago; this is often termed a ‘shifting baseline’ (e.g., Pinnegar and Engelhard 2008). Nonetheless, in the absence of earlier baseline data, it is important to estab-lish current, ‘typical’ parameters of reproductive ecology as soon as possible, however shifted they may be (Mal-lory et  al. 2010), to enable documenting and modelling future changes. Data from this study will allow future use of this species as a bioindicator of the marine health of the region as climate change continues and as human activities become increasingly pervasive throughout the Canadian High Arctic Archipelago.

Acknowledgements We are indebted to the many field assistants who helped with this project. K. Kuletz provided unpublished data on trends in Arctic Terns from Alaska, USA, and H. G. Gilchrist provided data from Southampton Island, NU. Financial and logistic support were provided by Environment Canada (Canadian Wild-life Service), Aboriginal Affairs and Northern Development Canada (Northern Contaminants Program), Natural Resources Canada (Polar Continental Shelf Program), University of New Brunswick, and Acadia University. All work was conducted under valid research per-mits (EC-PNR-11-020, NUN-SCI-09-01, WL 2010-042). Finally, we thank the anonymous referees who provided insightful reviews of this manuscript.

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