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There is increasing concern about the world's animal migrations. With many land‐use and climatological changes occurring simultaneously, pinning down the causes of large‐scale conservation problems requires sophisticated and data‐intensive approaches. Declining shorebird numbers along the East Asian–Australasian Flyway, in combination with data on habitat loss along the Yellow Sea (where these birds refuel during long‐distance migrations), indicate a flyway under threat. If habitat loss at staging areas indeed leads to flyway‐wide bird losses, we would predict that: (i) decreases in survival only occur during the season that birds use the Yellow Sea, and (ii) decreases in survival occur in migrants that share a reliance on the vanishing intertidal flats along the Yellow Sea, even if ecologically distinct and using different breeding grounds. Monitored from 2006–2013, we analysed seasonal apparent survival patterns of three shorebird species with non‐overlapping Arctic breeding areas and considerable differences in foraging ecology, but a shared use of both north‐west Australian non‐breeding grounds and the Yellow Sea coasts to refuel during northward and southward migrations (red knot Calidris canutus piersmai, great knot Calidris tenuirostris, bar‐tailed godwit Limosa lapponica menzbieri). Distinguishing two three‐month non‐breeding periods and a six‐month migration and breeding period, and analysing survival of the three species and the three seasons in a single model, we statistically evaluated differences at both the species and season levels. Whereas apparent survival remained high in north‐west Australia, during the time away from the non‐breeding grounds survival in all three species began to decline in 2011, having lost 20 percentage points by 2012. By 2012 annual apparent survival had become as low as 0·71 in bar‐tailed godwits, 0·68 in great knots and 0·67 in red knots. In a separate analysis for red knots, no mortality occurred during the migration from Australia to China. In the summers of low summer survival, weather conditions were benign in the Arctic breeding areas. We argue that rapid seashore habitat loss in the Yellow Sea is the most likely explanation of reduced summer survival, with dire (but uncertain) forecasts for the future of these flyway populations. This interpretation is consistent with recent findings of declining shorebird numbers at seemingly intact southern non‐breeding sites. Policy implications. Due to established economic interests, governments are usually reluctant to act for conservation, unless unambiguous evidence for particular cause–effect chains is apparent. This study adds to an increasing body of evidence that habitat loss along the Yellow Sea shores explains the widespread declines in shorebird numbers along the East Asian–Australasian Flyway and threatens the long‐term prospects of several long‐distance migrating species. To halt further losses, the clearance of coastal intertidal habitat must stop now.

Correlations between transequatorial migratory bird routes and bipolar biogeographic disjunctions in bryophytes suggest that disjunctions between northern and southern high latitude regions may result from bird-mediated dispersal; supporting evidence is, however, exclusively circumstantial. Birds disperse plant units (diaspores) internally via ingestion (endozoochory) or externally by the attachment of diaspores to the body (ectozoochory). Endozoochory is known to be the primary means of bird-mediated dispersal for seeds and invertebrates at local, regional, and continental scales. Data supporting the role of bird-mediated endozoochory or ectozoochory in the long distance dispersal of bryophytes remain sparse, however, despite the large number of bryophytes displaying bipolar disjunctions. To determine if transequatorial migrant shorebirds may play a role in the ectozoochory of bryophyte diaspores, we developed a method for screening feathers of wild birds. We provide the first evidence of microscopic bryophyte diaspores, as well as those from non-bryophyte lineages, embedded in the plumage of long distance transequatorial migrant birds captured in their arctic breeding grounds. The number of diaspores recovered suggests that entire migratory populations may be departing their northern breeding grounds laden with potentially viable plant parts and that they could thereby play significant roles in bipolar range expansions of lineages previously ignored in the migrant bird dispersal literature.

Long-distance migration, and the study of the migrants who undertake these journeys, has fascinated generations of biologists. However, many aspects of the annual cycles of these migrants remain a mystery as do many of the driving forces behind the evolution and maintenance of the migrations themselves. In this article we discuss nutritional, energetic, temporal and disease-risk bottlenecks in the annual cycle of long-distance migrants, taking a sandpiper, the red knot Calidris canutus, as a focal species. Red knots have six recognized subspecies each with different migratory routes, well-known patterns of connectivity and contrasting annual cycles. The diversity of red knot annual cycles allows us to discuss the existence and the effects of bottlenecks in a comparative framework. We examine the evidence for bottlenecks focusing on the quality of breeding plumage and the timing of moult as indicators in the six subspecies. In terms of breeding plumage coloration, quality and timing of prealternate body moult (from non-breeding into breeding plumage), the longest migrating knot subspecies, Calidris canutus rogersi and Calidris canutus rufa, show the greatest impact of bottlenecking. The same is true in terms of prebasic body moult (from breeding into non-breeding plumage) which in case of both C. c. rogersi and C. c. rufa overlaps with southward migration and may even commence in the breeding grounds. To close our discussion of bottlenecks in long-distance migrants, we make predictions about how migrants might be impacted via physiological 'trade-offs' throughout the annual cycle, using investment in immune function as an example. We also predict how bottlenecks may affect the distribution of mortality throughout the annual cycle. We hope that this framework will be applicable to other species and types of migrants, thus expanding the comparative database for the future evaluation of seasonal selection pressures and the evolution of annual cycles in long-distance migrants. Furthermore, we hope that this synthesis of recent advancements in the knowledge of red knot annual cycles will prove useful in the ongoing attempts to model annual cycles in migratory birds.

Migratory species rely on several distant sites during the annual cycle which makes their conservation more complex than that of non-migratory species. Even one of the most extensively studied migratory shorebirds - the Red Knot Calidris canutus - is currently ‘Near Threatened’ at the global level. Conflicting observations of migratory routes cast uncertainty on the subspecies classification and migratory connectivity of Red Knots in the Pacific coasts of the Americas. To fill essential information gaps, we present the first detailed population morphometrics of Red Knots during the non-breeding season in the southern Pacific coast, along with resightings of these birds throughout the Americas. We also estimated daily rate of weight gain during fuelling based on body mass at captures and known departure dates. Resightings demonstrate reliance on staging areas in both the Mid-continental and Atlantic flyways during the northward migration, and additionally in the Pacific Flyway during southward migration. In addition to the strong connection with several areas also used by C. c. rufa on the North American Atlantic coast, our results show morphometric differences within the ranges of both C. c. rufa and C. c. roselaari. Given the threats faced by Red Knots, the population in Chiloé Archipelago should be treated as a separate conservation unit within interhemispheric conservation programmes for endangered shorebirds within the Americas.

We investigated levels of arsenic mercury, lead, cadmium, and chromium in Red Knot ( Calidris canutus ), Semipalmated Sandpipers ( Calidris pusilla ), and Sanderling ( Calidris alba ) migrating through Delaware Bay, New Jersey to determine if contaminant levels are likely to be causing negative effects on the populations of these shorebirds, to compare among species, and to explore differences among individuals collected early and late during their migration stopover. We analyzed blood and feathers, both nonlethal ways of exploring contaminants in birds. Blood contaminant analysis provides a direct measure of recent dietary exposure, whereas feathers reflect body burden at the time of feather molt. We found some differences among species and between early and late samples. Levels of Hg and Pb were higher in Sanderling blood collected early (36.52 ± 8.45 and 145.00 ± 12.56 ng/g ww respectively) compared with later (16.21 ± 6.03 and 33.60 ± 4.05 ng/g ww respectively) during the migration stopover. Blood Pb levels of Sanderling in the early period were higher than those of the other two species (75.38 ± 15.52 ng/g ww in Red Knot and 42.39 ± 8.42 ng/g ww in Semipalmated Sandpipers). Semipalmated Sandpipers had lower blood As levels than the other two species (254.33 ± 40.15 and 512.00 ± 66.79 ng/g ww early and late respectively) but higher feather levels (914.01 ± 167.29 and 770.00 ± 116.21 ng/g dw early and late respectively), and their blood As was higher in the later sampling period compared with the early sampling period. Arsenic levels in shorebird tissues were relatively high and may reflect levels in horseshoe crab eggs, their primary diet item in Delaware Bay. In Red Knot, blood Cr levels were elevated in the later samples (572.17 ± 62.82 ng/g ww) compared to the early samples (382.81 ± 95.35 ng/g ww) and to the other species. The mean values of the metals analyzed were mostly below effect levels—the level that has a measurable negative impact—although relatively high As levels in Semipalmated Sandpiper feathers and some high levels of Pb need to be further explored.

Gut microbiota play a key role in host health. Mammals acquire gut microbiota during birth, but timing of gut microbial recruitment in birds is unknown. We evaluated whether precocial chicks from three species of arctic-breeding shorebirds acquire gut microbiota before or after hatching, and then documented the rate and compositional dynamics of accumulation of gut microbiota. Contrary to earlier reports of microbial recruitment before hatching in chickens, quantitative PCR and Illumina sequence data indicated negligible microbiota in the guts of shorebird embryos before hatching. Analyses of chick feces indicated an exponential increase in bacterial abundance of guts 0-2 days post-hatch, followed by stabilization. Gut communities were characterized by stochastic recruitment and convergence towards a community dominated by Clostridia and Gammaproteobacteria. We conclude that guts of shorebird chicks are likely void of microbiota prior to hatch, but that stable gut microbiome establishes as early as 3 days of age, probably from environmental inocula.

Dense congregations of shorebirds forage on tidal flats during long-distance migration, and their abundance is presumed to mirror the underlying ecological conditions. We quantified the nutritional content of intertidal biofilm (a thin layer of microalgae, bacteria, and other microorganisms embedded in a mucilaginous matrix that sits on the surface of tidal flats) to assess whether biofilm biomass, macronutrient content (lipid, protein, and carbohydrate), or both, provide a measure of habitat quality for migrating shorebirds. We compared shorebird use, biofilm biomass, macronutrient content, and stable isotope signatures at two mudflats on the Fraser River estuary, British Columbia, Canada, during summer 2020 (southward migration) and spring 2021 (northward migration). The abundances of Western Sandpiper (Calidris mauri) and Dunlin (Calidris alpina) were consistently higher (6–100×) at Brunswick Point relative to the Iona Foreshore site during both migration periods. Biofilm biomass (chlorophyll a) was 2× higher at Brunswick Point than Iona Foreshore during southward migration, and was similar between sites during northward migration, suggesting biofilm biomass alone was not a main determinant of shorebird use of these sites. Macronutrient content (lipid, protein, and carbohydrate) in intertidal biofilm was 1.4× to 3.8× higher at Brunswick Point than Iona Foreshore during both migration periods, indicating an association between macronutrient content and shorebird use. Carbohydrate content was higher during southward migration at both tidal flats, whereas protein and lipid content at Iona Foreshore did not show significant differences between years or migration periods. Carbon and nitrogen stable isotope signatures during spring 2021 did not differ between the two sites, suggesting that nutrients had similar relative inputs from marine and freshwater sources. While our comparative study does not eliminate alternative explanations that might act concomitantly (e.g., invertebrate abundance, predation risk, and disturbance), our results are consistent with the hypothesis that small-bodied shorebirds are more abundant at sites where intertidal biofilm has high macronutrient content. The latter can vary in complex ways depending on the taxonomic composition and physiological state of the microphytobenthos. Thus, the nutritional “quality” of intertidal biofilm can serve as a useful measure for determining restoration goals for tidal flats or assessing coastal areas for development projects.

The coast of Bohai Bay, north-western Yellow Sea, is critical for waterbirds migrating along the East Asia-Australasian Flyway. Between 1994 and 2010, a total of 450 km2 of offshore area, including 218 km2 of intertidal flats (one third of the original tidal area in the bay), has been reclaimed along the bay for two industrial projects. This has caused the northward migrants to become concentrated in an ever smaller remaining area, our core study site. The spring peak numbers of two Red Knot subspecies in the East Asia-Australasian Flyway, Calidris canutus piersmai and C. c. rogersi, in this so far little affected area increased from 13% in 2007 to 62% in 2010 of the global populations; the spring peak numbers of Curlew Sandpiper C. ferruginea increased from 3% in 2007 to 23% in 2010 of the flyway population. The decline in the extent of intertidal mudflats also affected Relict Gulls Larus relictus, listed by IUCN as ‘Vulnerable’; during normal winters 56% of the global population moved from the wintering habitats that were removed in Tianjin to the relatively intact areas around Tangshan. Densities of wintering Eurasian Curlew Numenius arquata, and spring-staging Broad-billed Sandpiper Limicola falcinellus and Sanderling Calidris alba have also increased in the remaining areas. With the proposed continuation of land reclamation in Bohai Bay, we predict waterbird densities in the remaining areas to increase to a point of collapse. To evaluate the future of these fragile, shared international resources, it is vital to promote an immediate conservation action plan for the remaining coastal wetlands in this region, and continued population monitoring to determine the effects of this action.

Competition intensity depends on the number of competitors and the amount of resources available. Coexistence of potential competitors can be enabled through niche differentiation or high resource availability. Using diet analysis, we investigated which of these 2 mechanisms was in play for coexisting shorebirds at a major staging site in the northern Yellow Sea, China, during northward migration in 2011 and 2012. Competition for food at this site is expected to be intense, with an estimated 250,000 migratory shorebirds gathering annually to refuel over a short period. Great Knots (Calidris tenuirostris), Eurasian Oystercatchers (Haematopus ostralegus osculans), and Red Knots (C. canutus) selected mostly the bivalve Potamocorbula laevis, whereas Bar-tailed Godwits (Limosa lapponica) had a broader diet and showed selection for polychaetes, even though most of their biomass intake was of P. laevis. Although all of these shorebirds fed on P. laevis, they showed different size selection and used different feeding methods. Bar-tailed Godwits, Great Knots, and Red Knots mainly swallowed P. laevis whole and preferred medium-sized P. laevis with relatively high ratios of flesh content to shell mass. By contrast, Eurasian Oystercatchers stabbed open P. laevis, ingested only the flesh, and preferred large P. laevis that provided the highest energetic return per prey taken. Despite evidence of niche differentiation in prey selection, the diets between the numerically dominant Bar-tailed Godwits and Great Knots overlapped substantially. Their coexistence seems to be enabled by high resource availability rather than niche separation.

Interannual mate and site fidelity is common in migratory shorebirds with monogamous mating systems. After long-distance migrations and separation during the winter, birds often relocate their former mate at their previous breeding territory. Although pairs frequently reunite, new pairs are also formed. Why birds change mates is still not completely understood. Mate change can involve active decisions, in which one or both mates actively chooses to divorce from a previous mate, but can also be related to arrival timing or mate availability at the breeding grounds. We explored possible causes of mate change in the Pacific subspecies of the migratory shorebird Dunlin (Calidris alpina pacifica) breeding at the subarctic Yukon-Kuskokwim Delta, Alaska, USA (61°36′N, 165°12′W). Interannual return rates of Dunlin to their breeding grounds were higher for males (74%) than for females (54%) and were 14% higher for birds with high previous breeding success. Mate change was rare if both birds returned to the breeding grounds in a consecutive breeding season: only 8% of all pairs divorced when previous mates were available. When former mates failed to return or returned late, however, many individuals formed new pairs (45% of males and 53% of females). Nest initiation dates were not delayed for new pairs compared to reuniting pairs, and nest survival did not differ between new and reuniting pairs; however, renesting after nest failure within a season was faster for reuniting pairs. We conclude that avoiding delayed nesting is a strong determinant of breeding decisions in Pacific Dunlin nesting in the short subarctic summer.