Potential Coincidence Between Wader/Shorebird Migration Flyways and Earth’s Gravity Field

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Khalil This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8398499/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract The present study identifies a potential link between wader/shorebird migratory flyways and the Earth’s gravity field, based on five key observations. The first four observations demonstrate a strong association between global migration routes and the stability of the underlying Earth’s gravitational field. The fifth observation presents a theoretical analysis of the relationship between Earth’s gravitational acceleration and the upward lift force during steady flight. Collectively, these findings suggest that (1) maintaining a constant gravitational acceleration is essential for efficient, low-energy flight, and (2) global wader/shorebird migratory flyways tend to follow routes characterized by relatively stable gravitational acceleration, potentially functioning as a form of gravitational navigation system. Biophysics Animal Science Geophysics Shorebird Migration flyways Earth’s gravity Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction Wader/shorebird migration flyways remain one of the enduring mysteries of animal navigation: how successive generations of birds are able to follow the same routes, spanning thousands of kilometers, year after year. Despite decades of research, our understanding of the mechanisms underlying this remarkable navigational precision remains limited and uncertain. A wide range of navigation cues has been proposed to explain how birds determine flight direction. These include the Earth’s magnetic field (Walker et al., 2002 ; Wiltschko, 2013 , 2014 , 2023 ), visual landmarks (Holland, 2003 , 2014 ), airborne chemical cues or odors (Papi et al., 1972 , 1978 ; Jorge et al., 2010 ), natural and anthropogenic sounds (Griffin and Hopkins, 1976 ; Mukhin et al., 2008 ; Hagstrum, 2013 ), the position of the sun and stars (Keeton, 1974; Armstrong et al., 2013 ), and the Earth’s gravitational field (Dornfeldt, 1991; Blaser et al., 2013 , 2014 ; Kanevskyi, 2009 , 2023). Despite the extensive literature on these proposed cues, no single mechanism has been universally accepted, and each hypothesis has both advocates and critics. In this article, I do not attempt to determine the definitive navigational cue, nor do I approach the problem from an ornithological perspective. Rather, as a geophysicist, I report a clear spatial coincidence between wader/shorebird migratory flyways and features of the Earth’s gravity field. In support of this observation, I also note several studies based on extensive experimental work that identify the Earth’s gravitational field as a strong candidate for explaining key navigational parameters (Dornfeldt, 1991). Discussion The study is based on four observational lines of evidence and a theoretical analysis examining the relationship between the Earth’s gravitational acceleration and wader/shorebird migratory flyways. 1- First observation Figure 1 (A) shows major flyways of migratory land and waterbirds, following Boere et al. ( 2006 ). The authors classified global migration into eight principal flyways: Pacific Americas, Mississippi Americas, Atlantic Americas, East Atlantic, Black Sea/Mediterranean, West Asia/East Africa, Central Asia, and East Asia/Australasia. (B) Earth’s gravitational field map at 700 km spatial resolution (NASA, 2003 ). (C) Earth’s gravitational field map at 200 km spatial resolution (NASA, 2003 ). Comparison of the flyway map with the gravitational field maps reveals a clear spatial correspondence. From west to east, the Pacific Americas flyway aligns with a broad region of high-gravity anomalies (yellow to red), whereas the Atlantic Americas flyway coincides with a region of lower-gravity anomalies (blue). The East Atlantic flyway is associated with high-gravity anomalies and exhibits a similar spatial pattern, particularly across Greenland and northern Europe. The Central Asia flyway corresponds largely to low-gravity anomalies, except in the Himalayan region. The East Asia/Australasia flyway is associated with high-gravity anomalies with a comparable spatial distribution. In contrast, inland flyways such as the Black Sea/Mediterranean and West Asia/East Africa do not exhibit stable gravity anomaly patterns. Given that the NASA gravity map has a spatial resolution of 200 km, a higher-resolution terrestrial gravity dataset may better highlight this correlation. 2- Second observation Figure 2 shows a comparison of the Earth’s gravitational field map at 200 km spatial resolution (A) (NASA, 2003 ) with a generalized schematic of major migratory routes from Hötker et al. ( 1998 ) (B). Beyond the overall correspondence between established migration routes (black solid lines) and gravity anomaly patterns, two notable features are highlighted. First, the East Asia/Australasia flyway in the Hötker et al. ( 1998 ) schematic map (B) branches into two routes between the Vietnamese coastline and Australia. This divergence corresponds closely to a similar divergence in high-gravity anomalies shown in the gravity map (A). Second, the Pacific Americas flyway splits into two routes in northwestern South America (Colombia, Ecuador, and Peru), aligning with comparable deviations in high-gravity anomalies observed in map (A). 3- Third observation The spatial distribution of the Least Sandpiper in the United States, based on data from the Cornell Lab of Ornithology (eBird), is shown in Fig. 3 A, with areas of high occurrence highlighted as bright spots. Figure 3 B presents the Bouguer gravity map of the United States after the Gravity Map Service (GMC), while Fig. 3 C shows the surface elevation (topographic) map of the United States after the U.S. Geological Survey (USGS). As theoretically expected, the Bouguer gravity map (Fig. 3 B) broadly mirrors the surface elevation map (Fig. 3 C), with regions of high topography corresponding to low Bouguer gravity anomalies. This relationship arises because the Bouguer gravity anomaly represents a correction to observed gravity measurements that accounts for the gravitational effect of surface topography. In addition, the Bouguer gravity map exhibits an inverse anomaly pattern relative to the satellite-derived NASA gravity maps. The similarity among the three maps suggests that Least Sandpipers may be tracking either low-elevation topographic features (consistent with a visual landmark navigation cue) or regions of relatively high gravity anomalies. Figure 4 shows the Upland Sandpiper flew nonstop for six days over the Pacific Ocean from Massachusetts, USA, to Venezuela without food, water, or significant sleep (Vermont Center for Ecostudies). Notably, this flight path follows a segment of the low-gravity Atlantic Americas flyway (Fig. 1 ). It is also noteworthy that the Upland Sandpiper traveled approximately 2,315 miles over open ocean, where no visual landmarks (coastlines) are present, except Puerto Rico, which is about 1,674 miles from Massachusetts, USA. Many published studies have determined that visual landmarks' cue is not an essential requirement for migration. Holland ( 2003 ) stated that Pigeons use landmarks around the familiar area of their loft, but not to greater distances, not even at very familiar sites. The experiments of Schmidt-Koenig and Walcott (1978) revealed that pigeons fitted with frosted lenses and released remotely were able to fly in the direction of their lofts. Their results support the view that pigeons use a navigation system that does not require detailed vision of the landscape and is accurate enough to lead birds to within 0.5 to 5 km of their goal (Schmidt-Koenig and Walcott 1978). Non-visual Top of Form Many published studies have shown that visual landmarks are not an essential requirement for migration. Holland ( 2003 ) noted that pigeons rely on landmarks only within the familiar area surrounding their loft, but not over longer distances, even at familiar sites. Experiments by Schmidt-Koenig and Walcott (1978) demonstrated that pigeons fitted with frosted lenses and released at remote locations were still able to orient toward their lofts. These results support the idea that pigeons employ a navigation system that does not require detailed visual information from the landscape and is sufficiently accurate to guide birds within 0.5 to 5 km of their goal. Non-visual navigation systems have also been proposed by Barlow ( 1964 ), Wallraff ( 1966 ), and Papi et al. ( 1971 , 1972 , 1973 ). 4- Fourth observation Figure 5 A presents the biogeographic population polygons of 276 migratory waterbird populations (the world’s largest flyway) representing 216 species, as compiled by Mundkur and Langendoen ( 2022 ). Figure 5 B shows the global marine gravity anomaly map derived from satellite radar altimetry, following Mueller et al. ( 2017 ). These gravity data were produced using measurements from the GEOSAT, ERS-1, Envisat, Jason-1, and CryoSat-2 missions. A comparison of the two maps reveals a clear correspondence between regions of dense migratory flyways (blue color) and the low-gravity tectonic trenches in East Asia and Australasia. This relationship is particularly evident along the Aleutian Trench flyway. Although the geographic distance between Alaska and Russia is relatively short, the flyway closely follows the Aleutian Trench rather than the shortest overwater route. Similarly, south of Australia and between Australia and India—regions where no major trenches or island chains exist—low-gravity anomalies coincide with major concentrations of migratory flyways. This observation raises an important question: are shorebirds primarily tracking island chains (visual landmarks cue) for feeding and resting opportunities, or are they following low-gravity anomalies associated with deep-seated geophysical structures? A closer examination of the two maps reveals a strong spatial congruence between low-gravity trenches and dense migratory flyways. Notably, the flyways run parallel to island chains rather than crossing them and remain closely aligned with the low-gravity anomalies. 5- Fifth observation (Theoretical analysis) The high-altitude migration equation, according to Tennekes ( 2009 ), is expressed as: L = 0.3 ρ V 2 S (1) Where L is the force made by lift, S is the bird’s wing area in square meters, V is the air speed in meters per second, and ρ is the air density in kilograms per meter. The 0.3 is related to the angle of attack (AOA) in long-distance flight, for which the average value is 6 °. The AOA, defined as the angle between the bird’s wing and the oncoming airflow, is a critical factor in maintaining flight equilibrium. In equilibrium flight, the upward lift ( L ) equals the downward weight ( W ), allowing Eq. (1) to be rewritten as: W = 0.3 ρ V 2 S (2) Since all flying occurs on Earth and is affected by terrestrial gravity, weight can be expressed as the magnitude of the downward gravitational force ( \(\:{F}_{g}\) ) using Newton’s second law: $$\:W=\left|{F}_{g}\right|=m.g$$ 3 Where m is the mass of the bird in kg and \(\:g\) is the Earth’s gravitational acceleration (9.8 m/s²). Combining equations (1), (2), and (3) yields; L = m. \(\:g\) = 0.3 ρ V 2 S (4) Equation (4) indicates that during gliding flight at constant velocity ( \(\:V\) ) and constant air density ( \(\:\rho\:\) ) at a fixed altitude, the upward lift ( \(\:L\) ) is balanced by the downward weight. Accordingly, flying under a constant gravitational acceleration ( \(\:g\) ) is critical for maintaining a balanced, low-energy flight. This is consistent with the temporal stability and spatial homogeneity of Earth’s gravitational field. Furthermore, Eq. (4) highlights that gravitational acceleration ( \(\:g\) ) is an essential component of the high-altitude migration equation and is expected to play a significant role in long-distance flight. Recent research and technological developments also suggest that gravitational navigation could, in certain contexts, serve as an alternative or complement to satellite-based navigation systems. Wang et al. ( 2018 ) demonstrated that homing pigeons are capable of sensing extremely small variations in gravitational force, as low as 0.0001 Gal (0.1 mGal), despite the presence of significant inertial forces generated by the mechanical movement of the bird’s body during flight—specifically, the vertical acceleration caused by wing flapping, which is approximately 0.7 Gal. Dornfeldt (1991) found that gravity anomalies are the strongest geophysical predictors of poor initial orientation and homing performance in pigeons. In most multivariate regression models, variables related to gravity provided the best fit to navigational parameters, suggesting that the observed patterns may warrant consideration of a causal relationship. These observations are consistent with the gravity vector hypothesis ( GVH ) proposed by Kanevskyi (2023). His experiments with homing pigeons revealed instances of disorientation at the boundaries of gravity anomalies, where strong horizontal gradients of the gravitational field influenced flight trajectories. Conclusion Based on observed patterns and the theoretical relationship between lift and gravitational acceleration, the author proposes a strong correlation between Earth’s gravitational field and wader/shorebird migratory flyways. These flyways tend to follow regions of relatively stable gravitational acceleration, suggesting a potential gravitational navigation system. A global-scale experiment is recommended to rigorously test and validate this proposed correlation, taking into account factors such as bird species, flight altitude, and bird body mass. Abbreviations NASA National Aeronautics and Space Administration GRACE Gravity Recovery and Climate Experiment GMC Gravity Map Service USGS United States Geological Survey L Force of lift S bird’s wing area V air speed ρ air density AOA Angle Of Attack W Weight \(\:{F}_{g}\) Force of Gravity m mass of the bird kg Kilogram \(\:g\) Earth’s gravitational acceleration mGal milligal Declarations Funding: The author declares he didn’t receive funding for this study. 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1","display":"","copyAsset":false,"role":"figure","size":1029012,"visible":true,"origin":"","legend":"\u003cp\u003e(A) Major flyways of migratory land and waterbirds after Boere et al. (2006), with permission from David Stroud. (B) shows Earth's gravitational field from NASA (GRACE) 2003 at 700 km spatial resolution. (C) Earth's gravitational field from NASA (GRACE) 2003 at 200 km spatial resolution.\u003c/p\u003e","description":"","filename":"floatimage1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8398499/v1/43c320a5aa96331541e11a3b.jpeg"},{"id":98776934,"identity":"df130d84-1524-4b72-aa79-b54a51549ec8","added_by":"auto","created_at":"2025-12-22 12:24:32","extension":"jpeg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1286889,"visible":true,"origin":"","legend":"\u003cp\u003eComparison between the Earth’s gravitational field map derived from NASA (GRACE) data at 200 km spatial resolution (A) and a generalized schematic of principal migration routes from Hötker et al. (1998) (B). Solid lines represent known migration routes, while dashed lines indicate suspected migration routes. Small white circles denote sites with \u0026gt;100,000 shorebirds and low primary productivity, and small white squares denote sites with \u0026gt;100,000 shorebirds and high primary productivity. Large purple circles highlight divergences in migration routes that correspond to similar diversions in the gravity field.\u003c/p\u003e","description":"","filename":"floatimage2.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8398499/v1/1a9f8af189fb74187f31e500.jpeg"},{"id":98733932,"identity":"b4772f34-cf3e-4179-8603-d549f06dee05","added_by":"auto","created_at":"2025-12-22 05:58:41","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":764966,"visible":true,"origin":"","legend":"\u003cp\u003eComparison between the spatial distribution of Least Sandpiper in the USA (A) after Cornell Lab of Ornithology (eBird), the Bouguer gravity map of the USA (B) after Gravity Map Service (GMC), and the topographic map of the USA (C) after U.S. Geological Survey (USGS).\u003c/p\u003e","description":"","filename":"floatimage3.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8398499/v1/cf4e13aa424719f9420fee0e.jpeg"},{"id":98733936,"identity":"1db58efe-bada-4867-9cf5-ca7403c25323","added_by":"auto","created_at":"2025-12-22 05:58:41","extension":"jpeg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":1009658,"visible":true,"origin":"","legend":"\u003cp\u003eComparison between sandpiper migration route (A) after Vermont Center for Ecostudies and the Earth’s gravitational field (B) after NASA (GRACE), 200 km spatial resolution.\u003c/p\u003e","description":"","filename":"floatimage4.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8398499/v1/2464979efaff09b325bec6ea.jpeg"},{"id":98733938,"identity":"b6df15dd-2f03-407f-85cc-29f7799382fe","added_by":"auto","created_at":"2025-12-22 05:58:41","extension":"jpeg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":421157,"visible":true,"origin":"","legend":"\u003cp\u003eComparison between the biogeographic population polygons of 276 migratory waterbirds of 216 species (A) after Mundkur and Langendoen, (2022) and the global marine gravity anomaly map (B) based on satellite radar altimetry (GEOSAT, ERS-1, Envisat, Jason-1, and Cryosat-2), after Mueller et al (2017).\u003c/p\u003e","description":"","filename":"floatimage5.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8398499/v1/5484e72df9f52a247312c673.jpeg"},{"id":98785795,"identity":"0904d7a7-436e-406f-9f7f-21288f325cd7","added_by":"auto","created_at":"2025-12-22 12:43:22","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4894873,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8398499/v1/439a0552-7383-4295-8292-5cebeafb81c7.pdf"}],"financialInterests":"The authors declare no competing interests.","formattedTitle":"\u003cp\u003e\u003cstrong\u003ePotential Coincidence Between Wader/Shorebird Migration Flyways and Earth’s Gravity Field\u003c/strong\u003e\u003c/p\u003e","fulltext":[{"header":"Introduction","content":"\u003cp\u003eWader/shorebird migration flyways remain one of the enduring mysteries of animal navigation: how successive generations of birds are able to follow the same routes, spanning thousands of kilometers, year after year. Despite decades of research, our understanding of the mechanisms underlying this remarkable navigational precision remains limited and uncertain.\u003c/p\u003e \u003cp\u003eA wide range of navigation cues has been proposed to explain how birds determine flight direction. These include the Earth\u0026rsquo;s magnetic field (Walker et al., \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Wiltschko, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2013\u003c/span\u003e, \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2014\u003c/span\u003e, \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2023\u003c/span\u003e), visual landmarks (Holland, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2003\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2014\u003c/span\u003e), airborne chemical cues or odors (Papi et al., \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e1972\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e1978\u003c/span\u003e; Jorge et al., \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2010\u003c/span\u003e), natural and anthropogenic sounds (Griffin and Hopkins, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e1976\u003c/span\u003e; Mukhin et al., \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Hagstrum, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2013\u003c/span\u003e), the position of the sun and stars (Keeton, 1974; Armstrong et al., \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2013\u003c/span\u003e), and the Earth\u0026rsquo;s gravitational field (Dornfeldt, 1991; Blaser et al., \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2013\u003c/span\u003e, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Kanevskyi, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2009\u003c/span\u003e, 2023). Despite the extensive literature on these proposed cues, no single mechanism has been universally accepted, and each hypothesis has both advocates and critics.\u003c/p\u003e \u003cp\u003eIn this article, I do not attempt to determine the definitive navigational cue, nor do I approach the problem from an ornithological perspective. Rather, as a geophysicist, I report a clear spatial coincidence between wader/shorebird migratory flyways and features of the Earth\u0026rsquo;s gravity field. In support of this observation, I also note several studies based on extensive experimental work that identify the Earth\u0026rsquo;s gravitational field as a strong candidate for explaining key navigational parameters (Dornfeldt, 1991).\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eThe study is based on four observational lines of evidence and a theoretical analysis examining the relationship between the Earth\u0026rsquo;s gravitational acceleration and wader/shorebird migratory flyways.\u003c/p\u003e\n\u003ch3\u003e1- First observation\u003c/h3\u003e\n\u003cp\u003eFigure \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e (A) shows major flyways of migratory land and waterbirds, following Boere et al. (\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). The authors classified global migration into eight principal flyways: Pacific Americas, Mississippi Americas, Atlantic Americas, East Atlantic, Black Sea/Mediterranean, West Asia/East Africa, Central Asia, and East Asia/Australasia. (B) Earth\u0026rsquo;s gravitational field map at 700 km spatial resolution (NASA, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2003\u003c/span\u003e). (C) Earth\u0026rsquo;s gravitational field map at 200 km spatial resolution (NASA, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2003\u003c/span\u003e). Comparison of the flyway map with the gravitational field maps reveals a clear spatial correspondence. From west to east, the Pacific Americas flyway aligns with a broad region of high-gravity anomalies (yellow to red), whereas the Atlantic Americas flyway coincides with a region of lower-gravity anomalies (blue). The East Atlantic flyway is associated with high-gravity anomalies and exhibits a similar spatial pattern, particularly across Greenland and northern Europe. The Central Asia flyway corresponds largely to low-gravity anomalies, except in the Himalayan region. The East Asia/Australasia flyway is associated with high-gravity anomalies with a comparable spatial distribution. In contrast, inland flyways such as the Black Sea/Mediterranean and West Asia/East Africa do not exhibit stable gravity anomaly patterns. Given that the NASA gravity map has a spatial resolution of 200 km, a higher-resolution terrestrial gravity dataset may better highlight this correlation.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e\n\u003ch3\u003e2- Second observation\u003c/h3\u003e\n\u003cp\u003eFigure \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e shows a comparison of the Earth\u0026rsquo;s gravitational field map at 200 km spatial resolution (A) (NASA, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2003\u003c/span\u003e) with a generalized schematic of major migratory routes from H\u0026ouml;tker et al. (\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e1998\u003c/span\u003e) (B). Beyond the overall correspondence between established migration routes (black solid lines) and gravity anomaly patterns, two notable features are highlighted.\u003c/p\u003e \u003cp\u003eFirst, the East Asia/Australasia flyway in the H\u0026ouml;tker et al. (\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e1998\u003c/span\u003e) schematic map (B) branches into two routes between the Vietnamese coastline and Australia. This divergence corresponds closely to a similar divergence in high-gravity anomalies shown in the gravity map (A). Second, the Pacific Americas flyway splits into two routes in northwestern South America (Colombia, Ecuador, and Peru), aligning with comparable deviations in high-gravity anomalies observed in map (A).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e\n\u003ch3\u003e3- Third observation\u003c/h3\u003e\n\u003cp\u003eThe spatial distribution of the Least Sandpiper in the United States, based on data from the Cornell Lab of Ornithology (eBird), is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eA, with areas of high occurrence highlighted as bright spots. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eB presents the Bouguer gravity map of the United States after the Gravity Map Service (GMC), while Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eC shows the surface elevation (topographic) map of the United States after the U.S. Geological Survey (USGS).\u003c/p\u003e \u003cp\u003eAs theoretically expected, the Bouguer gravity map (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eB) broadly mirrors the surface elevation map (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eC), with regions of high topography corresponding to low Bouguer gravity anomalies. This relationship arises because the Bouguer gravity anomaly represents a correction to observed gravity measurements that accounts for the gravitational effect of surface topography. In addition, the Bouguer gravity map exhibits an inverse anomaly pattern relative to the satellite-derived NASA gravity maps.\u003c/p\u003e \u003cp\u003eThe similarity among the three maps suggests that Least Sandpipers may be tracking either low-elevation topographic features (consistent with a visual landmark navigation cue) or regions of relatively high gravity anomalies.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e shows the Upland Sandpiper flew nonstop for six days over the Pacific Ocean from Massachusetts, USA, to Venezuela without food, water, or significant sleep (Vermont Center for Ecostudies). Notably, this flight path follows a segment of the low-gravity Atlantic Americas flyway (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). It is also noteworthy that the Upland Sandpiper traveled approximately 2,315 miles over open ocean, where no visual landmarks (coastlines) are present, except Puerto Rico, which is about 1,674 miles from Massachusetts, USA.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eMany published studies have determined that visual landmarks' cue is not an essential requirement for migration. Holland (\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2003\u003c/span\u003e) stated that Pigeons use landmarks around the familiar area of their loft, but not to greater distances, not even at very familiar sites. The experiments of Schmidt-Koenig and Walcott (1978) revealed that pigeons fitted with frosted lenses and released remotely were able to fly in the direction of their lofts. Their results support the view that pigeons use a navigation system that does not require detailed vision of the landscape and is accurate enough to lead birds to within 0.5 to 5 km of their goal (Schmidt-Koenig and Walcott 1978). Non-visual Top of Form\u003c/p\u003e \u003cp\u003eMany published studies have shown that visual landmarks are not an essential requirement for migration. Holland (\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2003\u003c/span\u003e) noted that pigeons rely on landmarks only within the familiar area surrounding their loft, but not over longer distances, even at familiar sites. Experiments by Schmidt-Koenig and Walcott (1978) demonstrated that pigeons fitted with frosted lenses and released at remote locations were still able to orient toward their lofts. These results support the idea that pigeons employ a navigation system that does not require detailed visual information from the landscape and is sufficiently accurate to guide birds within 0.5 to 5 km of their goal. Non-visual navigation systems have also been proposed by Barlow (\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e1964\u003c/span\u003e), Wallraff (\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e1966\u003c/span\u003e), and Papi et al. (\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e1971\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e1972\u003c/span\u003e, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e1973\u003c/span\u003e).\u003c/p\u003e\n\u003ch3\u003e4- Fourth observation\u003c/h3\u003e\n\u003cp\u003eFigure \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eA presents the biogeographic population polygons of 276 migratory waterbird populations (the world\u0026rsquo;s largest flyway) representing 216 species, as compiled by Mundkur and Langendoen (\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Figure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eB shows the global marine gravity anomaly map derived from satellite radar altimetry, following Mueller et al. (\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). These gravity data were produced using measurements from the GEOSAT, ERS-1, Envisat, Jason-1, and CryoSat-2 missions.\u003c/p\u003e \u003cp\u003eA comparison of the two maps reveals a clear correspondence between regions of dense migratory flyways (blue color) and the low-gravity tectonic trenches in East Asia and Australasia. This relationship is particularly evident along the Aleutian Trench flyway. Although the geographic distance between Alaska and Russia is relatively short, the flyway closely follows the Aleutian Trench rather than the shortest overwater route.\u003c/p\u003e \u003cp\u003eSimilarly, south of Australia and between Australia and India\u0026mdash;regions where no major trenches or island chains exist\u0026mdash;low-gravity anomalies coincide with major concentrations of migratory flyways.\u003c/p\u003e \u003cp\u003eThis observation raises an important question: are shorebirds primarily tracking island chains (visual landmarks cue) for feeding and resting opportunities, or are they following low-gravity anomalies associated with deep-seated geophysical structures?\u003c/p\u003e \u003cp\u003eA closer examination of the two maps reveals a strong spatial congruence between low-gravity trenches and dense migratory flyways. Notably, the flyways run parallel to island chains rather than crossing them and remain closely aligned with the low-gravity anomalies.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e\n\u003ch3\u003e5- Fifth observation (Theoretical analysis)\u003c/h3\u003e\n\u003cp\u003eThe high-altitude migration equation, according to Tennekes (\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2009\u003c/span\u003e), is expressed as:\u003c/p\u003e \u003cp\u003e \u003cem\u003eL\u0026thinsp;=\u0026thinsp;0.3 ρ V\u003c/em\u003e \u003csup\u003e \u003cem\u003e2\u003c/em\u003e \u003c/sup\u003e \u003cem\u003eS\u003c/em\u003e (1)\u003c/p\u003e \u003cp\u003eWhere \u003cem\u003eL\u003c/em\u003e is the force made by lift, \u003cem\u003eS\u003c/em\u003e is the bird\u0026rsquo;s wing area in square meters, \u003cem\u003eV\u003c/em\u003e is the air speed in meters per second, and \u003cem\u003eρ\u003c/em\u003e is the air density in kilograms per meter. The 0.3 is related to the angle of attack (AOA) in long-distance flight, for which the average value is 6 \u0026deg;. The AOA, defined as the angle between the bird\u0026rsquo;s wing and the oncoming airflow, is a critical factor in maintaining flight equilibrium.\u003c/p\u003e \u003cp\u003eIn equilibrium flight, the upward lift (\u003cem\u003eL\u003c/em\u003e) equals the downward weight (\u003cem\u003eW\u003c/em\u003e), allowing Eq.\u0026nbsp;(1) to be rewritten as:\u003c/p\u003e \u003cp\u003e \u003cem\u003eW\u0026thinsp;=\u0026thinsp;0.3 ρ V\u003c/em\u003e \u003csup\u003e \u003cem\u003e2\u003c/em\u003e \u003c/sup\u003e \u003cem\u003eS\u003c/em\u003e (2)\u003c/p\u003e \u003cp\u003eSince all flying occurs on Earth and is affected by terrestrial gravity, weight can be expressed as the magnitude of the downward gravitational force (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{F}_{g}\\)\u003c/span\u003e\u003c/span\u003e) using Newton\u0026rsquo;s second law:\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:W=\\left|{F}_{g}\\right|=m.g$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eWhere \u003cem\u003em\u003c/em\u003e is the mass of the bird in kg and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:g\\)\u003c/span\u003e\u003c/span\u003e is the Earth\u0026rsquo;s gravitational acceleration (9.8 m/s\u0026sup2;). Combining equations (1), (2), and (3) yields;\u003c/p\u003e \u003cp\u003e \u003cem\u003eL\u0026thinsp;=\u0026thinsp;m.\u003c/em\u003e \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:g\\)\u003c/span\u003e\u003c/span\u003e \u003cem\u003e= 0.3 ρ V\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e \u003cem\u003eS\u003c/em\u003e (4)\u003c/p\u003e \u003cp\u003eEquation (4) indicates that during gliding flight at constant velocity (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:V\\)\u003c/span\u003e\u003c/span\u003e) and constant air density (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\rho\\:\\)\u003c/span\u003e\u003c/span\u003e) at a fixed altitude, the upward lift (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:L\\)\u003c/span\u003e\u003c/span\u003e) is balanced by the downward weight. Accordingly, flying under a constant gravitational acceleration (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:g\\)\u003c/span\u003e\u003c/span\u003e) is critical for maintaining a balanced, low-energy flight. This is consistent with the temporal stability and spatial homogeneity of Earth\u0026rsquo;s gravitational field.\u003c/p\u003e \u003cp\u003eFurthermore, Eq.\u0026nbsp;(4) highlights that gravitational acceleration (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:g\\)\u003c/span\u003e\u003c/span\u003e) is an essential component of the high-altitude migration equation and is expected to play a significant role in long-distance flight. Recent research and technological developments also suggest that gravitational navigation could, in certain contexts, serve as an alternative or complement to satellite-based navigation systems.\u003c/p\u003e \u003cp\u003eWang et al. (\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2018\u003c/span\u003e) demonstrated that homing pigeons are capable of sensing extremely small variations in gravitational force, as low as 0.0001 Gal (0.1 mGal), despite the presence of significant inertial forces generated by the mechanical movement of the bird\u0026rsquo;s body during flight\u0026mdash;specifically, the vertical acceleration caused by wing flapping, which is approximately 0.7 Gal.\u003c/p\u003e \u003cp\u003eDornfeldt (1991) found that gravity anomalies are the strongest geophysical predictors of poor initial orientation and homing performance in pigeons. In most multivariate regression models, variables related to gravity provided the best fit to navigational parameters, suggesting that the observed patterns may warrant consideration of a causal relationship.\u003c/p\u003e \u003cp\u003eThese observations are consistent with the gravity vector hypothesis (\u003cem\u003eGVH\u003c/em\u003e) proposed by Kanevskyi (2023). His experiments with homing pigeons revealed instances of disorientation at the boundaries of gravity anomalies, where strong horizontal gradients of the gravitational field influenced flight trajectories.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eBased on observed patterns and the theoretical relationship between lift and gravitational acceleration, the author proposes a strong correlation between Earth\u0026rsquo;s gravitational field and wader/shorebird migratory flyways. These flyways tend to follow regions of relatively stable gravitational acceleration, suggesting a potential gravitational navigation system. A global-scale experiment is recommended to rigorously test and validate this proposed correlation, taking into account factors such as bird species, flight altitude, and bird body mass.\u003c/p\u003e"},{"header":"Abbreviations","content":"\u003cdiv class=\"DefinitionList\"\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eNASA\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eNational Aeronautics and Space Administration\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eGRACE\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eGravity Recovery and Climate Experiment\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eGMC\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eGravity Map Service\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eUSGS\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eUnited States Geological Survey\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eL\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eForce of lift\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eS\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003ebird\u0026rsquo;s wing area\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eV\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eair speed\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eρ\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eair density\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eAOA\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eAngle Of Attack\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eW\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eWeight\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{F}_{g}\\)\u003c/span\u003e\u003c/span\u003e\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eForce of Gravity\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003e\u003cem\u003em\u003c/em\u003e\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003emass of the bird\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003ekg\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eKilogram\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:g\\)\u003c/span\u003e\u003c/span\u003e\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eEarth\u0026rsquo;s gravitational acceleration\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003emGal\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003emilligal\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003c/div\u003e"},{"header":"Declarations","content":"\u003ch2\u003eFunding:\u003c/h2\u003e \u003cp\u003eThe author declares he didn\u0026rsquo;t receive funding for this study.\u003c/p\u003e\u003ch2\u003eAvailability of data and materials:\u003c/h2\u003e \u003cp\u003edata and materials used and/analyzed during the current study are available in:\u003c/p\u003e \u003cp\u003e \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://gravityservices.com/?page_id=92\u003c/span\u003e\u003cspan address=\"https://gravityservices.com/?page_id=92\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttp://www.youtube.com/watch?v=PHGRdovNGmM\u003c/span\u003e\u003cspan address=\"http://www.youtube.com/watch?v=PHGRdovNGmM\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e \u003cp\u003e \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://www.jpl.nasa.gov/images/pia04652-new-views-of-earths-gravity-field-from-grace/\u003c/span\u003e\u003cspan address=\"https://www.jpl.nasa.gov/images/pia04652-new-views-of-earths-gravity-field-from-grace/\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e \u003c/p\u003e \u003cp\u003e \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://www.usgs.gov/3d-national-topography-model\u003c/span\u003e\u003cspan address=\"https://www.usgs.gov/3d-national-topography-model\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e \u003c/p\u003e \u003cp\u003e \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://vtecostudies.org/blog/live-updates-tracking-upland-sandpiper-trans-hemispheric-migration/\u003c/span\u003e\u003cspan address=\"https://vtecostudies.org/blog/live-updates-tracking-upland-sandpiper-trans-hemispheric-migration/\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e \u003c/p\u003e \u003cp\u003e \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://vtecostudies.org/blog/live-updates-tracking-upland-sandpiper-trans-hemisphericmigration/\u003c/span\u003e\u003cspan address=\"https://vtecostudies.org/blog/live-updates-tracking-upland-sandpiper-trans-hemisphericmigration/\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e \u003c/p\u003e"},{"header":"References","content":"\u003cp\u003eArmstrong C, Wilkinson H, Meade J, Biro D, Freeman R, Guilford T. 2013. 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Pigeon homing in relation to geomagnetic, gravitational, topographical, and meteorological conditions. Behavioral Ecology Sociobiology 28:107-123.\u003c/p\u003e\n\u003cp\u003eGMS, Gravity Map Service, https://gravityservices.com/?page_id=92, accessed in June 2025.\u003c/p\u003e\n\u003cp\u003eGriffin DR, Hopkins CR .1976. Sounds audible to migrating birds. Animal Behavior 22: 672-678.\u003c/p\u003e\n\u003cp\u003eHagstrum JT .2013. Atmospheric propagation modeling indicates homing pigeons use loft-specific infrasonic ‘map’ cues. Journal of Exploration Biology 216: 687-699.\u003c/p\u003e\n\u003cp\u003eHolland R A. 2014. True navigation in birds: from quantum physics to global migration. Journal of Zoology, 293:1–15. h t t p s : / / d o i . o r g / 1 0 . 1 1 1 1 / j z o . 1 2 1 0 7. \u003c/p\u003e\n\u003cp\u003eHolland R A. 2003. The role of visual landmarks in the avian familiar area map. 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Report on the Conservation Status of Migratory Waterbirds of the East Asian – Australasian Flyway. First Edition. Report to the East Asian – Australasian Flyway Partnership. Wetlands International, Ede, The Netherlands. URL: https://www.wetlands.org/eaaf-conservation-status-review1/\u003c/p\u003e\n\u003cp\u003eNASA, 2003, New views of Earth’s gravity field from GRACE, https://www.jpl.nasa.gov/images/pia04652-new-views-of-earths-gravity-field-from-grace/, accessed in July 2025.\u003c/p\u003e\n\u003cp\u003ePapi F, Fiore L, Fiaschi V, Benvenuto S .1972. Olfaction and homing in pigeons. Monit Zool Ital (NS) 6:85–95.\u003c/p\u003e\n\u003cp\u003ePapi F, Ioalé P, Fiaschi V, Benvenuti S, Baldaccini NE .1978. Pigeon homing: cues detected during the outward journey influence initial orientation. In: Schmidt-Koenig K, Keeton WT (eds) Animal migration navigation and homing. Springer, Berlin, pp 63–77.\u003c/p\u003e\n\u003cp\u003ePapi F, Fiore L, Fiaschi V and Benvenuti S .1971. The influence of olfactory nerve section on the homing capacity of carrier pigeons. Monitore zool. Ital. (N.S.), 5, 265-267.\u003c/p\u003e\n\u003cp\u003ePapi F, Fiore L, Fiaschi V. and Benvenuti S. 1973. An experiment for testing the hypothesis of olfactory navigation of homing pigeons. J. comp. Physiol., 83, 93-102. \u003c/p\u003e\n\u003cp\u003eSchmidt-Koenig K, and Walcott C.1978. Tracks of pigeons homing with frosted lenses, Animal Behaviour, Volume 26, Part 2, Pages 480-486. https://doi.org/10.1016/0003-3472(78)90065-9.\u003c/p\u003e\n\u003cp\u003eTennekes H.2009. The simple science of flight: from insects to jumbo jets. The MIT Press. ISBN 978-0-262-51313-5. 201 pp. \u003c/p\u003e\n\u003cp\u003eUSGS, USA Topographic map, https://www.usgs.gov/3d-national-topography-model, accessed August 2025.\u003c/p\u003e\n\u003cp\u003eVermont Center for Ecostudies: https://vtecostudies.org/blog/live-updates-tracking-upland-sandpiper-trans-hemispheric-migration/. Accessed in July 2025.\u003c/p\u003e\n\u003cp\u003eWalker MM, Dennis TE, Kirschvink JL .2002. The magnetic sense and its use in long-distance navigation by animals. Curr Opin Neurobiol 12:735–744.\u003c/p\u003e\n\u003cp\u003eWallraff HG.1966. Über die Heimfondelleistungen von Brieftauben nach Haltung in verschiedenartig abgeschirmten Volieren . Z. vergl. Physiol., 52, 215-259.\u003c/p\u003e\n\u003cp\u003eWang Y, Tobalske BW, Deng X .2018. Gravitation-enabled Forward Acceleration during Flap-bounding Flight in Birds, Journal of Bionic Engineering, Volume 15, pages 505–515.\u003c/p\u003e\n\u003cp\u003eWiltschko R, Wiltschko W .2013. The magnetite-based receptors in the beak of birds and their role in avian navigation. J Comp Physiol A 199:89–98.\u003c/p\u003e\n\u003cp\u003eWiltschko R, Wiltschko W .2014. Sensing magnetic directions in birds: radical pair processes involving cryptochrome. Biosensors 4:221-242.\u003c/p\u003e\n\u003cp\u003eWiltschko R, Wiltschko W. 2023. Animal navigation: how animals use environmental factors to find their way. Eur Phys J Special Top. 232:237-52. h t t p s : / / d o i . o r g / 1 0 . 1 1 4 0 / e p j s / s 1 1 7 3 4 - 0 2 2 - 0 0 6 1 0 - w. \u003c/p\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"University of Nebraska-Lincoln","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Shorebird, Migration flyways, Earth’s gravity","lastPublishedDoi":"10.21203/rs.3.rs-8398499/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8398499/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe present study identifies a potential link between wader/shorebird migratory flyways and the Earth\u0026rsquo;s gravity field, based on five key observations. The first four observations demonstrate a strong association between global migration routes and the stability of the underlying Earth\u0026rsquo;s gravitational field. The fifth observation presents a theoretical analysis of the relationship between Earth\u0026rsquo;s gravitational acceleration and the upward lift force during steady flight. Collectively, these findings suggest that (1) maintaining a constant gravitational acceleration is essential for efficient, low-energy flight, and (2) global wader/shorebird migratory flyways tend to follow routes characterized by relatively stable gravitational acceleration, potentially functioning as a form of gravitational navigation system.\u003c/p\u003e","manuscriptTitle":"Potential Coincidence Between Wader/Shorebird Migration Flyways and Earth’s Gravity Field","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-12-22 05:58:36","doi":"10.21203/rs.3.rs-8398499/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"5baceb3c-af6c-46ca-b8d0-c5ec47e21ec9","owner":[],"postedDate":"December 22nd, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":59912586,"name":"Biophysics"},{"id":59912587,"name":"Animal Science"},{"id":59912588,"name":"Geophysics"}],"tags":[],"updatedAt":"2025-12-22T05:58:36+00:00","versionOfRecord":[],"versionCreatedAt":"2025-12-22 05:58:36","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8398499","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8398499","identity":"rs-8398499","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}