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5 April 2017 Migratory connectivity of Semipalmated Sandpipers and implications for conservation
Stephen Brown, Cheri Gratto-Trevor, Ron Porter, Emily L. Weiser, David Mizrahi, Rebecca Bentzen, Megan Boldenow, Rob Clay, Scott Freeman, Marie-Andrée Giroux, Eunbi Kwon, David B. Lank, Nicolas Lecomte, Joe Liebezeit, Vanessa Loverti, Jennie Rausch, Brett K. Sandercock, Shiloh Schulte, Paul Smith, Audrey Taylor, Brad Winn, Stephen Yezerinac, Richard B. Lanctot
Author Affiliations +
Abstract

Semipalmated Sandpiper (Calidris pusilla) populations have undergone significant declines at core nonbreeding sites in northeastern South America. Breeding populations have also declined in the eastern North American Arctic, but appear to be stable or increasing in the central and western Arctic. To identify vulnerable populations and sites, we documented the migratory connectivity of Semipalmated Sandpipers using light-level geolocators, deploying 250 at 8 Arctic sites across the species' breeding range from 2011 to 2015, plus 87 at a single wintering site in northeastern Brazil in 2013 and 2014. We recovered 59 units and resighted 7 more (26% return rate) on the breeding grounds, but none at the nonbreeding site. We recovered only ∼3% of units deployed in 2013 at eastern Arctic breeding sites, but recovered 33% of those deployed in 2015. Overall, birds with geolocators were 57% as likely to return as those carrying alphanumeric flags. Stopover durations at prairie sites (mean: 8.7 days southbound, 6.7 days northbound) were comparable with durations estimated by local banding studies, but geolocator-tagged birds had longer stopovers than previously estimated at James and Hudson Bay, the Bay of Fundy, and the Gulf of Mexico. Migration routes confirmed an eastern Arctic connection with northeastern South America. Birds from eastern Alaska, USA, and far western Canada wintered from Venezuela to French Guiana. Central Alaskan breeders wintered across a wider range from Ecuador to French Guiana. Birds that bred in western Alaska wintered mainly on the west coasts of Central America and northwestern South America, outside the nonbreeding region in which population declines have been observed. Birds that bred in the eastern Arctic and used the Atlantic Flyway wintered in the areas in South America where declines have been reported, whereas central Arctic–breeding populations were apparently stable. This suggests that declines may be occurring on the Atlantic Flyway and in the eastern Arctic region.

INTRODUCTION

Understanding the population dynamics of migratory animals requires knowledge of migration routes and patterns of connectivity among different stages of the life cycle (Webster et al. 2002, Marra et al. 2006). Understanding migration ecology is particularly important in situations in which one population of a species is declining or where declines are present in one portion of a species' range but not others. In these cases, knowing the extent to which individuals of different populations co-occur in different seasons is essential for implementing effective conservation strategies.

The Semipalmated Sandpiper (Calidris pusilla) was historically one of the most widespread and numerous shorebird species in the Western Hemisphere, breeding across the North American Arctic tundra (Brown et al. 2001). Western, central, and eastern breeding populations are suspected to occur across the North American Arctic based on a stepped cline in bill length, with longer-billed forms in the east and shorter-billed forms in the west (Manning et al. 1956, Harrington and Morrison 1979, Gratto-Trevor et al. 2012a), although genetically the species appears monotypic (Miller et al. 2013). Monitoring across the breeding range has shown variable population trends, with declines occurring at some sites in the eastern Arctic, but generally stable or increasing trends in the central and western Arctic (Jehl 2007, Andres et al. 2012a, Smith et al. 2012). Population declines have also been reported at staging sites along the east coasts of Canada and the United States (Howe et al. 1989, Morrison et al. 1994, 2001, Bart et al. 2007, Gratto-Trevor et al. 2012a, 2012b, Hicklin and Chardine 2012, Mizrahi et al. 2012). The Atlantic Flyway migration route is thought to be used by eastern Arctic breeding populations as well as by some central Arctic breeding birds. Major population-level declines have been documented in the eastern portion of the core nonbreeding range in Suriname and French Guiana since the mid-1980s (Gratto-Trevor et al. 2012b, Morrison et al. 2012).

Our current understanding of migration routes and connectivity is based on 2 major sources, bill lengths and band recoveries. A recent genetic study (Miller et al. 2013) was not useful for determining migratory connectivity between breeding and nonbreeding sites because populations were panmictic. Bill length analyses of migrating or nonbreeding birds can suggest breeding origin because of the cline in bill lengths across the Arctic (Gratto-Trevor et al. 2012a, Tavera et al. 2016). Band recoveries and resightings are available for a small proportion of the birds marked at breeding, staging, and nonbreeding sites over the past 40 yr, and connect the banding and recovery locations. Bill length data and available band recoveries suggest that most western breeders migrate south through the prairies, along with some birds from central Arctic populations (Lank 1979, 1983, Gratto-Trevor and Dickson 1994). The remaining central Arctic breeders, and all eastern Arctic birds, are thought to migrate south along the north Atlantic Coast of North America (Harrington and Morrison 1979, Gratto-Trevor et al. 2012a). Overall, western Arctic breeders appear to overwinter farther west in South America than eastern breeders, although there is considerable mixing among populations, particularly in French Guiana and Guyana (Lank 1983, Gratto-Trevor et al. 2012a). In spring, birds from the eastern Arctic usually migrate north along the U.S. Atlantic Coast. Central and western Arctic breeders primarily migrate north through the interior of North America (Morrison 1984, Gratto-Trevor et al. 2012a). Some populations exhibit elliptical migration, using western flyways for northbound migration and more easterly routes during southbound migration (Gratto-Trevor and Dickson 1994). However, knowledge of these movement patterns is incomplete because individuals have not been followed throughout the entire migratory cycle as this is not possible based on band recoveries alone, and resighting rates may be biased by the geographic distribution of banding locations or observers.

Our major objectives were to provide new and more accurate information about the migration routes of Semipalmated Sandpipers and to understand the connections between breeding and wintering areas so that causes of population declines can be studied and addressed. To effectively conserve Semipalmated Sandpipers, we need to understand the spatial and temporal relationships among nonbreeding sites, migration routes, and breeding areas (Brown et al. 2001). While geolocators cannot identify precise locations, with appropriate analytical techniques their accuracy is effective for investigating long-distance movements and for identifying the general regions used during migration and in winter by birds from different breeding locations (Porter and Smith 2013). Geolocators can provide improved information about routes and staging areas used during migration by individuals from specific breeding populations. This can allow a better understanding of how different populations may be affected by loss or degradation of habitat, hunting, and other threats occurring at particular wintering or staging sites, so that conservation efforts can be more effectively directed at key staging or nonbreeding sites used by birds from declining populations. The Arctic Shorebird Demographics Network (Brown et al. 2014), which operated study sites across the North American Arctic, provided an excellent opportunity to collect information about migration routes of birds from across the breeding range.

In addition to using geolocators to fulfil our major objectives, we also used geolocator data to determine the duration of Semipalmated Sandpiper stopovers during migration. Previous measurements of stopover duration have been calculated by recapturing or resighting birds at migration sites (e.g., Dunn et al. 1988, Lyons and Haig 1995, Iverson et al. 1996, Alexander and Gratto-Trevor 1997, Warnock and Bishop 1998, Henkel and Taylor 2015), but these observations have potentially underestimated stopover durations because of the unknown amount of time that birds were present at the study site either before their first capture or after their last capture or resighting. Unbiased stopover estimates will improve population estimates based on counts at migration sites.

Our large sample of geolocators deployed over multiple years also allowed us to examine whether geolocators themselves may have detrimental effects on small migrating shorebirds. Geolocators have been successfully used to track other migratory shorebirds (Minton et al. 2010, Niles et al. 2010, Hedenström et al. 2013, Hooijmeijer et al. 2013, Smith et al. 2014, Summers et al. 2014), but none had been placed on Semipalmated Sandpipers prior to this study. Because tags have had detrimental effects on some bird species (Costantini and Møller 2013, Gómez et al. 2014, Fairhurst et al. 2015), but not on others (Conklin and Battley 2010, Petersen et al. 2015, Weiser et al. 2016), and because the weight of the tag influences the impact on the bird carrying it (Barron et al. 2010, Streby et al. 2015, Weiser et al. 2016), we investigated whether geolocators might be a handicap for Semipalmated Sandpipers. To do so, we compared return rates across our network of study sites between birds fitted with geolocators and birds marked only with alphanumeric leg flags.

METHODS

Deployment of Geolocators

We deployed 192 geolocators (W65, Migrate Technology, Cambridge, UK) on adult Semipalmated Sandpipers breeding at 8 sites on the breeding grounds across the species' Arctic breeding range in the spring of 2013 (Figure 1). An additional 29 geolocators were deployed (18 of model W65A9UJ and 11 of Mk20A, British Antarctic Survey, Cambridge, UK) on adults breeding at Nome, Alaska, USA, in 2011 and 2012, and 29 more units (model W65) on birds from Coats Island, Nunavut, Canada, in 2015, for a total of 250 units deployed at breeding sites. Nests were located using area searches and by dragging a rope along the ground between 2 people to flush nesting birds within defined study plots following methods outlined by Brown et al. (2014). Individual birds were captured at nest sites with bownet and walk-in traps, and were marked with geolocators attached to leg flags on the left tibiotarsus, and either alphanumeric leg flags or some combination of uniquely identifiable color bands on the right tibiotarsus. Leg flags are similar to traditional color bands, but have an extension, in our case either with a 3-character alphanumeric code that uniquely identified the individual bird, or to which the geolocator was attached (see leg flag banding methods in Gratto-Trevor (2004), and geolocator attachment methods in Minton et al. (2010) and Niles et al. (2010)). The total weight of flag and geolocator varied among sites from 0.8 to 1.0 g, or between 3% and 4% of the average body weight of 26 g for the birds in our sample. We added a color band applied below the geolocator (spacer band) on birds from Coats Island in 2015 to help the geolocator flag move more freely and to reduce its impact.

FIGURE 1.

Locations of sites where geolocators were deployed on Semipalmated Sandpipers: (A) Nome, (B) Cape Krusenstern, (C) Barrow, (D) Ikpikpuk, and (E) Canning River, Alaska, USA; (F) Mackenzie Delta, Northwest Territories, Canada; and (G) Coats Island, and (H) Igloolik, Nunavut, Canada. Numbers of geolocators recovered/deployed follow site letters. All were deployed in 2013 and recovered in 2014, except at Coats Island and Nome, where recovery years and numbers are as shown.

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Birds were relocated and recaptured in the year following deployment of geolocators by systematically searching areas near marked birds' previous nest locations. We sexed birds by comparing the bill lengths of adults in mated pairs, with longer-billed birds in pairs being identified as female. Within a breeding population, sexing by bill length alone is 92% accurate, and is virtually 100% if both members of the pair are measured (Gratto and Cooke 1987). When only one individual was captured, we sexed birds if possible using bill length information from Harrington and Morrison (1979) and Sandercock (1998), and otherwise considered them to be of unknown sex.

We deployed an additional 37 W65A9UJ geolocators in January, 2013, and 50 more in January, 2014, on adult birds captured at a major roosting site on the wintering grounds at Coroa dos Ovos, a site within Reentrâncias Maranhenses, a hemispherically important Western Hemisphere Shorebird Reserve Network (WHSRN) site in the state of Maranhão, Brazil. Birds were captured with mist nets at night starting 4 hr before high tide. The same roosting site was revisited in 2014 and 2015 and efforts were made to relocate and recapture birds fitted with geolocators using mist nets and a modified cannon net during 8 days of trapping each year.

The 0.65-g W65 tags were set to full level light recording in Mode 3, with a 5-min interval and minimum and maximum temperature recording, as well as wet vs. dry count and conductivity records. The 0.8-g Mk20A units were also set to full level light recording with a 5-min interval, but with no temperature recording. Locations of birds were determined from the light-level data using the threshold methods within Bastrak software (British Antarctic Survey; see  www.birdtracker.co.uk). No precalibration was conducted due to logistical constraints. Subsequent refinements were employed using the orientations of coastlines, weather, equinox equidistance, salt water conductivity, temperature, and higher thresholds in northern Canada, following techniques outlined by Porter and Smith (2013). Errors in latitude away from coastal sites are more variable due to different light characteristics, weather, topography, or the number of fixes obtained for a given position. Thus, positions of tagged birds reported from interior locations should be viewed as approximate, especially with respect to latitude. Errors in longitude may be greatly reduced by averaging across all locations in a given site, since the greatest component of error for shorebirds away from the breeding grounds is shading due to cloud cover. Shaded sunrise fixes are offset in the opposite direction from shaded sunset fixes, but across many days a shaded sunrise is likely to be offset by a shaded sunset of a different day. For a typical bird in this study, C470, the longitudes and 95% confidence intervals following the annual cycle were: Alberta, Canada, 110.1 (109.4–110.8; n = 10); North Dakota, USA, 100.0 (99.5–100.5; n = 33); Venezuela, 69.2 (68.8–69.6; n = 28); French Guiana, 52.0 (51.9–52.1; n = 451); Guyana, 59.5 (59.2–59.8; n = 24); and Cuba, 78.9 (78.5–79.3; n = 14). It is not appropriate to average latitude to improve precision because all shading dislocates the fixes in the same direction. We caution that all locations should be viewed as approximate, and we have purposely avoided providing latitude and longitude values to prevent ascribing too much certainty to these locations. Our primary study objectives were to identify the general regions used by migrating and wintering birds and the migratory connectivity between breeding and wintering regions.

To determine northbound and southbound migration stopover sites, we mapped tracks and evaluated them one fix at a time to determine periods when a bird was traveling vs. stationary. Periods of travel were identified by consistent directional movement and distances between consecutive fixes of >200 km. We assumed that a bird had stopped at a new location when there was an absence of consistent directional change in latitude and longitude, and distances between consecutive fixes were <200 km. We plotted direct connections between stopover sites, but actual travel routes may have deviated from direct-line paths. Our approach allowed us to discern the start and end of each stopover or travel period with a resolution of 0.5 to 1.0 day.

Effects of Geolocators on Return Rates

We compared the 1-yr return rates of the birds carrying geolocators with those of a control group captured at the same breeding sites. The control group consisted of Semipalmated Sandpipers that were individually marked with an alphanumeric flag, but not fitted with geolocators. Aside from application of the geolocator, both groups of birds were subject to similar capture and handling effects and were captured at nests in the same study plots at our field sites. Control birds included those captured in 2010–2013 and resighted the next year in 2011–2014 at all sites, and birds captured in 2015 and resighted in 2016 on Coats Island. No control birds were banded on Coats Island in 2013, the first year of geolocator deployment, so the control group for birds fitted with geolocators on Coats Island in 2013 consisted of birds without geolocators that were marked in 2004 and 2005, and resighted in 2005 and 2006. We used birds marked with flags in 2015 as controls for the birds marked with geolocators on Coats Island in the same year. All Semipalmated Sandpipers captured at Igloolik, Nunavut, Canada, received a geolocator, so this site did not have a control group. At other sites, resighting effort was similar between groups, although observers may have been more likely to pursue and accurately record resightings of birds carrying geolocators relative to control birds.

For each group of birds at each site, we estimated the 1-yr return rate as the proportion of marked birds that were resighted or recaptured 1 yr after initial capture in systematic searches of the areas in each site in which geolocators were deployed. We did not include any resighting records from subsequent years because at most sites we had only a single year of resighting data for birds with geolocators (deployed in 2013, resighted in 2014). Return rates are the product of 4 probabilities, and a marked bird not seen in the year after capture could have died, dispersed to breed elsewhere, skipped a year of breeding, or been overlooked by observers.

We tested for an overall effect of geolocator and other potential covariates on return rates with a generalized linear mixed-effects model (GLMM, with a binomial link function). The other covariates (sex, nest success, timing of capture relative to season or incubation, body mass upon capture, whether or not an alphanumeric leg flag was applied) were included in case the birds fitted with geolocators represented a biased sample with respect to some factor that may have affected return rates. We also included a random effect of year on the intercept, and a random effect of site on both the intercept and slope of the geolocator effect. Our model thus controlled for effects of year and site while still testing for an overall effect of geolocator that was interpretable for the species as a whole.

To make effect sizes comparable across covariates with different scales, we standardized explanatory covariates by centering on the mean and dividing by 2 standard deviations with the standardize function in R package arm (Gelman and Su 2013). We tested all possible submodels of this full model using the dredge function in R package MuMIn (Bartoń 2013), and considered any model with a difference from the top model in Akaike's Information Criterion corrected for small sample sizes (ΔAICc) <2 to be a competitive model. Controlling for other potential covariates required using subsets of the data, so if a factor did not have an important effect on return rates (see below), we excluded it and repeated the model-selection procedure. Once all remaining covariates were included in the top model set, we used model averaging (Burnham and Anderson 2002) with function model.avg in package MuMIn to estimate effect sizes for variables in the top model set while accounting for model uncertainty. We made inferences from the averaged model. Return rates varied among sites, so we also compared return rates between groups within each site with a Fisher's exact test. We concluded that there was a site-specific effect of geolocators when the 95% confidence interval of the odds ratio (probability that a bird fitted with a geolocator returned / probability that a control bird returned) did not include 1.0.

RESULTS

We recovered 59 geolocators from Semipalmated Sandpipers that returned to the breeding grounds where they were tagged the previous year (Figure 1). We also recovered 3 geolocators from dead birds or remains prior to fall migration in the year that they were deployed (2 at Mackenzie Delta, Canada, and 1 at Nome), but had no information about the cause of death. Two of the 59 returned units had malfunctioned and recorded no useful light data, so we had migration tracks for 57 birds captured at sites across their Arctic breeding range. One bird at Nome was recovered 2 yr after deployment, and its geolocator had recorded both a round trip and an additional southbound trip. Not all recovered geolocators had recorded a full year of light data, and only 46 had recorded full northbound tracks, which affected the sample sizes of some analyses. Geolocator tags recorded movement data for an average of 329 ± 73.3 SD days. Of the birds with recovered functioning geolocators, 19 were classified as female, 33 as male, and 5 as unknown sex (Table 1). We also identified an additional 7 birds with geolocators that returned to the breeding grounds, but were not recaptured, using their unique color band combinations or alphanumeric flags, and these individuals were included in the 26% return rate calculated across all breeding sites.

TABLE 1.

Wintering regions of Semipalmated Sandpipers equipped with geolocators at 7 breeding sites in the Arctic (see Figure 1 for breeding site locations) by sex. Western South America includes Peru, Ecuador, Colombia, and Venezuela; northeastern South America includes Suriname, French Guiana, and Brazil.

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Of the 37 birds fitted with geolocators in Brazil in January, 2013, at least 5 were observed again at their wintering location and 3 were observed during migration, but although they were uniquely identified using their color bands, none were recaptured. One bird was observed at Squaw Creek, Missouri, USA (40.08°N, 95.27°W), in May, 2013, a second at Wells, Maine, USA (43.31°N, 70.57°W), in July, 2013, and a third at Mispillion Harbor, Delaware, USA (38.95°N, W75.31°W), in May, 2014. Thus, it appears that 1 of these birds used the Central Flyway during northbound migration, 1 used the Atlantic Flyway during northbound migration, and 1 used the Atlantic Flyway during southbound migration. None of the 50 birds fitted with geolocators in January, 2014, were recaptured or resighted on the wintering grounds, or during northbound or southbound migration.

Migration Routes

The Semipalmated Sandpipers marked in this study wintered across the species' entire wintering range, with varying levels of connectivity between breeding and wintering areas (Figure 2, Table 1). Birds captured at breeding sites at Nome and Cape Krusenstern, Alaska, USA (western Arctic breeding population), wintered primarily on the west coast of South America (Ecuador, Colombia, and Peru) as well as at sites in Central America (Guatemala, Honduras, Nicaragua, and Panama; Figure 2A, Table 1). Birds from Barrow and Ikpikpuk, Alaska (western Arctic breeding population), wintered across almost the entire nonbreeding range of the species, from Peru to French Guiana (Figure 2B, Table 1). Birds from the Canning River, Alaska (western Arctic breeding population), and Mackenzie Delta, Canada (central Arctic breeding population), primarily wintered on the northeastern coast of South America (Suriname and French Guiana), as well as in western South America (on the Caribbean Coast of Colombia) and the Caribbean (Figure 2C, Table 1). The 12 birds from Coats Island, Nunavut, Canada (eastern Arctic breeding population), also wintered in northeastern South America (Suriname, French Guiana, and Brazil; Figure 2D, Table 1). The migration routes of western breeders went through the interior of North America for both northbound and southbound migrations, often staging in the Canadian prairies or the Dakotas, USA (Figures 2, 3, Table 2). All birds from Alaska and the Mackenzie Delta stopped on the coasts of Texas and Louisiana, USA, during northbound migration, often for several weeks, but only 40% used these areas during southbound migration (Figures 2, 3, Table 2).

FIGURE 2.

Southbound movement patterns of 57 geolocator-tagged Semipalmated Sandpipers from 7 Arctic breeding sites in 2011, 2012, 2013, and 2015. Different symbols represent location fixes obtained for birds from different breeding sites. Lines show direct connections between stopover sites, not paths taken by birds.

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FIGURE 3.

Northbound movement patterns of 46 geolocator-tagged Semipalmated Sandpipers from 7 Arctic breeding sites in 2012, 2013, 2014, and 2016. Symbols and lines as in Figure 2.

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TABLE 2.

Duration of northbound and southbound migration in days (d) for the entire migration (All) and for specific staging areas of Semipalmated Sandpipers equipped with geolocators at 7 breeding sites in the Arctic. Sample sizes vary based on the number of birds using an area.

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All 12 of the eastern-breeding Coats Island birds stopped at James or Hudson Bay, Canada, during southbound migration (Figure 2D, Table 2). One bird flew directly from James Bay (after staging there for 31 days) to Venezuela's Orinoco River Delta region, a distance of approximately 5,270 km, and then flew to the Amazon Delta region of Brazil. One bird stopped north of the Bay of Fundy, Canada, for 25 days, and 3 stopped in the Bay of Fundy (for 18, 27, and 31 days) before departing for the southern over-ocean flight. The remaining 7 birds stopped for 15–30 days at Delaware Bay, New Jersey, USA, before departure. All but 1 followed their arrival in South America with movements eastward to wintering locations (Figure 2D); 1 moved west from French Guiana to Suriname. All of the northbound tracks recorded from Coats Island birds (Figure 3D) followed the U.S. Atlantic coast, except for 1 that included the Florida panhandle, USA, and 1 that stopped in Georgia, USA, and then went as far inland as Saskatchewan, Canada, on the return trip to Coats Island. All but the bird that went to Saskatchewan stopped in James or Hudson Bay on the way north.

The duration of northbound and southbound migration varied greatly among individual birds, but was significantly longer on average during southbound migration (Table 2). Migration rates averaged 201.0 ± 66.2 SD km per day (n = 57), and total round-trip migration distance averaged 17,854 ± 4,424 SD km (n = 42). The influence of migratory direction on length of stay at common North American stopover sites differed among sites (Table 2). Length of stay along the Gulf Coast of the U.S. and on the North American prairies (western breeders) was similar between northbound and southbound migrants, but was longer for southbound than northbound migrants at Delaware Bay and James or Hudson Bay (eastern breeders; Table 2).

Females and males showed no significant differences in initiation date of northbound migration or arrival at the breeding site, nor were there significant differences in duration of northbound or southbound migration (Table 3). However, females started migrating south significantly earlier than males (Table 3). Among birds of known sex, females wintered throughout the entire range of the species, but no confirmed males were among the 3 birds found in the Caribbean region (Table 1).

TABLE 3.

Timing and duration (in days; d) of migration for male and female Semipalmated Sandpipers equipped with geolocators at 7 breeding sites in the Arctic.

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Effects of Geolocators on Return Rates

To compare return rates between birds with geolocators and birds carrying leg flags only, we analyzed 247 capture events of individuals in the geolocator group and 1,070 capture events of 978 individuals in the control group. We excluded 3 individuals from the geolocator group that were found dead without having left the breeding site. We encountered 66 (26%) birds with geolocators and 408 (38%) control birds at breeding sites in the year after capture. Return rates varied among sites and tended to be lower at sites in the eastern part of the breeding range (Figure 4). Overall, Semipalmated Sandpipers fitted with a geolocator were 57% as likely as those without a geolocator to return to breeding sites (Table 4). Two submodels were included in the top model set (ΔAICc < 2; Table 5). Averaging the 2 models resulted in final estimates of a strong negative effect of carrying a geolocator, a negative effect of carrying an alphanumeric flag, and a positive effect of having previously been banded at the capture site (Table 5).

FIGURE 4.

Proportion of Semipalmated Sandpipers that were resighted in the year following capture across their range. Sites are ordered from west to east across the North American Arctic (see Figure 1). Sample size (number of birds marked with geolocators or with uniquely coded leg flags) is shown above the corresponding bar for each site. Sample sizes for birds marked with geolocators at the Nome and Mackenzie sites differ from those in Figure 1 because 3 birds with geolocators were found dead prior to migration. An asterisk indicates a significant difference in return rate (P < 0.05; Table 4).

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TABLE 4.

Site-specific Fisher's exact tests for the difference in return rates between Semipalmated Sandpipers fitted with geolocators (n = 247) and control birds without geolocators (n = 1,070). The odds ratio is the proportional return rate of birds with vs. without a geolocator, and a value of 1 indicates no difference (e.g., in Nome, 0.293 birds with a geolocator returned for every 1 bird without a geolocator). Sites with a significant effect of geolocator are shown in bold font (see Figure 1 for site locations). Igloolik could not be tested because there were no data for birds without geolocators.

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TABLE 5.

Standardized effect of each covariate on 1-yr return rates of Semipalmated Sandpipers across all sites in the North American Arctic in 2012, 2013, 2014, and 2016. Values are on the logit scale and were obtained by averaging the top model set (ΔAICc < 2), which included 2 models: geolocator + alphanumeric flag + previously banded (wi = 0.505, ΔAICc = 0.00), and geolocator + previously banded (wi = 0.364, ΔAICc = 0.65). Each model included random effects of year (on intercept) and site (on intercept and geolocator effect).

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Site-specific tests indicated that geolocators had a significant negative effect on return rates at 2 sites: Nome and Ikpikpuk (Table 4). Field crews at Nome and Ikpikpuk applied geolocators only to birds that had not been previously banded, but had some control birds that had been marked in prior years. At the other study sites, both treatment groups included a mix of previously banded and unbanded birds. In our dataset, previously unbanded birds were less likely to return, which could partially explain why fewer birds marked with geolocators returned to Nome and Ikpikpuk. However, when we looked at only these 2 sites and used a subset of the data that included only previously unbanded birds in both the geolocator (n = 83) and control groups (n = 340), the negative effect of geolocator remained strong and significant (GLMM with random effects of site and year: intercept = −0.50 ± 0.08, geolocator effect = −0.91 ± 0.25, P < 0.001). Therefore, banding history did not explain the negative effects of carrying a geolocator at these 2 sites.

DISCUSSION

Our study provides the first large-scale analysis of migratory connectivity for Semipalmated Sandpipers among breeding, stopover, and nonbreeding sites. Our results confirm and refine the previous general understanding of population-specific migratory movements based on resightings, band recoveries, and bill length measurements (Harrington and Morrison 1979, Lank 1979, 1983, Gratto-Trevor and Dickson 1994, Gratto-Trevor et al. 2012a). Going well beyond current knowledge from prior studies, deploying geolocators simultaneously across the entire breeding range of this species provided unprecedented information about the timing of migration and clarified specific connectivity among breeding, migration, and nonbreeding areas, which will help to assess the relative conservation importance of sites and regions for Semipalmated Sandpipers.

Migration Routes and Connectivity

Geolocators recovered from Coats Island showed movements consistent with previous data suggesting that birds that breed in the eastern Arctic overwinter in the areas of northern South America where large population declines have been observed, including Suriname, French Guiana, and Brazil. Connectivity between these sites suggests that declines documented in northern South America may be linked to population declines in the eastern Arctic. On the other hand, at least some birds from the northwestern part of the breeding range also wintered in northern South America, including birds from the northern coast of Alaska as far west as Barrow and the Canning River. Birds from these sites spread out across the entire wintering range from Ecuador to Brazil. In contrast, all of the birds breeding in far western Alaska spent the boreal winter in South America in areas no farther east than western Venezuela.

Birds from breeding sites farther east also generally wintered farther east in South America (Table 1), which is consistent with the results of previous studies (Harrington and Morrison 1979, Lank 1979, 1983, Morrison 1984, Gratto-Trevor and Dickson 1994, Gratto-Trevor et al. 2012a). Previous work demonstrated that some Alaskan breeders wintered in northeastern South America, but our results indicate that