# Challenges in population monitoring: Bar-tailed Godwits (Limosa lapponica) on the East Atlantic Flyway defy assumed population structure

Jesse R Conklin, Roeland A Bom, Åke Lindström, Pierrick Bocher, Theunis Piersma

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Jesse R Conklin, Roeland A Bom, Åke Lindström, Pierrick Bocher, Theunis Piersma. Challenges in population monitoring: Bar-tailed Godwits (Limosa lapponica) on the East Atlantic Flyway defy assumed population structure. Ibis, 2025, 10.1111/ibi.13458. hal-05323127

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# Challenges in population monitoring: Bar-tailed Godwits (*Limosa lapponica*) on the East Atlantic Flyway defy assumed population structure

JESSE R. CONKLIN,<sup>*1,2</sup> ID ROELAND A. BOM,<sup>2,3</sup> ID
AKE LINDSTRÖM,<sup>4</sup> PIERRICK BOCHER<sup>5</sup> &
THEUNIS PIERSMA<sup>1,2,3</sup>

Estimating and monitoring migratory bird populations involves significant challenges, even in the most well-studied and easily-counted species, and can be further complicated by long-held but unverified assumptions and by shifting distributions in changing flyway conditions. We illustrate these challenges using Bar-tailed Godwits *Limosa lapponica* on the East Atlantic Flyway. The subspecies *L. l. lapponica* and *taymyrensis* are considered full leap-frog migrants along this flyway, and all population estimates and status assessments since the 1970s have been based on this premise. However, recent population genetics work has revealed that Bar-tailed Godwits wintering in Europe, previously thought to be 100% *lapponica*, contain significant

numbers—possibly a majority—of *taymyrensis*. In addition, satellite-tracking of godwits is accumulating evidence for adult *taymyrensis* individuals that do not migrate to West Africa as expected. Consequently, the actual population size of *lapponica* is probably only a fraction of the currently accepted estimate of 150 000–180 000 individuals, giving greater credibility to breeding-range estimates (consistently < 30 000 individuals) that historically had been discounted in favour of winter census data. Accordingly, the population size of *taymyrensis* has probably been greatly underestimated, and the trends in both subspecies should be revisited, as the perceived increasing size of *lapponica* and decline of *taymyrensis* may be further confounded by an actual northward redistribution from African to European non-breeding grounds. To more accurately evaluate the population sizes and geographical distribution of the two subspecies on the East Atlantic Flyway, we recommend continued and expanded tracking efforts combined with a large-scale genetic assignment of non-breeding individuals. This case study demonstrates that accurately monitoring of migratory populations requires comparative and multi-disciplinary approaches, and that underlying assumptions should be periodically revisited, particularly in light of rapid environmental change.

**Keywords:** differential migration, individual tracking, migratory flyway, population genetics, population monitoring, status assessment, waders.

Population status assessments, and consequent conservation efforts, require reliable estimates of the size and trajectory of populations. For migratory bird populations, a key challenge is identifying the season and method best suited to population estimation. Depending on the ecology and geography of the species, the most accurate results might come from, for example, extrapolating density from breeding bird surveys across a well-defined breeding range, direct counts at easily observable areas of non-breeding concentrations, or indirect estimation from mark-recapture methods (Bart 2005, McClintock & White 2012). Of course, each method comes with its own caveats, weaknesses and assumptions, and the most robust inferences may involve combining and comparing results from multiple approaches (e.g. Emmet et al. 2023, Piironen et al. 2023) or seasons (e.g. Schuckard et al. 2020, Ruthrauff et al. 2021), when possible.

For many arctic-breeding wader species, owing to the difficulty of monitoring low-density breeding populations across largely inaccessible and often poorly defined breeding ranges (but see, e.g., Lindström et al. 2019), population estimates typically derive from non-breeding censuses, in which large proportions of the population

<footnote n="1"><sup>1</sup>Conservation Ecology Group, Groningen Institute for Evolutionary Life Sciences (GELIFES), University of Groningen, PO Box 11103, Groningen, 9700 CC, the Netherlands</footnote>

<footnote n="2"><sup>2</sup>BirdEyes, Centre for Global Ecological Change at the Faculties of Science & Engineering and Campus Fryslân, University of Groningen, Zaaland 110, Leeuwarden, 8911 BN, the Netherlands</footnote>

<footnote n="3"><sup>3</sup>Department of Coastal Systems, NIOZ Royal Netherlands Institute for Sea Research, PO Box 59, Den Burg, Texel, 1790 AB, the Netherlands</footnote>

<footnote n="4"><sup>4</sup>Division of Biodiversity and Evolution, Department of Biology, Lund University, Ecology Building, Lund, SE-223 62, Sweden</footnote>

<footnote n="5"><sup>5</sup>Littoral Environnement et Sociétés Laboratory (LIENSs), CNRS, La Rochelle University, La Rochelle, France</footnote>

<footnote>*Corresponding author.
Email: conklin.jesse@gmail.com</footnote>
congregate in flocks that can be directly counted (Hansen *et al.* 2022, van Roomen *et al.* 2025). Reliance on population monitoring by the latter necessarily assumes that: (1) most or all non-breeding areas are known and accessible, and (2) the migratory connections with breeding areas (i.e. subspecies assignment) of these non-breeding congregations are known. Long-term mark-recapture and mark-resight programmes, and more recently, individual remote tracking and genetic assignment, have provided extensive knowledge regarding the connectivity between breeding and non-breeding ranges in many wader species. However, some assumptions or 'expert opinions' regarding population distributions have been long accepted without proper verification even in some of the most well-studied species (Wetlands International 2025), and the issue is potentially complicated by geographical redistributions amid changing global environmental conditions (e.g. Lehikoinen *et al.* 2013, Pavón-Jordán *et al.* 2019). We present an instructive example: Bar-tailed Godwits *Limosa lapponica* on the East Atlantic Flyway (EAF).

Globally, there are six recognized subspecies of Bar-tailed Godwits, two of which (*L. l. lapponica* and *taymyrensis*) are found on the EAF (Fig. 1). These two subspecies were once considered differentially migrating populations of the nominate subspecies *L. l. lapponica*, until the description of plumage and morphometric differences between godwits breeding in Fennoscandia (including the White Sea and Kanin Peninsula, Russia) and north-central Russia (from the West Siberian Plain to the Taimyr Peninsula) led to the designation of the latter as *L. l. taymyrensis* (Engelmoer & Roselaar 1998). This taxon was itself later split, with the recognition that godwits breeding on the West Siberian Plain migrated along the Central Asian Flyway to non-breeding areas in the Middle East and East Africa (*L. l. yamalensis*; Bom *et al.* 2022).

*Lapponica* and *taymyrensis* are generally considered 'leap-frog' migrants, with the entirety of *taymyrensis* wintering in West Africa, south of European-wintering *lapponica* (Drent & Piersma 1990). This corresponded with the identification of two 'ecotypes': those that moulted their flight feathers during autumn in Europe and remained there for the non-breeding season, and those that passed through Europe on southward migration without moulting, and carried on to West Africa before initiating moult. Further supported by historical evidence from marked individuals (reviewed in Duijns *et al.* 2012), this led to the conclusion that all Bar-tailed Godwits found in Europe during winter were *lapponica*, whereas all *taymyrensis* individuals spent the non-breeding season in Africa. Based on this, the currently accepted population estimates and trends for both subspecies are derived entirely from non-breeding census data (roughly coordinated annual counts in January) from Europe and West Africa, respectively, assuming no

mixing of subspecies. Accordingly, the *lapponica* population estimate is 150 000–180 000 individuals and has been considered stable or increasing since the 1980s, while *taymyrensis* is estimated at 380 000–420 000 after experiencing a significant long-term decline (van Roomen *et al.* 2025, Wetlands International 2025).

Information about these populations is not limited to non-breeding census data. In recent decades, there have been various efforts to estimate the *lapponica* population from the northern European breeding grounds. These have resulted in estimates of 1640–7400 breeding pairs (data 1986–2000) reported by Thorup (2006), 6898 (range 4500–11 000) breeding pairs (data 2008–2018) by the European Red List of Birds (BirdLife International 2021, Herrando *et al.* 2024) and 8591 breeding pairs (data 2013–2017) by the second European Breeding Bird Atlas (Keller *et al.* 2020, Herrando *et al.* 2024). Each of these independent efforts indicated a total *lapponica* population of < 30 000 individuals. Owing to the relatively inaccessible and less-studied breeding range of *taymyrensis*, there have been no comparable estimates from the breeding grounds for that population. However, Spaans *et al.* (2011) used mark-resight methods to calculate a population estimate for *taymyrensis*, based on ring densities of godwits encountered during December 2007 at the main non-breeding site at Banc d'Arguin, Mauritania, having been previously marked on northward migration through the Wadden Sea (2000–2007). This produced a population estimate of 240 000 individuals (95% CI = 177 000–302 000) for *taymyrensis*. For both populations, these estimates could not account for the greater numbers directly counted in non-breeding censuses, and therefore the latter were considered more accurate and persisted as the accepted population estimates.

However, results of a recent genetic study of global population structure in Bar-tailed Godwits (Conklin *et al.* 2024) seriously undermine the practice of assigning godwits on the EAF to breeding areas based on non-breeding location. Because few genetic samples exist from Bar-tailed Godwit breeding areas, that study mostly used godwits captured at non-breeding areas to represent the *lapponica* and *taymyrensis* subspecies, and relied on long-term mark-resight data to assign these individuals to population. For *lapponica*, the authors selected 27 adult birds that were captured in the Dutch Wadden Sea while undergoing wing moult during August–October (and thus are expected to spend the winter in Europe), and had never been recorded during intensive resight efforts in West Africa (primarily at Banc d'Arguin, Mauritania, where c. 77% of African-wintering Bar-tailed Godwits are found; Smit & Piersma 1989). These were grouped *a priori* with four reference samples from known *lapponica* breeding grounds in Murmansk, Russia. For *taymyrensis*, the authors selected 27 adult godwits that were captured
<figure><img src="image_5.png" /><figcaption>**Figure 1.** Breeding areas and migrations of Bar-tailed Godwits on the East Atlantic Flyway. Each of three recognized migration patterns is represented by one tracked adult individual: *lapponica* (yellow line), captured in the Wadden Sea, 12 March 2024; *taymyrensis* Africa (dark green line), captured in the Wadden Sea, 11 May 2022; *taymyrensis* Europe (light green line), captured in France, 13 October 2015 (Rousseau *et al.* 2021). Open circles = capture sites; filled circles = breeding sites derived from tracking data. Arrows indicate direction of travel. Breeding polygon sources: *lapponica* (BirdLife International 2024) and *taymyrensis* (Lappo *et al.* 2012).</figcaption></figure>

during the non-breeding season at Banc d'Arguin, had been subsequently resighted in West Africa during three to seven non-breeding seasons, and had never been resighted in Europe outside of migration periods. The reference for these samples were two individuals sampled at known *taymyrensis* breeding areas on the Taimyr Peninsula, Russia (Conklin *et al.* 2024).

In analyses of variation at c. 14 000 genome-wide single-nucleotide polymorphisms (SNPs), known breeding samples of *lapponica* and *taymyrensis* were clearly distinguishable (Fig. 2), although *taymyrensis* were essentially indistinguishable from the *yamalensis* subspecies (Conklin *et al.* 2024). However, only six (22.2%) of the 27 purported *lapponica* samples from the Wadden Sea clustered with the known breeding *lapponica* samples

from Murmansk. Nineteen others (70.4%) clustered tightly with the known *taymyrensis* samples from Taimyr, and two more (7.4%) formed an intermediate cluster between *lapponica* and *taymyrensis*, and appear to be $F_1$ hybrids between the two subspecies (Fig. 2). Of the 27 purported *taymyrensis* samples from Mauritania, all clustered tightly with the Taimyr reference samples, and none showed evidence of hybridization.

These surprising results have important implications for our understanding of Bar-tailed Godwit populations on the EAF. First, it appears possible to assign most godwits on the EAF to breeding areas using genome-wide genetic markers, despite a previously demonstrated lack of differentiation at mitochondrial DNA (Bom *et al.* 2022). Second, if the genotyped sample of godwits
<figure><img src="image_6.png" /><figcaption>**Figure 2.** Global genetic structure in Bar-tailed Godwits, reflecting principal component (PC) analysis of variation at genome-wide single nucleotide polymorphisms across five subspecies (*L. i. anadyrensis* is not represented). Individuals from the East Atlantic Fly-way formed two major clusters (*L. i. lapponica* and *taymyrensis*), with two intermediate individuals representing apparent F₁ hybrids. Open circles indicate individuals sampled at Russian breeding grounds in Murmansk (yellow) and Taimyr (green). Modified from figure 2 in Conklin *et al.* (2024).</figcaption></figure>

from the Dutch Wadden Sea is representative of the European wintering population in general, then at least 70% of these birds belong to the *taymyrensis* subspecies, not 100% to *lapponica* as believed previously. Third, the two populations are hybridizing, although it is impossible to know whether the detected hybrids originated from breeding grounds in Fennoscandia or north-central Russia.

While these genetic results are impossible to ignore, indications of our incomplete understanding of Bar-tailed Godwits on the EAF had actually been accumulating for years. Examining museum specimens collected on known breeding grounds, Engelmoer and Roselaar (1998) found that *lapponica* godwits of both sexes were, on average, larger than *taymyrensis* in all morphometrics, and Duijns et al. (2012) found differences of similar magnitude between live captures in the Wadden Sea and Mauritania during the non-breeding season. Despite statistically significant mean differences, there is substantial overlap in the ranges of all morphometrics between the two populations, and most individuals cannot be reliably assigned to population by morphometrics or by plumage. However, Engelmoer (2008) employed a statistical analysis of morphology and moult to predict proportions of *lapponica* and *taymyrensis* captured throughout the year in the Dutch Wadden Sea, and concluded that c. 20% of Bar-tailed Godwits present there during winter (i.e. November–

February) were likely to be *taymyrensis*. Although this finding seems to have been largely ignored at the time, it now appears to be a substantial understatement of the 'problem'.

Information from individual Bar-tailed Godwits has also increasingly supported the presence of *taymyrensis* in Europe during winter. In May 2020, a leg-flagged individual captured in France in November 2010 and observed there numerous times during December–February of 2011–2013 was recovered by a hunter in northwest Khanty-Mansia, Russia (63.7°N, 61.9°E; Rousseau *et al.* 2021), well east of the *lapponica* range and migration path (Fig. 1). When reviewing all available evidence for the purported leap-frog migration pattern, Duijns *et al.* (2012) also found a handful of records of presumed *lapponica* individuals recovered further east in Russia (70–93°E), but the authors cautiously disregarded the records because they involved individuals marked at age 1–2 years. We now suggest that excluding those records may have been too conservative. Since that review, satellite-tracked Bar-tailed Godwits have clearly demonstrated that some adult *taymyrensis* share European winter sites with *lapponica*. One adult godwit captured and tagged at Île de Ré, France in October 2015 remained in that area until March 2016, and then migrated to the Taimyr Peninsula (Fig. 1), while a second adult tagged at that site migrated to the *lapponica* breeding range (Rousseau *et al.* 2021). Similarly, among
six adult godwits tagged in the Dutch Wadden Sea during November–March of 2024/2025, outside the expected passage periods for *taymyrensis* (Drent & Piersma 1990), three individuals migrated to Taimyr in 2025 (NIOZ unpubl. data).

Consistent with these observations, every recent assessment of the population status of *lapponica* has been forced to acknowledge the 3–5-fold difference between breeding and non-breeding estimates (Stroud *et al.* 2004, Thorup 2006, Scott & Scheiffarth 2009). The clear discrepancy prompted speculations in these reports that breeding estimates were too low, or that an unknown proportion of the European winter counts were actually *taymyrensis*, but adherence to simply using the European winter census data to represent *lapponica* persisted in those and all later assessments (BirdLife International 2021, van Roomen *et al.* 2025, Wetlands International 2025). There is cause to treat the *lapponica* breeding estimates with caution, owing to uncertainty in the precise extent of the breeding range, lack of data from some regions (particularly in the Russian portion of the range), and the difficulty of extrapolating observations over large areas of low or unknown density. It should also be noted that current breeding estimates might be slightly inflated by the inclusion of small numbers of the *yamalensis* subspecies (Bom *et al.* 2022), as the European Breeding Bird Atlas survey area extends well east of the known *lapponica* range and quite close to the *yamalensis* breeding range (Keller *et al.* 2020). Regardless, if we accept that 70% of the 150 000–180 000 European-wintering Bar-tailed Godwits are in fact *taymyrensis*, this would give an estimate of 45 000–54 000 *lapponica*, which approaches the magnitude of the breeding estimates.

Globally, Bar-tailed Godwits are considered Near Threatened (BirdLife International 2017), based on observed long-term declines on all flyways. The exception among the subspecies, *lapponica* has been listed as stable or increasing in every assessment during 1975–2018 (Wetlands International 2025), based on generally increasing winter numbers in Europe. During the same period, *taymyrensis* has been considered to be declining, with non-breeding census numbers in West Africa dropping from c. 700 000 to 500 000 individuals (Wetlands International 2025). The most recent census data show a continuing decline in West Africa and the first signs of a possible decline in Europe during 2014–2023 (van Roomen *et al.* 2025). However, if the winter numbers in Europe contain a large proportion of *taymyrensis*, the perceived trends in the two non-breeding regions should no longer be considered independent, and it is currently impossible to determine the true trends—past or present—in either subspecies.

Furthermore, if large numbers of *taymyrensis* are found in Europe during winter, it implies a historical misunderstanding of the two ecotypes observed in the

Wadden Sea: one group that initiates pre-basic moult upon arrival after the breeding season and remains in Europe, and another that stages briefly before continuing migration to moult on more southerly non-breeding grounds. We must now consider that at least three groups exist on the EAF (Fig. 1): (1) *lapponica* as it has been previously understood, a short-distance migrant wintering in Europe; (2) a medium-distance migrant wintering in Europe but breeding in Taimyr (*taymyrensis* Europe); and (3) a long-distance migrant found in West Africa in the non-breeding season (*taymyrensis* Africa). Although the first group appears genetically distinguishable from the latter two, there was no discernible genetic structure, and thus no expected demographic separation, between the two segments of *taymyrensis* (Fig. 2; Conklin *et al.* 2024).

There is yet potential for more unrecognized complexity in the system. For example, it is possible that some post-breeding adult godwits conduct partial or complete moult in Europe before continuing migration to non-breeding grounds in Africa. Thus far, out of > 360 godwits captured during moult in the Wadden Sea, only one individual was reported from Africa later in the same year (NIOZ unpubl. data); as this bird was of uncertain age and in ill health when recaptured, it is unclear how to interpret the observation. Among tracked individuals, there is currently no evidence of Africa-wintering individuals stopping in Europe sufficiently long to have completed moult there: length-of-stay in Europe was 6–47 days (mean = 26 days; *n* = 52 tracks of 40 individuals; NIOZ unpubl. data), compared to an expected moult duration of c. 100–120 days (Conklin & Battley 2012, Jackson & Underhill 2022). However, a handful of marked individuals have been captured during September while moulting in the Wadden Sea, and then been reported in Africa during December–January of a subsequent year (Duijns *et al.* 2012). Most of these birds were first captured as sub-adults, suggesting that some members of *taymyrensis* Africa do not continue past Europe on their first southward migration. For *lapponica*, there has been no concerted effort to track southward migration from the breeding grounds, so the precise extent of its non-breeding range is not well described. No individual yet tracked northward from West Africa has flown to the *lapponica* breeding range (*n* = 32; Bom *et al.* 2022; NIOZ unpubl. data), and no individual sampled in West Africa has been genetically assigned to *lapponica* (*n* = 27; Conklin *et al.* 2024), so there is presently no reason to suspect that the two subspecies mix outside of Europe. However, it is now clear that non-breeding location and moult strategy do not necessarily indicate an individual’s breeding origin. In light of this, reassessment of the results and conclusions from previous studies relying on this subspecies distinction (Scheiffarth
et al. 2002, Scheiffarth 2003, Duijns et al. 2009, 2014, Rakhimberdiev et al. 2018) may be required.

We do not know for how long *taymyrensis* has been wintering in Europe—possibly for decades, centuries or millennia. An intriguing possibility is that a previously strong temporal and geographical separation between the subspecies is deteriorating as a consequence of recent and ongoing changes along the flyway. If the proportion of *taymyrensis* spending the winter in Europe has significantly increased during the modern period of population monitoring, both the perceived population decline in *taymyrensis* and the perceived increase in *lapponica* could largely reflect a general northward shift in the non-breeding range of *taymyrensis*. Such northward shifts, with an associated decrease in migration distance, have been linked to climate change in numerous migratory species (e.g. La Sorte & Thompson 2007, Visser et al. 2009), have been detected at small scales within Europe in several wader species, including Bar-tailed Godwits (Maclean et al. 2008), and may explain broad changes in wader numbers on the EAF in recent decades (Schekkerman et al. 2022). Range shifts may naturally result from demographic patterns, such as differential survival among non-breeding regions (Lok et al. 2013, Reneerkens et al. 2020), but can also occur through generational or within-individual spatial changes (Gill et al. 2019). For Bar-tailed Godwits on the EAF, there is little evidence of adult individuals changing their wintering areas between years (Duijns et al. 2012; NIOZ unpubl. data). Therefore, we suggest that the most likely mechanism for a gradual northward shift in the non-breeding range of *taymyrensis* would be juveniles short-stopping on their first southward migration from Taimyr, and then proceeding to winter in Europe as adults.

Interestingly, there is some genetic evidence for a recent demographic change (on the scale of generations) in the system. Conklin et al. (2024) estimated that the divergence between *lapponica* and the ancestor of *taymyrensis/yamalensis* occurred c. 16 000 years ago (95% CI 9000–29 000) with no strong evidence of subsequent gene flow. However, that study also detected two apparent F₁ hybrids among godwits wintering in Europe, but no other individuals of intermediate ancestry (Fig. 2). This suggests that (successful) interbreeding of the subspecies is a rare or very recent phenomenon, perhaps in the last few generations, and could have resulted from a newly emerging contact zone. If increasing spatial overlap in the non-breeding season or increasingly similar migration timing (such as proposed between *taymyrensis* and *yamalensis*; Born et al. 2024) has facilitated individuals of either subspecies to migrate to the ‘wrong’ breeding area, we would expect that hybridization could increase significantly in coming decades, and could eventually erode the genetic differentiation between the subspecies.

Clarifying the status of Bar-tailed Godwits on the EAF will require much more complete descriptions of

the geographical ranges and annual-cycle variation within both subspecies, which is achievable with a targeted combination of large-scale satellite-tracking and population genetic approaches. To help confirm and describe the annual cycle of *taymyrensis* individuals that do not migrate to West Africa, we recommend more extensive satellite-tracking of godwits wintering in Europe, including from more areas outside the Wadden Sea (i.e. UK, France, Iberia, Mediterranean). To describe the full non-breeding range of *lapponica*, we recommend tracking southward migration from as many Fennoscandia breeding areas as possible. Of course, all tracked individuals should be genetically assigned to subspecies, to better understand the frequency of ‘flyway-switching’ and identify circumstances promoting population mixing and potential hybridization. To more precisely estimate the proportions of the European census numbers that should be ascribed to *taymyrensis*, and to explore the possibility of the subspecies mixing outside Europe, we recommend genotyping a large number of godwits across the non-breeding range in both Europe and Africa, and assigning them to subspecies. Ideally, this genetic assignment effort would include: (1) temporal coverage of the entire non-breeding season (including both migration and residency periods); and (2) the greatest range of years possible, in order to detect geographical variation and potential changes through time in the subspecies proportions and degree of interbreeding. This would include the period when major capture and DNA-sampling efforts occurred (i.e. 1990s to present), but in principle could go deeper in time with the additional use of museum specimens. Finally, with additional information from these combined efforts, there should be a complete reassessment of the population sizes, trends and conservation status of both subspecies.

Among avian species, Bar-tailed Godwit populations appear relatively easy to monitor, as they display conspicuously in open habitats during the breeding season (McCaffery & Gill 2020, Lyons et al. 2024), congregate in large numbers at predictable, coastal non-breeding sites, often at multiple stages of the year (Schuckard et al. 2020, Ruthrauff et al. 2021), and have well-described sub-populations and migratory routes (Bom et al. 2022, Conklin et al. 2024). Still, we have demonstrated how the combination of unverified assumptions and changing flyway conditions can thwart efforts to monitor even such a ‘well-known’ system. Particularly among migratory waterbirds, there are numerous other scenarios in which an unknown degree of subspecies overlap may complicate population monitoring. Among waders, a few examples include Black-tailed Godwits *L. limosa* on the EAF (Lopes et al. 2013) and Central Asian Flyway (Das et al. 2025), and Red Knots *Calidris canutus* on both the EAF and East Asian–Australasian Flyway (Conklin et al. 2022). For these cases, we now have the means to apply multiple approaches (combining genetic, tracking
and census data) to fully describe year-round distribu-
tions, test long-held assumptions, uncover cryptic struc-
ture and strengthen population monitoring efforts for
conservation purposes.

We thank NIOZ, and specifically Job ten Horn, Luc de Monte and Anne Dekinga, for maintaining the long-term database on Bar-tailed Godwit captures and recoveries. The unpublished tracking data presented in this paper were collected within the WAAKVOGELS project. We thank the editor and reviewers for insightful comments that greatly improved the manuscript.

AUTHOR CONTRIBUTIONS

Jesse R. Conklin: Conceptualization; writing – original draft; writing – review and editing; visualization; investigation. Roeland A. Bom: Conceptualization; writing – original draft; writing – review and editing; visualization; investigation. Åke Lindström: Writing – original draft; writing – review and editing; investigation. Pierrick Bocher: Investigation; writing – review and editing; writing – original draft. Theunis Piersma: Conceptualization; writing – original draft; writing – review and editing; resources; investigation.

FUNDING

Funded by Waddenfonds, the Dutch Ministry of LVVN and the Dutch provinces of Groningen, Fryslân and Noord-Holland.

CONFLICT OF INTEREST

The authors declare no conflicts of interest.

ETHICAL NOTE

None.

DATA AVAILABILITY STATEMENT

The data presented in Figure 2 are available at: https://
datadryad.org/dataset/doi:10.5061/dryad.73n5tb352.

REFERENCES

Bart, J. 2005. Monitoring the abundance of bird populations.
Auk 122: 15-25.

BirdLife International. 2017. *Limosa lapponica* (amended version of 2016 assessment). The IUCN Red List of Threatened Species 2017: e.T22693158A111221714. https://doi.org/10.2305/IUCN.UK.2017-1.RLTS.T22693158A111221714.en

**BirdLife International.** 2021. *European Red List of Birds*. Available at: https://www.birdlife.org/wp-content/uploads/2022/05/BirdLife-European-Red-List-of-Birds-2021.pdf (accessed 1 June 2025).

**BirdLife International.** 2024. *Bird species distribution maps of the world*. Version 2024.2. Available at: http://datazone.birdlife.org/species/requestdis (accessed 1 June 2025).

Bom, R.A., Conklin, J.R., Verkuil, Y.I., Alves, J.A., de
Fouw, J., Dekinga, A., Hassell, C.J., Klaassen, R.H.G.,
Kwarteng, A.Y., Rakhimberdiev, E., Rocha, A., ten Horn,
J., Tibbitts, T.L., Tomkovich, P.S., Victor, R. & Piersma,
T. 2022. Central-West Siberian-breeding Bar-tailed godwits
(Limosa lapponica) segregate in two morphologically distinct
flyway populations. *Ibis* 164: 468–485.

Bom, R.A., Piersma, T., Alves, J.A. & Rakhimberdiev, E. 2024. Global temperature homogenization can obliterate temporal isolation in migratory animals with potential loss of population structure. *Glob. Chang. Biol.* 30: e17069.

Conklin, J.R. & Battley, P.F. 2012. Carry-over effects and compensation: late arrival on non-breeding grounds affects wing moult but not plumage or schedules of departing bar-tailed godwits *Limosa lapponica* baueri. *J. Avian Biol.* 43: 252–263.

Conklin, J.R., Verkuil, Y.I., Battley, P.F., Hassell, C.J., ten Horn, J., Johnson, J.A., Tomkovich, P.S., Baker, A.J., Piersma, T. & Fontaine, M.C. 2022. Global flyway evolution in red knots *Calidris canutus* and genetic evidence for a Nearctic refugium. *Mol. Ecol.* 31: 2124–2139.

Conklin, J.R., Verkuil, Y.I., Lefebvre, M.J.M., Battley, P.F., Bom, R.A., Gill, R.E. Jr., Hassell, C.J., ten Horn, J., Ruthrauff, D.R., Tibbitts, T.L., Tomkovich, P.S., Warnock, N., Piersma, T. & Fontaine, M.C. 2024. High dispersal ability versus migratory traditions: fine-scale population structure and post-glacial colonisation in bar-tailed godwits. *Mol. Ecol.* 33: e17452.

Das, D.K., Vansteelant, W.M.G., Zhu, B., Islam, S., Khandakar, N., van der Velde, M., Hassell, C.J., Conklin, J.R., Bocher, P., Hooijmeijer, J.C.E.W., Verkuil, Y.I. & Piersma, T. 2025. Three subspecies of Black-tailed Godwit share non-breeding sites in the world's largest river delta. *Avian Res.* 16: 100226.

Drent, R. & Piersma, T. 1990. An exploration of the energetics of leap-frog migration in arctic breeding waders. In Gwinner, E. (ed) *Bird Migration: Physiology and Ecophysiology*. 399-412. Berlin: Springer.

Duijns, S., van Dijk, J.G.B., Spaans, B., Jukema, J., de Boer, W.F. & Piersma, T. 2009. Foraging site selection of two subspecies of Bar-tailed Godwit *Limosa lapponica*: time minimizers accept greater predation danger than energy minimizers. *Ardea* 97: 51–59.

Duijns, S., Jukema, J., Spaans, B., van Horssen, P. & Piersma, T. 2012. Revisiting the proposed leap-frog migration of Bar-tailed Godwits along the East-Atlantic flyway. *Ardea* 100: 37–43.

Duijns, S., van Gils, J.A., Spaans, B., ten Horn, J., Brugge, M. & Piersma, T. 2014. Sex-specific winter distribution in a sexually dimorphic shorebird is explained by resource partitioning. *Ecol. Evol.* 4: 4009–4018.

Emmet, R.L., Benson, T.J., Allen, M.L. & Stodola, K.W. 2023. Integrating multiple data sources improves prediction
and inference for upland game bird occupancy models. *Ornithol. Appl.* **125**: duad005.

Engelmoer, M. 2008. *Breeding origins of wader populations utilizing the Dutch Wadden Sea as deduced from body dimensions, body mass and primary moult*. PhD thesis, University of Groningen, The Netherlands.

Engelmoer, M. & Roselaar, C.S. 1998. *Geographical Variation in Waders*. Dordrecht: Kluwer Academic Publishers.

Gill, J.A., Alves, J.A. & Gunnarsson, T.G. 2019. Mechanisms driving phenological and range change in migratory species. *Philos. Trans. R. Soc. Lond. Ser. B Biol. Sci.* **374**: 20180047.

Hansen, B.D., Rogers, D.I., Watkins, D., Weller, D.R., Clemens, R.S., Newman, M., Woehler, E.J., Mundkur, T. & Fuller, R.A. 2022. Generating population estimates for migratory shorebird species in the world's largest flyway. *Ibis* **164**: 735-749.

Herrando, S., Fraixedas, S., Brotons, L., Martí, D., Staneva, A., Keller, V., Vorísek, P. & Burfield, I.J. 2024. Improving national bird population estimates in Europe: insights from comparisons with atlas abundance data. *Bird Conserv. Int.* **34**: e16.

Jackson, C.H.W. & Underhill, L.G. 2022. Primary moult strategies in adult migrant waders (Charadrii). *Wader Study* **129**: 126-137.

Keller, V., Herrando, S., Vorísek, P., Franch, M., Kipson, M., Milanesi, P., Martí, D., Anton, M., Klvanová, A., Kalyakin, M.V. & Bauer, H.G. 2020. In European Bird Census Council (ed) *European Breeding Bird Atlas 2: Distribution, Abundance and Change*. Barcelona: Lynx Edicions.

La Sorte, F.A. & Thompson, F.R. 2007. Poleward shifts in winter ranges of north American birds. *Ecology* **88**: 1803-1812.

Lappo, E.G., Tomkovich, P.S. & Syroechkovskiy, E.E. 2012. *Atlas of Breeding Waders in the Russian Arctic*. Moscow: UF Ofsetnaya Pechat.

Lehikoinen, A., Jaatinen, K., Vähätalo, A.V., Clausen, P., Crowe, O., Deceuninck, B., Hearn, R., Holt, C.A., Hornman, M., Keller, V., Nilsson, L., Langendoen, T., Tománková, I., Wahl, J. & Fox, A.D. 2013. Rapid climate driven shifts in wintering distributions of three common waterbird species. *Glob. Chang. Biol.* **19**: 2071-2081.

Lindström, A., Green, M., Husby, M., Kalas, J.A., Lehikoinen, A. & Stjernman, M. 2019. Population trends of waders on their boreal and arctic breeding grounds in northern Europe. *Wader Study* **126**: 200-216.

Lok, T., Overdijk, O. & Piersma, T. 2013. Migration tendency delays distributional response to differential survival prospects along a flyway. *Am. Nat.* **181**: 520-531.

Lopes, R.J., Alves, J.A., Gill, J.A., Gunnarsson, T.G., Hooijmeijer, J.C.E.W., Lourenço, P.M., Masero, J.A., Piersma, T., Potts, P.M., Rabaçal, B., Reis, S., Sánchez-Guzman, J.M., Santiago-Quesada, F. & Villegas, A. 2013. Do different subspecies of Black-tailed Godwit *Limosa limosa* overlap in Iberian wintering and staging areas? Validation with genetic markers. *J. Ornithol.* **154**: 35-40.

Lyons, J.E., Brown, S.C., Saalfeld, S.T., Johnson, J.A., Andres, B.A., Sowl, K.M., Gill, R.E., McCaffery, B.J., Kidd, L.R., McGarvey, M., Winn, B., Gates, H.R.,

Granfors, D.A. & Lanctot, R.B. 2024. Alaska's climate sensitive Yukon-Kuskokwim Delta supports seven million Arctic-breeding shorebirds, including the majority of six north American populations. *Ornithol. Appl.* **126**: duad066.

Maclean, I.M.D., Austin, G.E., Rehfisch, M.M., Blew, J., Crowe, O., Delany, S., Devos, K., Deceuninck, B., Günther, K., Laursen, K., van Roomen, M. & Wahl, J. 2008. Climate change causes rapid changes in the distribution and site abundance of birds in winter. *Glob. Chang. Biol.* **14**: 2489-2500.

McCaffery, B.J. & Gill, R.E. 2020. Bar-tailed Godwit (*Limosa lapponica*), version 1.0. In Billerman, S.M. (ed) *Birds of the World*. Ithaca, NY: Cornell Lab of Ornithology.

McClintock, B.T. & White, G.C. 2012. From NOREMARK to MARK: software for estimating demographic parameters using MARK-resight methodology. *J. Ornithol.* **152**: S641-S650.

Pavón-Jordán, D., Clausen, P., Dagys, M., Devos, K., Encarnação, V., Fox, A.D., Frost, T., Gaudard, C., Hornman, M., Keller, V., Langendoen, T., Ławicki, Ł., Lewis, L.J., Lorentsen, S.H., Luigujoe, L., Meissner, W., Molina, B., Musil, P., Musilova, Z., Nilsson, L., Paquet, J.Y., Ridzon, J., Stipniece, A., Teufelbauer, N., Wahl, J., Zenatello, M. & Lehikoinen, A. 2019. Habitat- and species-mediated short- and long-term distributional changes in waterbird abundance linked to variation in European winter weather. *Divers. Distrib.* **25**: 225-239.

Piironen, A., Fox, A.D., Kampe-Persson, H., Skyllberg, U., Therkildsen, O.R. & Laaksonen, T. 2023. When and where to count? Implications of migratory connectivity and nonbreeding distribution to population censuses in a migratory bird population. *Popul. Ecol.* **65**: 121-132.

Rakhimberdiev, E., Duijns, S., Karagicheva, J., Camphuysen, C.J., VRS Castricum, Dekinga, A., Dekker, R., Gavrilov, A., ten Horn, J., Jukema, J., Saveliev, A., Soloviev, M., Tibbitts, T.L., van Gils, J.A. & Piersma, T. 2018. Fuelling conditions at staging sites can mitigate Arctic warming effects in a migratory bird. *Nat. Commun.* **9**: 4263.

Reneerkens, J., Versluijs, T.S.L., Piersma, T., Alves, J.A., Boorman, M., Corse, C., Gilg, O., Hallgrimsson, G.T., Lang, J., Loos, B., Ntiamoa-Baidu, Y., Nuoh, A.A., Potts, P.M., ten Horn, J. & Lok, T. 2020. Low fitness at low latitudes: wintering in the tropics increases migratory delays and mortality rates in an Arctic breeding shorebird. *J. Anim. Ecol.* **89**: 691-703.

van Roomen, M., Reneerkens, J., Citegetse, G., Crowe, O., Gueye, K., Langendoen, T., Dodman, T., Meise, K. & Schekkerman, H. 2025. *East Atlantic Flyway Assessment 2023*. The status of coastal waterbird populations and their sites. Wadden Sea Flyway Initiative p/a CWSS, Wilhelmshaven, Germany, Wetlands International, Wageningen, The Netherlands, BirdLife International, Cambridge, UK. https://doi.org/10.5281/zenodo.15355685

Rousseau, P., Robin, F., Delaporte, P., Jourdan, C. & Bocher, P. 2021. A Bar-tailed Godwit that wintered in France migrates to the Taimyr Peninsula. *Ornithos* **28**: 367-369.

Ruthrauff, D.R., Pohlen, Z.M., Wilson, H.M. & Johnson, J.A. 2021. Bar-tailed Godwits *Limosa lapponica* in Alaska: revisiting population estimates from the staging grounds. *Wader Study* **128**: 255-264.
**Scheiffarth, G.** 2003. The interaction between migration strategy and population dynamics: is there higher winter mortality in short-distance migrants and lower juvenile survival in long-distance migrants? *Wader Study Gr. Bull.* **100**: 157–158.

**Scheiffarth, G., Wahls, S., Ketzenberg, C. & Exo, K.-M.** 2002. Spring migration strategies of two populations of bar-tailed godwits, *Limosa lapponica*, in the Wadden Sea: time minimizers or energy minimizers? *Oikos* **96**: 346–354.

**Schekkerman, H., Nagy, S., Gueye Fall, K., Langendoen, T. & van Roomen, M.** 2022. Patterns in trends of waterbird populations using the coastal East Atlantic Flyway, update 2020. In van Roomen, M., Citegetse, G., Crowe, O., Dodman, T., Hagemeijer, W., Meise, K. & Schekkerman, H. (eds) *East Atlantic Flyway Assessment 2020*. The status of coastal waterbird populations and their sites: 17–25. Wadden Sea Flyway Initiative p/a CWSS, Wilhelmshaven, Germany, Wetlands International, Wageningen, The Netherlands, BirdLife International, Cambridge, UK.

**Schuckard, R., Melville, D.S., Riegen, A., Driscoll, P., Driessen, J. & Kidd, L.R.** 2020. Numbers of bar-tailed godwits (*Limosa lapponica baueri*) in New Zealand and Australia during the austral summer of 2019–2020. *Notornis* **67**: 643–650.

**Scott, D. & Scheiffarth, G.** 2009. Bar-tailed Godwit *Limosa lapponica*. In Delany, S., Scott, D., Dodman, T. & Stroud, D. (eds) *An Atlas of Wader Populations in Africa and Western Eurasia*: 291–297. Wageningen: Wetlands International.

**Smit, C.J. & Piersma, T.** 1989. *Numbers, mid-winter distribution, and migration of waders populations using the*

*East Atlantic flyway*. Pp. 24–63 in: Flyways and Reserve Networks for Water Birds: Developed from Papers Presented at the Third Meeting of the Conference of Contracting Parties to the Ramsar Convention, Held in Regina, Saskatchewan, Canada, 28-29 May 1987. IWRB Special Publication No. 9.

**Spaans, B., van Kooten, L., Cremer, J., Leyrer, J. & Piersma, T.** 2011. Densities of individually marked migrants away from the marking site to estimate population sizes: a test with three wader populations. *Bird Study* **58**: 130–140.

**Stroud, D.A., Davidson, N.C., West, R., Scott, D.A., Haanstra, L., Thorup, O., Ganter, B. & Delany, S.** 2004. The status of migratory wader populations in Africa and Western Eurasia in the 1990s. *Int. Wader Stud.* **15**: 1–259. Thetford, UK: International Wader Study Group.

**Thorup, O.** 2006. Breeding waders in Europe 2000. *Int. Wader Stud.* **14**: 59. Thetford, UK: International Wader Study Group.

**Visser, M.E., Perdeck, A.C., van Balen, J.H. & Both, C.** 2009. Climate change leads to decreasing bird migration distances. *Glob. Chang. Biol.* **15**: 1859–1865.

**Wetlands International.** 2025. *Waterbird population estimates*. Available at: wpe.wetlands.org (accessed 1 June 2025).

Received 30 June 2025;
Revision 17 September 2025;
revision accepted 18 September 2025.
Associate Editor: Jennifer Gill