Every year, long-distance migratory birds travel vast distances to find better foraging opportunities, avoid predation and inclement weather, and improve fitness (Alerstam et al. 2003). Although the conditions at their destination (i.e., its breeding and wintering grounds) are significant determinants of fitness and survival (Rappole and McDonald 1994; Latta and Baltz 1997), there is mounting evidence to suggest that conditions encountered on migration are important drivers of population dynamics (Baker et al. 2004; Newton 2006; Hewson et al. 2016). In particular, factors that influence the timing of northbound migration, such as weather (Sinelschikova et al. 2007) and prey availability (Zwarts 1990), are linked to the fitness of migratory birds. This is because early arrival on the breeding grounds is associated with access to higher quality territories and mates, earlier breeding, bigger clutches, higher offspring survival, additional time for re-nesting, and, consequently, greater reproductive success (Norris et al. 2004; Smith and Moore 2005).

Despite the clear link between early arrival to breeding areas and fitness, a complete understanding of the migration phenology of many species is lacking. Moreover, although recent studies have revealed differences in the timing of northbound and southbound avian migration (e.g., Lislevand et al. 2016; Carneiro et al. 2018; Covino et al. 2020), less is known about whether or how individuals adjust their migration timing within a season. Studying long-distance migrants has been complicated by the logistical difficulty of tracking individuals (particularly small-bodied species) across widely dispersed, but biologically linked, breeding, staging, and wintering sites (Marra et al. 2015). Recent advances in automated radio-telemetry, including the development of automated receivers and the miniaturization of radio-tags, has facilitated the tracking of small migrants across broad geographical scales (Taylor et al. 2017).

Our goal was to examine the timing of northbound migration and linkages between major staging areas in Sanderling (Calidris alba), a long-distance migratory shorebird species that has declined at a rate of -3.3%/ year since the 1970s (Environment and Climate Change Canada 2019). New World Sanderling can migrate up to 10,000 km annually between their wintering grounds in South and Central America and the Gulf of Mexico (GOM) and their breeding grounds in the Canadian Arctic (Macwhirter et al. 2020). We recently found that departure dates vary among Sanderling trapped in different staging sites in the GOM, which is linked to individual condition and oil spill contaminant levels (Bianchini and Morrissey 2018a). Here, we investigated whether these individuals from different GOM staging sites continue to show differences in arrival and subsequent departure timing as they migrate farther north.

Our objective was to compare the travel times and northbound migration timing of Sanderling trapped in three staging sites in the USA portion of the GOM: North Padre, Texas, Bolivar Flats, Texas, and Louisiana. Banding records suggest that most Sanderling staging in the GOM migrate north along the Central and Mississippi Flyways (hereafter the ‘Midcontinental Flyway’) through the Canadian Prairies (Myers et al. 1990), with up to 75,000 Sanderling staging at Chaplin and Reed Lakes in Saskatchewan, Canada (Howell et al. 2019). This represents approximately 58% of the total estimated numbers of Sanderlings using the Midcontinental Flyway or 12% of the global population (Howell et al. 2019). Chaplin and Reed Lakes are recognized as globally significant sites under the Important Bird Areas program (Schmutz 2000) and as sites of hemispheric importance in the Western Hemispheric Shorebird Reserve Network (Beyersbergen and Duncan 2007). In this study, travel time was defined as the total time of migration (i.e., a combination of any time spent in flight and staging) between the GOM and a more northern staging site, and migration timing was defined as the dates when individuals were migrating (i.e., arrival and departure dates). We tracked Sanderling as they travelled north using the Motus Wildlife Tracking System (Motus), an international array of automated radio-telemetry receivers (Taylor et al. 2017). In a previous study, we found that individuals trapped in North Padre, Texas, departed earlier from the GOM than individuals trapped in Bolivar Flats, Texas, and Louisiana (Bianchini and Morrissey 2018a). Therefore, here we examined whether Sanderling departing from North Padre would continue to exhibit differences in travel times to and departure timing from Saskatchewan relative to individuals departing from the other GOM sites.

Methods

Animal Capture and Handling

We trapped Sanderling during the day using cannon nets and noose carpets (capture dates are given in Table 1). Upon capture, we weighed, measured, and gave a unique numbered metal band and a coded alpha numeric flag to each individual. We attached Lotek coded nanotags (NTQB-3-2, Newmarket, Ontario, Canada) to Sanderling in North Padre (n = 59), Bolivar Flats (n = 31) and in Louisiana (n = 30). Tags weighed 0.67 g or < 1.5% of the estimated lean body mass of Sanderling (Scott et al. 2004) and were programmed with pulse lengths of 2.5 ms and pulse intervals of approximately 6 s (2015 and 2016) and 8 sec (2017), at a single frequency (166.380 MHz). Sanderling undergo extreme variations in body mass during migration (mass ranges from ∼ 40 to > 100 g during a single staging event; Robinson et al. 2003), which makes using harnesses a poor option for attaching nanotags to Sanderlings. Therefore, we affixed nanotags to each bird's upper back by parting the feathers and gluing tags to the bird's skin using a quick-setting marine epoxy (J-B Weld ClearWeld 5 Min Epoxy). We aged Sanderlings according to Pyle (1997), and we only attached nanotags to adult after second year (ASY) individuals, as second year (SY) juvenile shorebirds can exhibit alternative migration schedules, with juveniles tending to either migrate after adults on northward migration or to forego migration and stay on the wintering grounds (Newton 2011).

Figure 1.

Migration tracks of 35 radio-tagged Sanderling (Calidris alba) that were detected outside of the Gulf of Mexico (white circles indicate detection locations). For these birds, dashed lines connect the great circle distances between an individual's last detection in the Gulf of Mexico and its first detection at a more northern site (lines do not necessarily indicate flight paths). The map inset indicates the area in North America shown in the larger map. Smaller panels give a higher magnification of the Chaplin and Reed Lake, Saskatchewan (SK), North Padre (NP), Bolivar Flats (BF), and Louisiana (LA) staging areas, showing the receiver locations where Sanderling were detected (smaller white circles) and trapping sites (larger black circles) within each area.

Detection Data Collection and Processing

We tracked Sanderling migration movements using Motus (Taylor et al. 2017). Between 2015 and 2017, over 300 receiver stations were operational in North America. Nanotags were recorded by SRX receivers (Lotek Wireless, Newmarket, Ontario, Canada) or by Sensorgnome receivers (Compudata, London, Ontario, Canada), which can detect radio-tagged individuals within ∼20 or ∼50 km, respectively, of a receiver tower (Anderson et al. 2019). We removed false detections by filtering out detections with less than three consecutive bursts on a single antenna at a single receiver (Crewe et al. 2018). Our radio-tagged Sanderlings were detected at 26 receiver stations during their northward, spring migration (Fig. 1), giving us the time of detection and approximate location of individuals over the study period.

We tracked radio-tagged Sanderlings to Chaplin Lake (50° 26′ 28.36″ N, 106° 40′ 9.37″ W) and Reed Lake (42° 21′ 36.36″ N, 107° 6′ 7.1064″ W) in Saskatchewan by erecting six Motus receiver towers at this location. We positioned towers to give complete coverage of the staging area, so that arrival and departure dates could be determined with a high level of accuracy.

Table 1.

Summary of means (x̄ ) and ranges of Sanderling (Calidris alba) capture dates, departure dates from the Gulf of Mexico (GOM), travel times from the Gulf of Mexico to Saskatchewan, arrival and departure dates in Saskatchewan, and staging durations in Saskatchewana .

We calculated Sanderling departure dates from the GOM and arrival and departure dates in Chaplin and Reed Lakes, Saskatchewan, using Motus detection data. We classified departure dates as an individual's last detection at a site (Bianchini and Morrissey 2018a). For Sanderling detected in both the GOM and Saskatchewan (n = 20), we calculated travel time between these locations as the number of days between an individual's last detection in the GOM and an individual's first detection in Saskatchewan. For individuals that staged in Saskatchewan (n = 19), we calculated staging duration as the number of days between an individual's first and last detection in Chaplin and Reed Lakes (i.e., departure – arrival date). Here, we defined a staging event as a detection period lasting at least two days, and shorter durations were considered fly-bys (a two-day cut-off has been used for staging duration calculations in previous studies; e.g., Gómez et al. 2017). This eliminated nine birds that were detected for a median of 34 minutes (range = 1.42 min – 22 hours). It was not possible to reliably differentiate between migratory and regional movements with the current dataset; therefore, we assumed that the last and first detections at a site corresponded to the actual departure and arrival dates of actively migrating individuals. Our departure and arrival dates are consistent with previously measured departure and arrival timing of Sanderling in the GOM and in Saskatchewan (Myers et al. 1990; Macwhirter et al. 2020; Withers 2002). However, a limitation of the Motus system is that individuals can move out of the range of receiver stations, and it is possible that actual departures are later and actual arrivals are earlier than those reported here.

Results

Thirty-five of the 120 individuals tagged in the GOM were detected again by receivers in more northern latitudes. Of these, 94% (n = 33) were detected in Saskatchewan, and 6% (n = 2) were detected in the Atlantic Flyway. Migratory movements of Sanderling trapped in the GOM therefore indicated that most northward migration occurred along the Midcontinental Flyway (Fig. 1; online  Appendix: Fig. A1 (063.043.0301_Appendix.pdf) for the number of tags deployed and detected in each location).

We found differences in travel times and migration timing among Sanderling trapped in three GOM staging areas (Table 1). Motus detection data suggested that relative to Sanderling trapped in Bolivar Flats and Louisiana, Sanderling trapped in North Padre departed significantly earlier from the GOM (Bianchini and Morrissey 2018a; Fig. 2a) and travelled more slowly to Saskatchewan (β ± SE = 15.2 ± 5.8 days, P = 0.049; Fig. 2b; online  Appendix: Tables A1 (063.043.0301_Appendix.pdf) – A8 model selection results). North Padre migrants also arrived latest in Saskatchewan (β ± SE = 12.2 ± 3.4 days, P = 0.049; Fig. 3a). However, we found that North Padre Sanderling departed from Saskatchewan at the same time as individuals from the other staging areas (GOM staging area was not retained in the top model to explain variation in Saskatchewan departure dates; Fig. 3b) by shortening their staging durations (β ± SE = -0.66 ± 0.17 days, P = 0.002; Fig. 3c). Sanderling trapped in Bolivar Flats and Louisiana had similar patterns in the timing of migration, with no significant differences in departure dates from the GOM (β ± SE = -0.25 ± 2.42 days, P = 0.92; Fig. 2a), travel times to Saskatchewan (β ± SE = -4.2 ± 4.1 days, P = 0.36; Fig. 2b), Saskatchewan arrivals (β ± SE = 8.0 ± 3.6 days, P = 0.05; Fig. 3a), or Saskatchewan staging durations (β ± SE = -0.088 ± 0.188 days, P = 0.65; Fig. 3c).

Arrival dates in Saskatchewan were positively associated with GOM departure dates (β ± SE = 1.01 ± 0.02, P < 0.0001;) and travel times to Saskatchewan (β ± SE = 0.99 ± 0.02, P < 0.0001; online  Appendix: Fig. A2 (063.043.0301_Appendix.pdf)), such that individuals departing later from the GOM and traveling more slowly arrived later at the more northern site. Further examination of migration timing revealed no association between GOM departure date and travel time to Saskatchewan, such that Sanderling departing later from the GOM did not travel faster to the northern site. We saw no association between Saskatchewan arrival and departure dates. Instead, there was a negative association between Saskatchewan arrival dates and staging durations, such that later-arriving individuals spent less time staging in Saskatchewan (β ± SE = -0.046 ± 0.011, P = 0.0005; Fig. 4). Year explained some of the variance in GOM departure dates and travel time to Saskatchewan. On average, Sanderling tended to depart later and travel more slowly in 2015 relative to 2016 (departure: β ± SE = -12.4 ± 2.6, P < 0.0001; travel time: β ± SE = 20.7 ± 4.4, P = 0.005) and 2017 (departure: β ± SE = -17.0 ± 3.5, P < 0.0001; travel time: β ± SE = 19.1 ± 6.2, P = 0.03). There was no difference in Saskatchewan arrival dates, staging durations, or departure dates among years.

Figure 2.

Final detection dates (a) and calculated travel times to Saskatchewan (b) of Sanderling (Calidris alba) trapped in three Gulf of Mexico staging areas (NP = North Padre, BF = Bolivar Flats, LA = Louisiana). (a) Shaded polygons (from raw data) represent the probability density of departure (area under the curves = 1), and vertical lines indicate the model-averaged mean departure date from each area (NP: n = 18, BF: n = 17, LA: n = 13). (b) Boxes show model-averaged means ± SE, with vertical lines indicating the 95% confidence intervals. Circles indicate raw travel time values (NP: n = 10, BF: n = 4, LA: n = 6). The dotted horizontal line shows the model-averaged mean travel time of all individuals (n = 20). Note that the difference in departure dates among staging areas was revealed in our previous paper (Bianchini and Morrissey 2018a).

Figure 3.

(a) Sanderling (Calidris alba) trapped in North Padre (NP, n = 16) arrived later in Saskatchewan (SK) relative to birds trapped in Louisiana (LA, n = 6) and Bolivar Flats (BF, n = 6). (b) There was no difference in SK departure dates among individuals from the three Gulf of Mexico (GOM) staging areas (NP: n = 9, BF: n = 6, LA: n = 4), as (c) NP Sanderling (n = 9) had shorter staging durations in SK relative to birds from BF (n = 6) and LA (n = 4). In panels a and b, shaded polygons (from raw data) represent the probability density of SK arrival and departure, respectively (area under the curves = 1). In panel a, vertical lines indicate model-averaged mean arrival dates, and in panel b, vertical lines indicate median departure dates calculated (from raw data). In panel c, boxes show model-averaged means ± SE, with vertical lines indicating the 95% confidence interval. Circles show raw staging duration values, and the dotted horizontal line shows the model-averaged mean staging duration of all individuals.

Figure 4.

There was a negative association between Saskatchewan (SK) staging durations and arrival dates (n = 19). The line indicates a model-based estimate of the weighted model-averaged mean ± a 95% confidence interval. Each circle represents an individual.

We note that there were noticeable differences between the raw (Table 1) and model-predicted means (Figs. 2 and 3). This difference likely results from the relatively small number of detected individuals in our study. Nevertheless, our model-predicted means, which are adjusted for any other fixed effects in the best-approximating model and for the random effect of capture date, give our best prediction of the population means for each response variable given the available data.

Discussion

Recent advances in automated radio-telemetry systems allowed us to track individual long-distance migrants between staging sites separated by thousands of kilometers. Here, we followed Sanderling along the Midcontinental Flyway from three staging and/ or wintering areas in the GOM to a major staging area in Saskatchewan, which allowed us to capture detailed information about Sanderling migration timing across > 3000 km.

Despite greater Motus coverage along the Atlantic Flyway relative to the Midcontinental Flyway (Taylor et al. 2017), 33 of the 35 Sanderling detected in more northern sites (94%) were detected in Saskatchewan, and only 2 individuals were detected along the Atlantic Flyway, suggesting a strong preference for northward migration along the midcontinental route. Given the relatively high Motus coverage along the Atlantic Flyway (Tayler et al. 2017), it is unlikely for tagged individuals to use this route and not be detected. Conversely, there are large gaps in Motus coverage in the midcontinent (Taylor et al. 2017), and it is therefore more likely for tagged individuals to use this flyway undetected. Band resight data corroborate the importance of the Midcontinental Flyway for Sanderling and suggest that most individuals overwintering in Chile and Peru and staging and wintering in the Gulf Coast migrate north through the Canadian Prairies (Myers et al. 1990).

The small number of sites along the Midcontinental Flyway that are outfitted with Motus radio-telemetry receivers likely explains the low recovery rates of this study. Here, 29% of individuals tagged in the GOM were subsequently detected in more northern latitudes, which was lower than the recovery rates of previous studies using the Motus telemetry network (e.g., 32% in Gómez et al. 2017, 81% in Anderson et al. 2019). Sanderling have been sighted at stopover sites south of Saskatchewan in the U.S. Prairies (Myers et al. 1990), and shorebirds can also use dynamic wetland habitats opportunistically and can disperse broadly as they migrate across the midcontinent (Skagen and Knopf 1994). It is therefore possible that undetected individuals either lost their tags or used other staging sites along the Midcontinental Flyway that did not have Motus receivers. Regardless, the Motus network and rapid advancements in tracking technology have provided a tool that offers new insight into the midcontinental staging sites and migration routes of small shorebirds.

We showed that some of the variation in Sanderling migration timing along the Midcontinental Flyway was related to where individuals staged in the GOM. Overall, Sanderling trapped in North Padre appeared to use a different migration strategy than individuals trapped in Louisiana and Bolivar Flats. Our most notable result was that late-arriving individuals had shorter staging durations in Saskatchewan. Given the complete Motus receiver coverage of Chaplin and Reed Lakes, detection data from Saskatchewan reliably reflected actual arrival and departure dates. We were therefore able to evaluate actual staging durations in Saskatchewan, whereas most detection data only permit an evaluation of minimum staging duration (i.e., days between capture date and departure; e.g., Gómez et al. 2017; Wright et al. 2018). We found that Sanderling trapped in North Padre arrived in Saskatchewan later but had the shortest overall staging durations and ultimately departed from Saskatchewan at the same time as individuals trapped in Bolivar Flats and Louisiana. There are two possible explanations for these results, each of which would imply that the Sanderling trapped in North Padre use a different migration strategy.

First, it is possible that late-arriving individuals were following a time-minimization migration strategy. According to this explanation, individuals trapped in North Padre arrived later in Saskatchewan, but forwent longer stopovers, and likely refueled more quickly, in an effort to reach breeding areas earlier (Alerstam and Lindström 1990; Hendenström and Alerstam 1997). It is generally assumed that migratory birds are under pressure to arrive early on the breeding grounds (Alerstam 2006; Newton 2008), as early arrival is associated with higher quality territories and mates, earlier breeding, and additional time for renesting, which ultimately result in greater reproductive performance for earlier arriving individuals (Norris et al. 2004; Smith and Moore 2005).

Our first, time-minimization explanation assumes that Sanderling made a direct flight between the GOM and Saskatchewan. For all individuals, we saw large variations in travel times between the GOM and Saskatchewan. Some Sanderling made a direct flight to Saskatchewan in only 3.2 days, with the fastest individuals traveling at approximately 35 km/hr, a result comparable to the 40 km/hr ground speeds of migrating Sanderling measured by Scott et al. (2004). According to our travel time calculations, other individuals took up to 34 days, which would imply that some Sanderling made indirect flights, stopping over at unknown locations on route to Saskatchewan. Banding records (Myers et al. 1990) and survey data (Skagen et al. 1999), however, show that Sanderlings tend to use a “jumping” migration strategy along the Midcontinental Flyway, overflying most of the North American interior. Indeed, Sanderlings have been recorded in large numbers in Saskatchewan, but not elsewhere in the midcontinent (Skagen et al. 1999; Andrei et al. 2006). Banding records and survey data therefore suggest that Sanderling typically make a direct flight from the GOM to Saskatchewan (Myers et al. 1990; Skagen et al. 1999), which supports the possibility of a time-minimization migration strategy.

According to our first explanation, late arrivals in Saskatchewan would suggest that Sanderling departed later from North Padre than from the other two staging areas. This inference, however, contradicts the results from our previous study, which found that North Padre Sanderlings departed earliest from the GOM (Bianchini and Morrissey 2018a). It is possible that the earlier North Padre departure dates (and, consequently, the longer calculated travel times to Saskatchewan) reflected individuals in the North Padre staging area moving outside of the detection range of Motus receivers prior to departure. Unlike Bolivar Flats and Louisiana, which are more spatially discrete sites, the North Padre staging area is large and multidimensional (Withers 2002). In North Padre, shorebirds can easily access the nearby Laguna Madre, an extensive system of large tidal flats that is one of the most significant shorebird staging, wintering, and breeding habitats along the Gulf Coast (Withers 2002). Four Motus receivers are located along Padre Island and provide some coverage of the Laguna Madre, but most of this habitat falls outside of the range of Motus receivers (Taylor et al. 2017). Therefore, although an individual's last detection in North Padre was assumed to reflect its actual departure from the GOM, last detections could also reflect individuals moving outside of the range of Motus receivers prior to departure. However, we cannot reliably differentiate between actual departures and regional movements with the current dataset.

The second possible explanation for our results is that individuals trapped in North Padre departed earlier from the GOM and travelled more slowly to Saskatchewan, as was suggested by our Motus detection data. This explanation suggests that Sanderling trapped in North Padre tended to make less direct flights to Saskatchewan. Although shorebird survey and observational data from the mid-1990s suggested that Sanderling overfly much of the North American interior (Skagen et al. 1999), more recent eBird data report Sanderling sightings throughout the Midcontinental Flyway (eBird 2017). The Prairie wetlands of the North American interior are highly dynamic, and shorebirds tend to occupy available wetland habitats opportunistically during migration across the midcontinent (Skagen and Knopf 1994). It is therefore possible that the Sanderling trapped in North Padre staged and refuelled at unknown locations (without Motus receivers) on route to Saskatchewan, ultimately arriving later at this staging area. Individuals following an energy-minimization migration strategy are expected to stop more regularly, to make shorter flights, and to deposit less fuel than time-minimizers (Alerstam and Lindström 1990; Hedenström and Alerstam 1997). Therefore, if individuals from North Padre were following an energy-minimization migration strategy, they may have had shorter staging durations in Saskatchewan because they required less time to deposit smaller departure fuel loads.

Determining which of these two explanations is correct will require more data. For instance, Sanderling departure masses could provide evidence to support either explanation, as departure fuel loads can reflect individual refueling rates and can determine an individual's subsequent flight distance (Alerstam 2001). Individuals with faster fuel depositions rates tend to have larger departure fuel loads (Moore et al. 2017), and individuals with larger departure fuel loads tend to make longer and more direct flights to key staging areas (Gómez et al. 2017). Determining the food resources and predation risk at each location could also help to differentiate between these two explanations, as both factors can determine habitat selection during refuelling (Chudzińska et al. 2015). Notably, in our study, food availability or predation risk in the Laguna Madre relative to the North Padre shoreline may have influenced the likelihood of Sanderling moving into the Laguna Madre and out of range of the Motus receivers prior to departure from the North Padre staging area. Additionally, differences in food availability among years could also explain the interannual variations in GOM departure dates observed here, where higher food availability may have caused earlier departure dates in some years (Bridge et al. 2010).

Additionally, knowing the breeding and wintering locations of each individual could also provide evidence to support one of these explanations. A growing body of evidence suggests that breeding latitude is a key driver of northbound migration timing. Previous tracking studies in Collared Flycatchers (Ficedula albicollis; Briedis et al. 2016), Bar-tailed Godwits (Conklin et al. 2010), and Tree Swallows (Tachycineta bicolor; Gow et al. 2019) have found that individuals breeding in higher latitudes depart later from the wintering grounds, have longer staging durations, and arrive later on the breeding grounds. However, the breeding latitudes of Sanderling in this study are unknown. Non-breeding latitudes, in combination with sex or age, have also been shown to affect northbound migration timing in birds, with males wintering farther north than females and juveniles and arriving earlier on the breeding grounds (O'Hara et al. 2005; Woodworth et al. 2016). Based on banding and feather isotope records, we have information that suggests that a proportion of the Sanderling in Chaplin Lake, Saskatchewan overwinter in the Texas Gulf Coast (Labarrere 2016), but Bolivar Flats and Louisiana also provide overwintering habitat for Sanderling (Withers 2002). Nevertheless, individuals trapped in North Padre generally had earlier capture dates than Sanderling trapped in the other two staging areas. Of the Sanderling trapped in North Padre that were subsequently detected, 20/26 (77%) individuals were captured on or before day of the year 111 (April 20/21), whereas all of the Sanderling trapped in Bolivar Flats and Louisiana were captured after day of the year 114 (April 23/24; capture date range: 114 – 126). Although capture date was added as a random effect to control for seasonal variations, it is possible that Sanderling trapped in North Padre represented an earlier cohort of birds.

It is important to acknowledge that the population structure at each site in the GOM is unknown, and individuals of different ages and sexes, for example, may have inherently different migration schedules (Morrison 1984). Equally, the quality of staging sites as shorebird refueling habitat is not fully characterized in the GOM. The GOM hosts over one million wintering and migrating shorebirds every year (Cohen et al. 2017). Due to the oil extraction and transportation activities in this region, shorelines along the Gulf coast are exposed to frequent and repeated oil pollution from numerous oil leaks (Burger 2017) and from large-scale spills, like the Deepwater Horizon oil spill (Beyer et al. 2016). Oil pollutants have the potential to impair pre-migratory fueling (Bianchini and Morrissey 2018b) and we previously determined that this can delay departure (Bianchini and Morrissey 2018a). We also acknowledge that our sample size of detected individuals is low, and additional research (i.e., data from more individuals and years) will help inform the differences in migration timing among Sanderling from these three GOM staging areas.

Further research is therefore required to fully understand why differences in migration timing among Sanderling trapped in different GOM staging areas occur and to identify the factors driving the observed variations in migration pace. Our study underscores the need for more extensive Motus coverage in key shorebird habitats, like the Laguna Madre and North Padre areas in Texas. Our study also highlights the great potential of the Motus network to track shorebird migration, particularly along the relatively understudied Midcontinental Flyway that has revealed new insights into differential migration schedules and linkages across distant staging areas.