INTRODUCTION

Understanding interactions among events throughout the annual cycle is critical for the conservation of migratory birds. Birds may face different threats and pressures across breeding, migration, and non-breeding periods, and these threats can differ both among populations and among individuals within the same population (Rushing et al. 2016, Briedis and Bower 2018, Wilson et al. 2018). For example, in species displaying strong migratory connectivity, individuals from different breeding populations may migrate and spend the non-breeding period together in spatially separated non-breeding areas and thus experience different threats across populations (Huysman et al. 2022, McDuffie et al. 2022). On the other hand, under weak migratory connectivity, individuals from different breeding populations tend to mix during non-breeding periods (Skinner et al. 2022). A related concept is differential migration, whereby individuals from the same breeding population segregate by habitat or geographic area on the non-breeding grounds, often driven by individual differences such as sex, body condition, or body size (Catry et al. 2004, Briedis and Bower 2018). The above patterns can have important fitness consequences as exposure to different habitats of varying quality can carry over to influence individual performance in subsequent phases of the annual cycle (Rushing et al. 2016).

Shorebirds have experienced some of the steepest population declines of all North American avian taxa, but causes of declines are not well understood (NABCI 2019, Smith et al. 2023a). Although some of the most severe declines are reported for long distance Arctic-breeding migrants (NABCI 2019), some short distance migrants are also showing worrisome trends. The Marbled Godwit (Limosa fedoa) is both a short-distance migratory shorebird and a grassland-breeding bird. Grassland birds are one of the only groups with population declines exceeding those of shorebirds, primarily due to extensive loss of native grassland habitat (Rosenberg et al. 2019). Based on the North American Breeding Bird Survey, the Marbled Godwit has declined by -0.62%/year (95% CI: -1.1, -0.11) since 1970 and continues to show an even steeper short-term decline of -2.7%/year (-4.26, -1.1) over the past 10 years, with greater declines in Canada compared to the United States (Smith et al. 2023b). Within Canada, the cumulative decline over the past three generations (-44%) suggests this species would meet criteria for listing as Threatened under the Species at Risk Act (IUCN Standards and Petitions Committee 2024). In order to identify causal mechanisms and develop effective conservation strategies, especially given regional differences in population trends, we must identify when in the annual cycle population limitation occurs (Wilson et al. 2018). Although traditionally much research has focused on breeding season events, threats to shorebirds may occur outside of the breeding season, in particular during migration (Piersma et al. 2016, Brlík et al. 2022).

Several tracking studies have begun to reveal aspects of migratory connectivity among Marbled Godwit populations. Three disjunct breeding populations of Marbled Godwits exist, with the greatest geographic spread and population abundance occurring in the northern prairies of the United States and southern Canada, and two smaller populations in Alaska and James Bay (Gratto-Trevor 2020). Non-breeding populations occur on the Pacific Coast of the United States and Mexico, with greatest concentrations from central California to both coasts of Baja California and mainland Mexico, and on the Atlantic Coast from North Carolina south and in the Gulf of Mexico. Smaller numbers also occur on the Pacific Coast of Central America and into South America (Gratto-Trevor 2020). Available data on migration routes and non-breeding sites of the different breeding populations include a mark-resight study of godwits banded in southern Alberta, which found that resighted individuals occurred during the stationary non-breeding period in coastal Baja California (three females and two males) and Bodega Bay, California (one female), and during migration at coastal sites in northern (six females) and central California (two females and three males; Gratto-Trevor 2011). Olson et al. (2014) tracked 23 individuals with satellite transmitters at four staging and non-breeding locations in North America. Individuals tracked to breeding sites in the prairies spent the non-breeding season in Mexico. Contrary to what was previously thought, James Bay breeding birds crossed the continent and also spent the non-breeding period in Mexico (rather than traveling to the Atlantic coast), and godwits tagged during the non-breeding period in coastal Georgia “criss-crossed” that route and bred in the Dakotas (Olson et al. 2014). Ruthrauff et al. (2019) tracked nine godwits from the Alaska breeding population and documented that godwits from this population wintered at sites along the Pacific Coast from southern Washington to central California.

In this study, we tracked male and female godwits over multiple years from a prairie breeding population in southern Alberta, Canada, to examine potential differences in migration routes and non-breeding locations among individuals and protected area status of sites used across the annual cycle. Based on previous research from a banding study in the same area (Gratto-Trevor 2011) and the other tracking studies described above, we expected both sexes to winter in Mexico but to possibly separate during migration, given that banding data suggested only females stopped over in northern California.

METHODS

Our study area was located in southern Alberta, Canada, 100 km from the city of Brooks and approximately 160 km east of Calgary. Trapping occurred at two wetland complexes, Kitsim (50.488°N, 112.043°W) and Kininvie (50.323°N, 111.423°W), which are managed for grazing and waterfowl production by Ducks Unlimited Canada and described in detail in Gratto-Trevor (2006). We searched for nests in late May and early June via cable dragging with a 30 m cable or chain dragged slowly between a pair of all-terrain vehicles. Adults were captured on the nest during early incubation by lowering a mist net over the nest. We banded each bird with a USGS metal band and a uniquely coded leg flag. We took standard morphometrics (mass, bill, and wing length) and determined adult sex via plumage, bill color, and exposed culmen length (Ayala-Pérez et al. 2013, Gratto-Trevor 2020, Sandercock and Gratto-Trevor 2023). This was verified for a subset of birds through behavioral observations such as copulations and aerial displays. There was little overlap in culmen length in this breeding population (Gratto-Trevor 2011), and the eight satellite marked birds in this study were four mated pairs.

In 2013 and 2014, we deployed a total of eight solar-powered satellite transmitters (9.5g solar PTT, North Star Science and Technology, Virginia, United States) on four breeding pairs of Marbled Godwits. PTTs were attached via a modified leg-loop harness and Teflon ribbon following the methods of Ruthrauff et al. (2019) and represented 3.5–4.5% of individuals’ body mass at time of capture. We tagged two females and two males in 2013 and one female and one male in 2014 with PTTs programmed to operate on a duty cycle of 8hr of transmission and 38hr off to charge batteries, and one female and one male in 2014 with PTTs programmed to transmit whenever sufficiently charged (Table 1). The PTTs (when on) transmitted signals every 60s to the Argos satellite system which, when in contact with a PTT, used Doppler geolocation to determine locations and assign accuracy classes to locations (CLS 2016). Activity sensors onboard the transmitters indicated when movement ceased (suggesting either a shed transmitter or a dead bird), and we used that information to truncate tracks of three birds to their last moving or “live” location. In 2014, one tagged male’s PTT (128033) stopped transmitting within five days of deployment and thus contributed no data to movement summaries.

We filtered the Argos locations by applying the hybrid filter option of the Douglas-Argos Filter (Douglas et al. 2012) set to retain all standard class locations (i.e., classes 3, 2, and 1) and to remove any implausible auxiliary locations (i.e., classes 0, A, B, and Z) based on filtering parameters of 130 km/hr for maximum sustainable rate of movement and 10 km for maximum redundant distance (https://www.movebank.org/cms/webapp?gwt_fragment=page=studies,path=study1097938025). For each bird, we estimated the timing of departure and arrival to breeding, stopover, and stationary non-breeding sites by calculating duration of the outgoing (departure) or incoming (arrival) vector based on length of those vectors and a ground speed of 49.6 km/hr (the average speed during 28 bouts of obvious flight by the tracked birds). Duration of vectors were subtracted (departure) or added (arrival) to the dates/times of the first stationary location at a site. Long “off times” precluded detecting location of some stopovers that were < 24 hr long, however we did note when slow travel times over long distances suggested a bird had made a stop enroute. Off times tended to become longer as tags aged, and thus there was less information about length of stay at sites in later years for the individuals tracked > 2 years. After filtering, we had a total of 8389 locations including 20% standard and 80% auxiliary classes, with 87–2115 locations per individual.

Protected area status of each site was determined based on the World Database of Protected Areas (WDPA; UNEP-WCMC and IUCN 2024) and a visual assessment of whether ≥ 75% of points occurred within 5 km of the protected area boundaries. The 5 km buffer was applied to include back and forth movements of birds where stopover locations were centered within site boundaries. Back and forth movements were often associated with birds in flight so most stationary locations were within site boundaries. Protected areas in the database follow either the International Union for Conservation of Nature or the Convention on Biological Diversity’s definition of a protected area, which is generally defined as a geographically distinct space that is recognized and managed, through legal or other effective means, to achieve long-term conservation goals. Specifically, the sites used in our study fell into a small number of categories: WDPA Categories 1a (Strict Nature Reserve), IV (Habitat or species management area), and V (Protected landscape or seascape) (UNEP-WCMC and IUCN 2024).

Southbound and northbound migration began when birds embarked on directional movements away from breeding and stationary non-breeding areas, respectively. We deemed stopovers to have occurred when clusters of ≥ 3 points indicated rate of movement was < 10 km/hr and used the centroids of point clusters as the generalized location of a stop.

RESULTS

Individuals were tracked over a mean of 2.2 annual cycles (range 0.6–5.6; Table 1), with females on average being tracked for longer (mean 2.9 annual cycles) than males (1.1). One male was only tracked to its first stationary non-breeding location, whereas the female tracked for the longest was tracked up to its sixth non-breeding season, providing data from 2014 to 2019.

Marbled Godwits showed complete separation by sex upon leaving the breeding grounds and throughout migration (Fig. 1). There was no overlap in use of any stopover or stationary non-breeding locations between the sexes, although within each sex there were shared locations (Table 2). In general, females followed a direct south-westerly route to non-breeding locations in coastal California, whereas males took a more southbound route to overwinter in Baja California Sur. These different patterns also occurred during return trips to the breeding grounds (Appendices 1–4). On average, females made fewer detected stopovers during southbound migration (mean 0.4, range 0–1, for four individuals over 12 migrations) compared to males (mean 1.8, range 1–4, for three individuals over five migrations), but the same number during northbound migration (females: mean 1, range 0–3, for four individuals over 10 migrations; males: mean 1, range 1, for two individuals over three migrations). One female (ID 128030, Appendix 2) tracked over three southbound and two northbound migrations did not make any stops and the females that did stop used different sites across years (Table 1). In contrast, all three males stopped at Willard Bay in Great Salt Lake, Utah, during southbound migration, and one individual (ID 128032) tracked over three southbound and two northbound migrations used it as a stopover during each migration (Appendices 1, 2, 4). Mean length of stay at detected stopover locations was 4.9 days (2.9–6.9) for females and 11.2 days (0.5–31.4) for males during southbound migration, and was 15.9 days (1.2–34.2) for females and 4.2 days (0.3–8.7) for males during northbound migration, although values for males, especially during northbound migration, should again be interpreted with caution because of very small sample sizes. Mean distance travelled during southbound migration (i.e., great circle distance from breeding, to any stopover, to stationary non-breeding based on latitude and longitude data) was 1577 km (1444–1652) for females and 2564 km (2563–2619) for males, and during northbound migration was 1563 km (1456–1724) for females and 2595 km (2554–2636) for males (Fig. 2).

During the stationary non-breeding period, all females occurred in coastal California and all males in Baja California Sur. Specifically, females occurred at three locations: Humboldt Bay, California, used across six seasons by one individual; East Central San Francisco Bay, California, used across two seasons by one individual; and South San Francisco Bay, California, used across four seasons by two individuals (Table 2). All males overwintered at Laguna Ojo de Liebre, Baja California Sur, Mexico (used across five seasons by three individuals), and one of the males traveled twice to La Bocana, Baja California Sur, for the second part of two of its three stationary non-breeding seasons.

Marbled Godwits overwhelmingly made use of protected areas during migration and the stationary non-breeding season. All stationary non-breeding sites were found within protected areas, with the exception of East Central San Francisco Bay, although the female’s use of the site overlapped partially with the Eden Landing Ecological Reserve (Table 2). Five of 14 (36%) stopover sites and 19 of 27 (70%) individual stopovers occurred within protected areas (Table 2).

DISCUSSION

We tracked four mated pairs of Marbled Godwits breeding in southern Alberta, Canada, and uncovered a strong pattern of differential migration based on sex, with complete separation of females and males by about 1300 km during the stationary non-breeding period. Of males with at least one available stationary non-breeding location (n = 3), all traveled to Baja California Sur, Mexico, passing through their main stopover site at Great Salt Lake, Utah, United States. All females (n = 4) were tracked for at least one annual cycle, and all but one over multiple years; females spent the stationary non-breeding period along the coast of California, United States (three in San Francisco Bay and one in Humboldt Bay), with more variable use of stopover sites among individuals. Individuals made heavy use of protected areas during the non-breeding period, including National Wildlife Refuges and Biosphere Reserves.

Differential migration based on sex has been reported in several bird species and is generally explained by three main hypotheses (Ketterson and Nolan 1983). The body size hypothesis predicts that the larger-bodied sex will spend the non-breeding period closer to the breeding grounds because of greater cold tolerance. This hypothesis has since been expanded to include other forms of niche specialization, such as differential access to food resources based on sexual size dimorphism (Catry et al. 2012, Duijns et al. 2014). The dominance hypothesis predicts that the larger, more dominant sex is able to exclude the subordinate sex from preferred non-breeding locations. Finally, the arrival-time hypothesis assumes that early arrival to the breeding grounds is more advantageous for one sex, which will spend the non-breeding period closer to the breeding grounds (Cristol et al. 1999). The rate of sexual segregation in shorebirds during the non-breeding period is not well studied; however, it has been reported in a few species that exhibit long-distance migrations (Myers 1981, Gill et al. 1995, Shepherd et al. 2001). For example, Western Sandpipers (Calidris mauri) show a strong latitudinal cline in sex ratio, with females, which are slightly larger, more often occupying sites further south (Nebel 2003). The pattern was hypothesized to be caused by resource partitioning, with longer-billed females being able to access deeper prey at warmer southern latitudes, or possibly due to differential susceptibility to predation, because females have higher wing loading (lower escape ability) and benefit from being further south where less fat needs to be carried (Nebel 2003, 2005, Mathot et al. 2007).

Marbled Godwits are characterized as short-distance migrants and are most similar in their life histories to the sympatric Long-billed Curlew (Numenius americanus) and Willet (Tringa semipalmata), which are also grassland-breeding shorebirds with bi-parental care and short to moderate-distance migrations, and in which the females tend to be the larger sex (Dugger and Dugger 2020, Lowther et al. 2020). Both Long-billed Curlews and Willets display a general pattern of strong migratory connectivity, and neither show evidence of spatial segregation of the sexes (Page et al. 2014, Huysman et al. 2022). It is not clear why the godwits we tracked showed such strong spatial segregation, and in the opposite pattern from what has been described in most other shorebirds (i.e., larger-bodied females further south; Myers 1981). One possibility is the larger size of female godwits allows them to winter further north, thus expending less energy on migration (i.e., body size hypothesis). It is also possible that differences in bill length between females and males allow the sexes to better take advantage of different food resources in California and Mexico (i.e., niche segregation hypothesis), but we do not have detailed data on their feeding ecology or prey availability at these sites. We have no data in support of the dominance hypothesis, although this hypothesis is difficult to show (Cristol et al. 1999). Our findings are in contrast to the arrival-time hypothesis, because we have evidence of male godwits arriving earlier than female godwits to our study site (e.g., 69% of males but only 29% of females arrived before the end of April, 2000; authors, unpublished data). Hedh and Hedenström (2020) observed a pattern similar to ours in the Common Ringed Plover (Charadrius hiaticula), with males wintering further south than females, but in this species the males are larger, and the authors concluded their findings could be explained by temporal differences in energy requirements between the sexes.

Sex ratios of Marbled Godwits vary across their non-breeding distribution, and our tracking results for prairie-breeding godwits show some consistencies, but also some major differences, with previous tracking and surveying research. In a large sample of godwits from the main non-breeding site used by our tracked males (Laguna Ojo de Liebre-Guerrero Negro wetland complex), Ayala-Pérez et al. (2013) observed a 2:1 ratio of males to females, consistent with our results. Similarly, from eight individuals tracked from the Alaska breeding population, six females wintered further north than two males, and all were distributed from the coast of California and north to Washington (Ruthrauff et al. 2019). Interestingly, this indicates that males from Alaska overlapped in their non-breeding areas with females from our study. On the other hand, other satellite tracking and band-resighting studies did not find strong evidence of segregation by sex during the non-breeding period (Gratto-Trevor 2011, Olson et al. 2014). Both sexes were represented when captured and tagged with satellite transmitters at Great Salt Lake, and individuals spent the non-breeding period at various sites in Mexico (Olson et al. 2014). However, these represented individuals from spatially separate areas of the prairie breeding population, whereas ours was the first study to track mated pairs from a single site. It is not clear why the propensity for non-breeding sexual segregation would vary across the breeding range, or why, within a breeding population, it would be beneficial for members of a pair to segregate. Given our limited sample size, it is also likely the patterns we uncovered may not be completely representative of the breeding birds in our study area.

Our results have implications for the threats and pressures experienced by female and male godwits across their annual cycle. For example, different conditions faced by the sexes at stopover or stationary non-breeding sites could result in carry-over effects, or even direct mortality events, that influence females and males differently, thus influencing individual condition or sex ratio on the breeding grounds (Catry et al. 2006, de Zwaan et al. 2019). Indeed, male godwits at our study site had slightly higher apparent survival (0.944 ± 0.018) than females (0.924 ± 0.021), although this could be due to greater breeding site fidelity in males (Sandercock and Gratto-Trevor 2023). Interestingly, the godwits tracked in our study predominantly used protected areas throughout the migration and non-breeding periods, with 80% of stationary non-breeding areas and 70% of individual stops occurring in protected areas. This implies that these birds concentrate at areas without direct human impact outside of the breeding season, in contrast to the human-modified agricultural landscapes that make up the majority of their breeding grounds. Ultimately, the drivers of Marbled Godwit population declines and when they occur in the annual cycle remain uncertain, and future research should aim to understand the consequences of differential migration, especially as they may relate to survival.

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