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The use of stable isotopes of carbon (δ13C) and nitrogen (δ15N) from feces and breath offers potential as non-destructive tools to assess diets and nutrition. How stable isotope values derived from breath and feces compare with those from commonly used tissues, such as blood fractions and liver, remains uncertain, including understanding the metabolic routing of dietary nutrients. Here, we measured δ13C and δ15N from feces and δ13C of breath from captive Red-necked Stints (Calidris ruficollis) and 26 species of wild-caught migratory shorebirds (n = 259 individuals) and compared them against isotopic values from blood and feathers. For captive birds fed either cereal- or fish-based diets, differences in δ13C between feces and lipid-free diet were small, − 0.2 ± 0.5‰ and 0.1 ± 0.3‰, respectively, and differences in δ15N, − 0.7 ± 0.5‰ and − 0.5 ± 0.5‰, respectively. Hence, δ13C and δ15N values from feces can serve as proxies for ingested proteinaceous tissues and non-soluble carbohydrates because isotopic discrimination can be considered negligible. Stable isotope values in plasma and feces were strongly correlated in wild-caught shorebirds, indicating feces can be used to infer assimilated macronutrients. Breath δ13C was 1.6 ± 0.8‰ to 5.6 ± 1.2‰ lower than bulk food sources, and breath C derived from lipids was estimated at 47.5% (cereal) to 96.1% (fish), likely underlining the importance of dietary lipids for metabolism. The findings validate the use of stable isotope values of feces and breath in isotopic assays to better understand the dietary needs of shorebirds.

A new field of shorebird feeding ecology has been opened, stemming directly from natural history observations of previously overlooked morphological and behavioral features. We describe how the pieces of this puzzle were assembled to reveal a fascinating story of small shorebird migration, trophic level shift, and direct feeding upon mudflat superficial biofilm just prior to breeding. Many unexplored avenues of research have arisen, including a trampoline effect of resource timing, the potential trade‐off between trophic imperatives and toxic co‐products, and the particular utilization of this resource in small, as opposed to large, shorebirds.

Intertidal biofilm is a thin layer of microbes and meiofauna enmeshed in an extracellular polymeric matrix within and on top of mudflat sediment. This medium provides a dynamic resource for a variety of consumers in estuarine habitats, and is rich in essential fatty acids that birds require for long-distance migration. We measured seasonal changes in biofilm fatty acid content from spring to summer on the Fraser River Estuary, one of the richest and most important ecosystems for migrant and wintering waterbirds in Canada. Fatty acid content in biofilm showed a strong seasonal pattern with a peak in the spring that is associated with the northward migration of Western Sandpipers (Calidris mauri) to their breeding grounds. This peak is linked to the abundance and physiological state, and hence nutritional condition of epipelic diatoms, which, in turn, depend on a combination of mudflat topography, salinity, temperature, and nutrients that can fluctuate widely with the freshwater inputs from the Fraser River. Specifically, areas with higher elevation (i.e. ~1 m) had longer periods of exposure to ambient conditions (i.e. light, warm temperatures, gas exchange into/out of biofilms) that facilitated more biofilm growth and higher fatty acid content. Moreover, springtime changes in water chemistry (i.e. salinity/osmotic stresses and nutrients) and temperature facilitated the production of higher overall total lipid/fatty acid contents in the mudflat biofilms compared to summer. Effective conservation of migrating shorebirds depends on the protection of underlying processes at important stopover sites that promote biofilm communities to escalate their production of lipids including essential fatty acids during key times of the year.

We show that a higher vertebrate can graze surficial intertidal biofilm, previously only considered a food source for rasping invertebrates and a few specialized fish. Using evidence from video recordings, stomach contents, and stable isotopes, we describe for the first time the grazing behavior of Western Sandpipers (Calidris mauri) and estimate that biofilm accounts for 45—59% of their total diet or 50% of their daily energy budget. Our finding of shorebirds as herbivores extends the trophic range of shorebirds to primary consumers and potential competitors with grazing invertebrates. Also, given individual grazing rates estimated at seven times body mass per day and flock sizes into the tens of thousands, biofilm-feeding shorebirds could have major impacts on sediment dynamics. We stress the importance of the physical and biological processes maintaining biofilm to shorebird and intertidal conservation.

Biofilm communities on intertidal mudflats are recognized as major producers of nutrients, especially fatty acids. The rising threats posed by both climatic and anthropogenic stressors increase the necessity of understanding and conserving these communities. Shorebirds provide a proxy for studying the complex ecology of biofilm communities because of their heavy reliance on fatty acids from diatomaceous biofilm for successful long-distance migration. Herein, we review biofilm feeding patterns by migratory shorebirds, experimental design considerations for sampling and studying the fatty acid content of biofilm, and the literature describing established and emerging analytical methodology. Techniques for fatty acid analysis include the commonly employed gas chromatography–flame ionization detection (GC/FID) and gas chromatography–mass spectrometry (GC/MS) with derivatization. Liquid chromatography–mass spectrometry (LC/MS) and liquid chromatography–quadrupole time of flight (LC/QTOF) are newly emerging techniques that enable derivatization to be eliminated. In addition, Fourier transform infrared spectroscopy (FT/IR), a common instrument in chemistry laboratories, has applications in fatty acid research, specifically for screening. Using a combination of sampling and analytical methods is necessary for improved understanding of intertidal biofilm, both as a source of essential fatty acids in aquatic systems and a critical food for shorebirds.

Western Sandpipers and Dunlin are capable of grazing biofilm. As there has been no assessment of this dietary constituent in stomach contents, the stomachs of 89 Western Sandpipers and 56 Dunlin collected during breeding migration through the Fraser River delta, British Columbia, Canada, were examined. Invertebrates, traditionally regarded as the principal prey, comprised a minor fraction of mean stomach volumes (Dunlin: <25%; Western Sandpiper <10%). Three phyla accounted for most of these invertebrates: molluscs, annelids and arthropods. In comparison, sediment (a mixture of sediment particles, broken and unbroken diatoms plus organic detritus) comprised the major component of stomach volumes (Dunlin: >40%; Western Sandpipers: >75%). Although the mean volume of sediment was significantly greater in Western Sandpipers than Dunlin, there was no effect of sex for either species. Stomach sediment volume appears a convenient index of biofilm intake and sediment loads indicative of biofilm grazing have been reported in stomach contents from other shorebird species. Re-examination of shorebird diets appears necessary given that conditions promoting biofilm are not necessarily conducive for invertebrate prey.

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.

Soft‐bottomed intertidal flats are essential foraging areas for shorebirds but are severely impacted by threats such as coastal development and climate change. Notwithstanding the urgency for humanintervention (conservation, restoration and creation) of tidal flats, few ecologically based technical guidelines exist for the artificial (clearly intended human intervention) intertidal flats, and none explicitly consider the unique properties of intertidal biofilm as a critical food source for small‐bodied shorebirds. We propose that effective human intervention in intertidal flat ecosystems can be developed through mirroring the needs of small‐bodied shorebirds. Scientific evidence from intertidal flat recovery projects in Japan is summarized, and foraging requirements of shorebirds are reviewed with a focus on intertidal biofilm as a critical food source. These findings are used to propose the primary goal of intervention, that is maximizing total energy intake for population recovery of small‐bodied shorebirds through biofilm. Three sub‐goals are presented for creating environmental conditions in which (1) a broad spectrum of food sources is available, but particularly intertidal biofilm; (2) maximizing energy intake rate per individual; and (3) maximizing foraging activity. We then describe seven key ecologically based technical attributes for artificial intertidal flats that promote use by small‐bodied shorebirds: depositional environment, complex shoreline, gentle slope, gradient of grain sizes from muddy to sandy, maximum water depth at the lowest tide 5 cm or less, freshwater inflow and unobstructed sight‐lines. Critical questions remain for effective intervention in intertidal flat ecosystems, including food web dynamics, variation in the quality and quantity of food sources, especially biofilm, optimal sedimentary environment systems (interaction between grain size, bed slope and elevation), monitoring involving comparisons with appropriate benchmark (control) habitats, quantifying foraging behaviour and the synergy and trade‐offs among ecosystem services. 干潟生態系における小型シギ・チドリ個体群の回復−バイオフィルムを考慮した7つの生態工学的要件 干潟はシギ・チドリ類にとって不可欠な採餌場であるが、沿岸開発や気候変動などによって深刻な影響を受けている。したがって、干潟への人為的介入 (保全・修復・創造) が急務である。それにもかかわらず、人為的介入を企図した生態工学的根拠に基づく技術ガイドラインはほとんど存在しない。あるいは、小型シギ・チドリ類の重要な餌源であるバイオフィルムの特性を明確に考慮したガイドラインもない。 そこで、本研究では最初に、日本の干潟再生プロジェクトから得られた科学的証拠をとりまとめ、重要な食料源としての干潟のバイオフィルムに焦点を当てて、シギ・チドリの採餌に必要な条件を整理した。 次に、これらの知見をもとに、小型シギ・チドリ類の個体群回復のための人為介入の最終目標として、「バイオフィルムを含めた総エネルギー摂取量の最大化」を本研究では提案した。さらに、最終目標を達成するために3つの副目標、「バイオフィルムをはじめとする様々な餌を採ることが可能な環境条件」、「個体あたりのエネルギー摂取量の最大化」、「採餌活動率の最大化」を設定し、副目標を達成するための鍵となる7つの干潟の属性、すわなち「砂泥が堆積しやすい環境」、「複雑な汀線」、「緩やかな海底勾配」、「泥質と砂泥質の両方を含む多様な海底」、「最干潮時の最大水深が5cm以下」、「淡水の流入」、「遮るもののない見通しの良さ」を、生態工学的根拠に基づき提案した。 最後に、干潟生態系へ効果的に介入するためには、食物網の動態、バイオフィルムを中心とした食物源の質と量の変化、最適な堆積物環境 (粒径、海底勾配、標高の3者間の相互作用) 、適切な対照生息地との比較などを含めたモニタリング、採餌行動の定量化、生態系サービスの相乗効果とトレードオフなど、様々な研究課題が残されていることを指摘した。 We describe seven key attributes for artificial intertidal flats that promote use by small‐bodied shorebirds: depositional environment, complex shoreline, gentle slope, appropriate sediment grain size, shallow water, freshwater inflow and unobstructed.

Coastal landscapes with extensive intertidal mudflats provide non-breeding habitat for Arctic shorebirds. Few attempts have been made to develop and test landscape-level models predicting the intertidal distribution of these birds. We modelled the distribution of a Holarctic species, Dunlin (Calidris alpina), at a hemispherically important non-breeding site, the Fraser River Delta, British Columbia, Canada, in seasons with different predator landscapes. We trained the models during a season when nocturnal predators were common and tested temporal transferability of the models on independent datasets when nocturnal predators were absent. Snowy Owls (Nyctea scandiaca) influenced Dunlin distribution and thus model transferability. After accounting for their presence, models displayed good to excellent discrimination, i.e. prediction of the instantaneous and cumulative (over low tide period) probability of mudflat use by Dunlin, in fore- and backcasting applications. Model calibration was good or else, where over-prediction was observed, the reason for the bias was identified. The distribution models may predict mudflat use by Dunlin and possibly related species given relevant data describing the intertidal landscape. The models are amenable to GIS application, describe the amount of use per hectare of the intertidal zone and can be used to determine and visualise relative and absolute suitability of intertidal areas.

We report that a latitudinal cline in intertidal food distribution is associated with the nonbreeding distribution of the Western Sandpiper (Calidris mauri). This novel result is the first to demonstrate a clear relationship between patterns of differential nonbreeding distribution and food availability for any shorebird species. Within each age class and sex, longer-billed Western Sandpipers winter further south. Moreover, females, the longer-billed sex, tend to winter south of males. Thus, both inter- and intra-sexual clines in bill morphology result in an overall gradient of increasing bill length from north to south. Longer-billed birds are able to extract prey that are buried more deeply in the sediment; therefore, we predicted shifts in the vertical distribution of food resources to coincide with the clines in bill morphology across the nonbreeding range. We tested our prediction by measuring biofilm density and the vertical distribution of macrofaunal invertebrates at six nonbreeding sites. Although no latitudinal trend was observed for biofilm, the vertical distribution of invertebrates was consistent with our prediction and revealed that the greatest relative abundance of surface prey occurred at northern nonbreeding sites and declined with decreasing latitude. We discuss the potential implications of these findings in the context of competing evolutionary hypotheses of differential migration and bill dimorphism in shorebirds.