On the use of stable oxygen isotope (δ18O) measurements for tracking avian movements in North America

Tracking migratory animals has benefitted using measurements of naturally occurring stable isotopes of hydrogen (δ²H) in keratinous tissues such as hair and feathers to link animal origins to continental patterns or isoscapes of δ²H in precipitation. However, for most taxa, much less information exists on the use of stable oxygen isotope ratios (δ¹⁸O) despite the fact that δ²H and δ¹⁸O are strongly linked in environmental waters through the meteoric relationship and the possibility of using both isotopes to infer greater information on origins and climatic conditions where tissues are grown. A fundamental requirement of using stable isotopes to assign individuals and populations to origins is the development of a rescaling function linking environmental food web signals to the tissue of interest and for birds, this has not been carried out. Here, we derived the relationship between H and O isotopes in known source feathers of 104 individuals representing 11 species of insectivorous passerines sampled across the strong precipitation isoscape of North America. We determined again a strong expected relationship between feather δ²H (δ²H_f) and long‐term amount‐weighted precipitation δ²H (δ²H_p; r² = 0.77), but the corresponding relationship between δ¹⁸O_f and δ¹⁸O_p was poor (r² = 0.32) for the same samples. This suggests that δ²H measurements are currently more useful for assignment of insectivorous songbirds to precipitation isoscapes but does not preclude other uses of the δ¹⁸O_f data. Currently, mechanisms responsible for the decoupling of H and O isotopes in food webs is poorly known, and we advocate a much broader sampling of both isotopes in the same keratinous tissues across precipitation isotope gradients and across taxa to resolve this issue and to increase the power of using water isotopes to track migratory animals.     

Natal origins and timing of migration of two passerine species through the southern Alps: inferences from multiple stable isotopes (δ2H, δ13C, δ15N, δ34S) and ringing data

Understanding spatial linkages between areas used by migratory animals during the annual cycle is fundamental to their conservation. Stable isotope measurements of animal tissues can be a valuable tool in understanding spatial connectivity and migration phenology of migratory wildlife. We inferred natal origins of two migratory passerines, European Pied Flycatcher Ficedula hypoleuca and European Robin Erithacus rubecula, captured during autumn migration in the Italian Alps, by combining feather δ²H (δ²H_f) and ring recovery data. We used a spatially explicit likelihood-based method to assign individuals to a precipitation δ²H surface calibrated to represent feather δ²H, together with the directional probability of origin from ring recoveries. The highest probabilities of origin for most individuals of both species were in central and north-eastern Europe. Seasonal trends in δ²H_f, which described the species’ migratory phenology through the Italian Alps, were correlated with feather δ¹³C, δ¹⁵N and δ³⁴S values, indicating strong spatial discrimination related to continental patterns for these isotopes. We demonstrate how this combined information can define catchment areas and migratory connectivity of birds intercepted in the Alps. We highlight the importance of ringing data in defining directional priors to define Bayesian-based probability surfaces using continental δ²H_f isoscapes, and how such information can be used to inform estimates of migratory connectivity.     

Tracking long‐distance migration of marine fishes using compound‐specific stable isotope analysis of amino acids

The long‐distance migrations by marine fishes are difficult to track by field observation. Here, we propose a new method to track such migrations using stable nitrogen isotopic composition at the base of the food web (δ¹⁵N_Base), which can be estimated by using compound‐specific isotope analysis. δ¹⁵N_Base exclusively reflects the δ¹⁵N of nitrate in the ocean at a regional scale and is not affected by the trophic position of sampled organisms. In other words, δ¹⁵N_Base allows for direct comparison of isotope ratios between proxy organisms of the isoscape and the target migratory animal. We initially constructed a δ¹⁵N_Base isoscape in the northern North Pacific by bulk and compound‐specific isotope analyses of copepods (n = 360 and 24, respectively), and then we determined retrospective δ¹⁵N_Base values of spawning chum salmon (Oncorhynchus keta) from their vertebral centra (10 sections from each of two salmon). We then estimated the migration routes of chum salmon during their skeletal growth by using a state‐space model. Our isotope tracking method successfully reproduced a known chum salmon migration route between the Okhotsk and Bering seas, and our findings suggest the presence of a new migration route to the Bering Sea Shelf during a later growth stage.     

Incorporating site and year‐specific deuterium ratios (δ2H) from precipitation into geographic assignments of a migratory bird

The study of migratory connectivity is rapidly growing in ornithology, as is the technology used to measure it. While use of extrinsic markers, such as archival tags, is becoming more prevalent, for many small species the best tool available for tracking birds remains intrinsic markers, such as stable‐hydrogen isotope ratios (δ²H). Many researchers have raised concerns that spatial and temporal environmental variation introduces a large amount of error into isotope‐based assignments, limiting their utility. Here, using feathers, we sought to address these issues in developing δ²H base maps for assigning pied flycatchers Ficedula hypoleuca of known origin to 15 sites across the breeding range (approx. 4 020 800 km²). We evaluated the effects of including random site variation and year‐specific precipitation δ²H (δ²H_P) maps on assignments, compared to using mean annual growing season δ²H_p and no site effects. We found a positive correlation between feather δ²H (δ²H_F) and mean annual δ²H_P, resulting from large scale geographic variation. Repeatability of feather δ²H for individuals sampled in multiple years was strong overall, but variable among populations. Annual variation in δ²H_P explained 21% of within individual variation in δ²H_F. Neither year‐ nor site‐specific methods improved assignment precision or accuracy. All three methods assigned flycatchers of unknown origin captured at an African overwintering site to similar breeding areas. However, methods using long‐term means of δ²H_p assigned birds more precisely than year‐specific methods. Our results suggest that annual variation in this system is primarily a result of food web or individual level processes and that random site effects are not strong enough to drastically impact accuracy. We conclude that improvements in isotope based geographic assignments will rely on the addition of prior information, such as relative abundance in a Bayesian framework, or additional intrinsic markers.     

Small‐mammal isotope ecology tracks climate and vegetation gradients across western North America

Stable carbon, nitrogen, hydrogen and oxygen isotopes have been used to infer aspects of species ecology and environment in both modern ecosystems and the fossil record. Compared to large mammals, stable isotopic studies of small‐mammal ecology are limited; however, high species and ecological diversity within small mammals presents several advantages for quantifying resource use and organism–environment interactions using stable isotopes over various spatial and temporal scales. We analyzed the isotopic composition of hair from two heteromyid rodent species, Dipodomys ordii and Perognathus parvus, from localities across western North America in order to characterize dietary variation in relation to vegetation and climatic gradients. Significant correlations between the carbon isotopic composition (δ¹³C) of these species and several climatic variables imply that seasonal temperature and precipitation control the composition and distribution of dietary resources (grass seeds). Our results also suggest a moisture influence on the nitrogen isotopic composition (δ¹⁵N) of heteromyid diets. Population‐ and species‐level variation in δ¹³C and δ¹⁵N values record fine‐scale habitat heterogeneity and significant differences in resource use between species. Using classification and regression‐tree techniques, we modeled the geographic variation in heteromyid δ¹³C_diet values based on 10 climatic variables and generated an isotope landscape model (‘isoscape’). The isoscape predictions for δ¹³C_diet differ from expectations based on observed C₄ distributions and instead indicate that D. ordii and P. parvus record seasonally abundant grass resources, with additional model deviations potentially attributed to geographic variation in dietary selection. The oxygen and hydrogen isotopic composition of D. ordii is enriched relative to local meteoric water and suggests that individuals rely on highly evaporated water sources, such as seed moisture. Based on the climatic influences on vegetation and diet documented in this study, the isotopic composition of small mammals has high potential for recording ecological responses to environmental changes over short and long time scales.     

A stable isotope (δ 2H) approach to deriving origins of harvested woodcock (Scolopax rusticola) taken in France

A crucial but elusive aspect of the effective conservation and management of migratory birds is the determination of which regions or habitats contribute to the recruitment of juvenile birds into the adult breeding population or, in the case of hunted species, the annual harvest. Although ring recoveries have provided important information for several game species, limitations of this approach include the bias to only marked populations and the tremendous scale of effort required to recover enough individuals. Here, we explored the use of an intrinsic marker, the stable hydrogen isotope composition of feathers (δ ²H_f) of juvenile woodcock (Scolopax rusticola), as a means of assigning birds to natal origin using a woodcock-specific δ ²H_f isoscape. We applied this approach to 987 juvenile woodcock during the winters of 2005–2006 and 2006–2007, and 1875 juvenile woodcock sampled during autumn migration of 2005 and 2006. We used a likelihood-based assignment approach to determine origins from four regions. Considering only the feather isotope data, the majority (50 %) were assigned to origins in the Baltic—Western European Russia region (including western Russia) and 44 % assigned to central European origins. Few (6 %) were considered residents in France and <1 % were assigned to northern Fennoscandia and northern European Russia. Using the amount of forest cover available within zones as a prior resulted in greater emphasis on origins in the Baltic and western European Russia (62 %) and less emphasis (29 %) on central Europe as a potential origin. We also depicted origins of birds on a continuous isoscape surface which emphasised more extant forests of central and eastern Europe and western Russia. Results were relatively similar regardless of whether we considered samples collected during migration versus those collected during the wintering period. Spatial assignment of 51 woodcock sampled in Switzerland was consistent with the Baltic and central Europe. Our results are in general agreement with ring-recovery data and emphasise the utility of an isotope approach to assignment of gamebirds to origin.     

Intra‐population variation in the natal origins and wing morphology of overwintering western monarch butterflies Danaus plexippus

Understanding the natal origins of migratory animals is critical for understanding their population dynamics and conservation. However, quantitative estimates of population recruitment from different natal habitats can be difficult to assess for many species, especially those with large geographic ranges. These limitations hinder the evaluation of alternative hypotheses about the key movements and ecological interactions of migratory species. Here, we quantitatively investigated intra‐population variation in the natal origins of western North American monarch butterflies Danaus plexippus using spatial analyses of stable isotope ratios and correlations with wing morphology. A map of hydrogen isotope values in western monarch butterfly wings (δ²H_m) was estimated using a transfer function that relates the δ²H_m values of monarch butterfly wing keratin to a long‐term dataset of precipitation isotope (δ²H_p) values across the western United States. Isotopic analyses of 114 monarch butterfly wings collected at four California overwintering locations indicated substantial individual variation in natal origins, with most recruitment coming from broad regions along the Pacific coast, the southwestern US and the northern intermountain region. These observed patterns may partially resolve and reconcile several past hypotheses about the natal origins of western monarch butterflies, while also raising new questions. More negative δ²H_m values (associated with longer migratory distance) were significantly correlated with larger forewing sizes, consistent with expectations based on the aerodynamic and energetic costs of long‐distance migration, while analyses of wing shape suggest potential differences in the movement behaviors and constraints observed in the western range, compared with previous observations in eastern North America. Taken together, the results of this study indicate substantial individual variation in the natal origins of overwintering western monarch butterflies, suggesting both local and long‐distance movement to overwintering sites.     

Flyway population delineation in Taiga Bean Geese Anser fabalis fabalis revealed by multi‐element feather stable isotope analysis

Fundamental to effective management of migratory waterbird populations is an understanding of their flyway delineation. Taiga Bean Geese Anser fabalis fabalis wintering in NW Denmark, Scotland and England are considered to originate from northern and central Sweden, southern and central Norway (‘Western flyway’), those wintering in southern Sweden, NE and southern Denmark are considered to originate from northern Fennoscandia and western Russia (‘Central flyway’), and those wintering in eastern Germany and Poland (which show far less favourable conservation status) are thought to come from western Siberia (‘Eastern 1 flyway’), although evidence to demonstrate this has largely been lacking. Evidence for different natal and moult origins of Taiga Bean Geese was investigated using stable isotope analyses of feathers of four elements (δ²H, δ¹³C, δ¹⁵N and δ³⁴S). There were significant differences in isotopic composition of feathers from Swedish (Central) and German (Eastern 1) wintering populations and those moulting in Sweden in late summer (Western), which validated the three proposed major management flyway units above. The strong continental gradient in the stable hydrogen isotope ratios in precipitation (δ²H_p) across the region was used to assign wintering birds geospatially to natal and moulting origin, indicating separate natal and moulting areas for German (n = 37, from western Siberia) and Swedish (n = 20, Fennoscandia and more western Russia) wintering birds. These results confirm the largely discrete nature of these three flyways and contribute significantly to our ability to deliver effective targeted and appropriate research, monitoring and management actions throughout the ranges of these flyways.     

Connecting breeding and wintering grounds of Neotropical migrant songbirds using stable hydrogen isotopes: a call for an isotopic atlas of migratory connectivity

There is an overdue and urgent need to establish patterns of migratory connectivity linking breeding grounds, stopover sites, and wintering grounds of migratory birds. Such information allows more effective application of conservation efforts by applying focused actions along movement trajectories at the population level. Stable isotope methods, especially those using stable hydrogen isotope abundance in feathers (δ²H_f) combined with Bayesian assignment techniques incorporating prior information such as relative abundance of breeding birds, now provide a fast and reliable means of establishing migratory connectivity, especially for Neotropical migrants that breed in North America and molt prior to fall migration. Here we demonstrate how opportunistic sampling of feathers of 30 species of wintering birds in Cuba, Venezuela, Guatemala, Puerto Rico, and Mexico, regions that have typically been poorly sampled for estimating migratory connectivity, can be assigned to breeding areas in North America through both advanced spatial assignment to probability surfaces and through simpler map lookup approaches. Incorporating relative abundance information from the North American Breeding Bird Survey in our Bayesian assignment models generally resulted in a reduction in potential assignment areas on breeding grounds. However, additional tools to constrain longitude such as DNA markers or other isotopes would be desirable for establishing breeding or molt origins of species with broad longitudinal distributions. The isotope approach could act as a rapid means of establishing basic patterns of migratory connectivity across numerous species and populations. We propose a large‐scale coordinated sampling effort on the wintering grounds to establish an isotopic atlas of migratory connectivity for North American Neotropical migrants and suggest that isotopic variance be considered as a valuable metric to quantify migratory connectivity. This initiative could then act as a strategic template to guide further efforts involving stable isotopes, light‐sensitive geolocators, and other technologies.     

Conservation through connectivity: can isotopic gradients in Africa reveal winter quarters of a migratory bird?

Conservation of migratory wildlife requires knowledge of migratory connectivity between breeding and non-breeding locations. Stable isotopes in combination with geographical isotopic patterns (isoscapes) can provide inferences about migratory connectivity. This study examines whether such an approach can be used to infer wintering areas in sub-Saharan Africa, where we lack such knowledge for many species, but where this method has not been used widely. We measured δ ²H, δ ¹³C and δ ¹⁵N in winter-grown feathers of a breeding Swiss and Spanish population of European hoopoe Upupa epops—a typical Palaearctic-Afrotropical migrant. δ ²H values predicted that ~70 % of the hoopoes spent the non-breeding season in the western portion of their potential winter range. This was corroborated by a shallow east–west gradient in feather-δ ²H values of museum specimens from known African origin across the potential winter range and by the recovery of Swiss hoopoes marked with geolocators. Hoopoes categorized as from eastern versus western regions of the wintering range were further delineated spatially using feather δ ¹³C and δ ¹⁵N. δ ¹⁵N showed no trend, whereas adults were more enriched in ¹³C in the western portion of the range, with eastern adults being in addition more depleted in ¹³C than eastern juveniles. This suggests that eastern juveniles may have occupied more xeric habitats than sympatric adults. We demonstrated that stable isotopes, especially δ ²H, could only very roughly delineate the winter distribution of a trans-Saharan Palaearctic migrant restricted primarily to the Sahelian and savanna belt south of the Sahara. Further refinements of precipitation isoscapes for Africa as well the development of isoscapes for δ ¹³C and δ ¹⁵N may improve assignment of this and other migrants.     

Identifying critical habitat of the endangered vaquita (Phocoena sinus) with regional δ13C and δ15N isoscapes of the Upper Gulf of California,Mexico

The vaquita (Phocoena sinus) is the world's most endangered cetacean and has experienced a 60% reduction in the size of its population in the past decade. Knowledge of its basic ecology is essential for developing successful management plans to protect and conserve this species. In this study, we identified vaquita foraging areas by creating an isoscape of the Upper Gulf of California (UGC) based on sediment and zooplankton carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope values. Our results confirm that this species is confined to the western region of the UGC, which is characterized by relatively high δ¹⁵N values (sediments: 10.2‰ ± 2.0‰, zooplankton: 15.8‰ ± 1.3‰), higher sea surface temperatures (～16°C–25°C), higher concentrations of silt in sediments, and the highest turbidity. In contrast, the eastern region of the UGC had relatively low sediment (7.7‰ ± 2.4‰) and zooplankton (14.6‰ ± 1.0‰) δ¹⁵N values, and the highest concentrations of sand in sediments. Our approach is an effective use of marine isoscapes over a small spatial scale (<200 km) to identify the environmental characteristics that define the critical habitat for an extremely endangered marine mammal.     

Tracing site‐specific isotopic signatures along a Blue Tit Cyanistes caeruleus food chain

Food chains culminating with temperate insectivorous passerines are well described, yet whether trophic webs can be site‐specific remains a largely unexplored question. In the case of site‐ or habitat‐specificity of food webs, stable isotope signatures of bird feathers may enable assignment of unmarked individuals to a site or a habitat of origin. We address this question in landscapes that include contrasting forest habitat patches with either deciduous Downy Oak Quercus humilis or evergreen Holm Oak Quercus ilex as dominant tree species. First, we examine the spatial variation across habitats and sites in the stable isotope ratios of carbon (δ¹³C) and nitrogen (δ¹⁵N) along the oak leaf–Tortrix moth Tortrix viridana caterpillar–Blue Tit Cyanistes caeruleus food chain. Secondly, we assess whether the isotopic signatures allow for correct assignment of individual birds to their site or habitat of origin. At the scale of the landscape, stable isotope values enabled identification of the different components of the Blue Tit food chain: from oak leaves to Blue Tit nestlings and yearling birds. However, isotopic signatures were site‐specific (i.e. geographical) more than habitat‐specific (i.e. deciduous vs. evergreen oaks). Discriminant analyses correctly assigned 85% of nestlings and 83% of resident yearling birds, indicating a pronounced effect of site on Blue Tit feather isotopic signatures. We thereby demonstrate that isotopes reflect a stronger association of locally born birds to the local features of their habitat than that of un‐ringed yearling birds, whose plumage may have grown while in a wider geographical area. This study provides evidence of site‐specific isotopic signatures from oak leaves to Blue Tit feathers at a fine spatial scale.     

Wintering areas predict age‐related breeding phenology in a migratory passerine bird

Understanding connections between breeding, stopover and wintering grounds for long‐distance migratory birds can provide important insight into factors influencing demography and the strength of carry‐over effects among various periods of the annual cycle. Using previously described, multi‐isotope (δ¹³C, δ¹⁵N, δ²H) feather isoscapes for Africa, we identified the most probable wintering areas for house martins Delichon urbica breeding at Badajoz in southwestern Spain. We identified two most‐probable wintering areas differing in isotopic signature in west Africa. We found that the probability to winter in the isotopic cluster two was related to age and sex of individuals. Specifically, experienced males (i.e. two years or older) winter in the isotopic cluster two with a greater probability than experienced females, whereas first‐year females winter in the isotopic cluster two with a greater probability than first‐year males. In addition, wintering area was correlated with breeding phenology, with individuals wintering in the isotopic cluster two initiating their clutches earlier than those wintering in the isotopic cluster one. For birds wintering in the isotopic cluster two, there was no relationship between age and clutch initiation date. In contrast, young birds wintering in the isotopic cluster one initiated their clutches earlier than experienced birds wintering in this area. There was no significant correlation between wintering area and clutch size or the number of fledglings produced. We hypothesize that the relationship among social status, population density and winter habitat quality should be the most important driver of the carry‐over effect we found for this population.     

Toward a Deuterium Feather Isoscape for Sub-Saharan Africa: Progress,Challenges and the Path Ahead

A key challenge to the application of continent-wide feather isoscapes for geographic assignment of migrant birds is the lack of ground-truthed samples. This is especially true for long-distance Palearctic-Afrotropical migrants. We used spatially-explicit information on the δ ²H composition of archived feathers from Green-backed/Grey-backed Camaroptera, to create a feather δ ²H isoscape for sub-Saharan Africa. We sampled from 34 out of 41 sub-Saharan countries, totaling 205 sampling localities. Feather samples were obtained from museum collections (n = 224, from 1950 to 2014) for δ ²H assay. Region, altitude, annual rainfall and seasonal patterns in precipitation were revealed as relevant explanatory variables for spatial patterns in feather δ ²H. Predicted feather δ ²H values ranged from -4.0 ‰ to -63.3 ‰, with higher values observed in the Great Rift Valley and South Africa, and lower values in central Africa. Our feather isoscape differed from that modelled previously using a precipitation δ ²H isoscape and an assumed feather-to-precipitation calibration, but the relatively low model goodness fit (F_10,213 = 5.98, p<0.001, R² = 0.18) suggests that other, non-controlled variables might be driving observed geographic patterns in feather δ ²H values. Additional ground-truthing studies are therefore recommended to improve the accuracy of the African feather δ ²H isoscape.     

Geographical origin of dabbling ducks wintering in Iberia: sex differences and implications for pair formation

Natural and anthropogenic Iberian wetlands in southern Europe are well known for supporting large numbers of migratory Palaearctic waterbirds each winter. However, information on the geographical origin of dabbling ducks overwintering in these wetlands is scarce and mostly limited to data from ringing recoveries. Here, we used intrinsic isotopic markers to determine the geographical origin of male and female Northern Pintails Anas acuta and Eurasian Teal Anas crecca in Extremadura, inland Iberia, a key site for overwintering dabbling ducks. Additionally, we fitted six Northern Pintails with GPS‐GSM tags to complement the data derived from stable isotope analysis. Most (> 70%) first calendar‐year Northern Pintails were assigned to regions above 55°N, flying 2600–5600 km from their main natal regions to Extremadura. Mean values of δ²H_f varied significantly between male and female Northern Pintails, suggesting that the sexes had different geographical origins. Data from tagged adult Northern Pintails supported the isotopic data, one male flying more than 5000 km to the coast of the Pechora Sea (Russia). Most (> 70%) first calendar‐year Eurasian Teal were assigned to the region between 48° and 60°N, travelling 1500–4500 km to arrive in Extremadura. Male and female Eurasian Teal showed marginal differences in mean values of δ²H_f. In migratory dabbling ducks, pairing typically occurs on the wintering grounds, and ducks in their first winter can breed the following spring. For Northern Pintails, pair formation in Extremadura could occur between individuals with different geographical origins, which could contribute to the genetic variability of their offspring.     

The oxygen isotopic signature of soil‐ and plant‐derived sulphate is controlled by fertilizer type and water source

The oxygen isotope signature of sulphate (δ¹⁸O_sulphate) is increasingly used to study nutritional fluxes and sulphur transformation processes in a variety of natural environments. However, mechanisms controlling the δ¹⁸O_sulphate signature in soil–plant systems are largely unknown. The objective of this study was to determine key factors, which affect δ¹⁸O_sulphate values in soil and plants. The impact of an ¹⁸O‐water isotopic gradient and different types of fertilizers was investigated in a soil incubation study and a radish (Raphanus sativus L.) greenhouse growth experiment. Water provided 31–64% of oxygen atoms in soil sulphate formed via mineralization of organic residues (green and chicken manures) while 49% of oxygen atoms were derived from water during oxidation of elemental sulphur. In contrast, δ¹⁸O_sulphate values of synthetic fertilizer were not affected by soil water. Correlations between soil and plant δ¹⁸O_sulphate values were controlled by water δ¹⁸O values and fertilizer treatments. Additionally, plant δ³⁴S data showed that the sulphate isotopic composition of plants is a function of S assimilation. This study documents the potential of using compound‐specific isotope ratio analysis for investigating and tracing fertilization strategies in agricultural and environmental studies.     

Placing butterflies on the map – testing regional geographical resolution of three stable isotopes in Sweden using the monophagus peacock Inachis io

Stable isotope analyses of tissues have been used to help delineate natal regions and routes of migratory animals. The foundations of such studies are isotopic gradients or differences representing geographic regions and habitat used by the organism that are retained in selected tissues. We sampled peacock butterflies Inachis io on a regional level in southern Sweden to study natural variation and the resolving power of the stable isotope method to delineate individuals from known areas on a smaller scale than has typically been used in previous studies. Hydrogen (δD), carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopes were obtained from butterflies at seven different locations in an area of 250×250 km over three years (2002–2004). We found sufficient isotopic differences on this regional scale to delineate approximate origins. Of the three isotopes, deuterium showed good discrimination between sites, carbon isotopes showed weaker differentiation, whereas nitrogen isotopes proved unsuitable for small scale studies in this region due to high and unpredictable variation. We found there was enough variation in δD between years to prevent a general application of the technique to resolve sub‐regional variation. Substantial part of this variation was probably caused by seasonal changes in δD of precipitation. These differences produce significant variation in δD between years in animals having short and variable tissue development times, and are difficult to estimate in natural situations. We conclude that stable isotopes are potentially powerful predictors for studies of migratory butterflies in Europe. However, without good knowledge about the sampled individuals’ previous life‐history, a lot of the natural environmental variation in tissue δD cannot be controlled for. In the case of migratory species, this information is difficult to obtain, making the confidence intervals for prediction of natal areas fairly wide and probably only suitable for longer distance migration.     

Dual‐tracer‐based isotope turnover rates in a highly invasive mysid Limnomysis benedeni from Lake Constance

Understanding the ecological patterns of invasive species and their habitats require an understanding of the species’ foraging ecology. Stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope values provide useful information into the study of animal ecology and evolution, since the isotope ratios of consumers reflect consumer's dietary patterns. Nevertheless, the lack of species‐ and element‐specific laboratory‐derived turnover rates could limit their application. Using a laboratory‐based dual stable isotope tracer approach (Na¹⁵NO₃ and NaH¹³CO₃), we evaluated the δ¹⁵N and δ¹³C isotope turnover rates in full‐grown adult invasive Limnomysis benedeni from Lake Constance. We provide δ¹⁵N and δ¹³C turnover rates based on nonlinear least‐squares regression and posterior linear regression models. Model precisions and fit were evaluated using Akaike's information criterion. Within a couple of days, the δ¹⁵N and δ¹³C of mysids began to change. Nevertheless, after about 14 days, L. benedeni did not reach equilibrium with their new isotope values. Since the experiment was conducted on adult subjects, it is evident that turnover was mainly influenced by metabolism (in contrast to growth). Unlike traditional dietary shifts, our laboratory‐based dual stable isotope tracer approach does not shift the experimental organisms into a new diet and avoids dietary effects on isotope values. Results confirm the application of isotopic tracers to label mysid subpopulations and could be used to reflect assimilation and turnover from the labeled dietary sources. Field‐based stable isotope studies often use isotopic mixing models commonly assuming diet‐tissue steady state. Unfortunately, in cases where the isotopic composition of the animal is not in equilibrium with its diet, this can lead to highly misleading conclusions. Thus, our laboratory‐based isotopic incorporation rates assist interpretation of the isotopic values from the field and provide a foundation for future research into using isotopic tracers to investigate invasion ecology.     

Inferring origins of migrating insects using isoscapes: a case study using the true armyworm,Mythimna unipuncta,in North America

1. Many important insect pests undertake seasonal migrations at continental scales in response to changes in resource quality and availability. The frequency and timing of these events could be influenced by the impact of climate change on the suitability of the different sites exploited throughout the year, yet, in many cases, little is known about the origin of seasonal populations, as tracking insect movements is extremely challenging due to their small body size. 2. The use of stable isotope measurements in insect tissues combined with the development of tissue‐specific ‘isoscapes’ of modelled geographic isotope patterns presents a potentially valuable but rarely used approach for obtaining such information on important pest species. In this paper it is illustrated how stable hydrogen isotope analyses (δ²H) in wing chitin of the true armyworm (Mythimna (Psuedaletia) unipuncta Haworth), a seasonal migrant, clearly delineated between 2016 spring immigrants and later locally produced moths in southern Ontario, Canada. 3. It is shown that adult moths captured in early fall in Texas were immigrants from farther north, the first direct confirmation of a southward return migration of this species. Stable carbon isotope (δ¹³C) measurements indicated that spring immigrants in Ontario and autumn immigrants in Texas were from exclusively C3 biomes. Stable nitrogen isotope (δ¹⁵N) measurements also provided information on the probability of individuals coming from agriculturally intensive (i.e. higher δ¹⁵N) sites. Finally, several recommendations are provided regarding future research that could improve the Bayesian assignment models and thus improve assignment accuracy.     

Transpiration rate relates to within‐ and across‐species variations in effective path length in a leaf water model of oxygen isotope enrichment

Stable oxygen isotope ratio of leaf water (δ¹⁸O_L) yields valuable information on many aspects of plant–environment interactions. However, current understanding of the mechanistic controls on δ¹⁸O_L does not provide complete characterization of effective path length (L) of the Péclet effect, – a key component of the leaf water model. In this study, we collected diurnal and seasonal series of leaf water enrichment and estimated L in six field‐grown angiosperm and gymnosperm tree species. Our results suggest a pivotal role of leaf transpiration rate (E) in driving both within‐ and across‐species variations in L. Our observation of the common presence of an inverse scaling of L with E in the different species therefore cautions against (1) the conventional treatment of L as a species‐specific constant in leaf water or cellulose isotope (δ¹⁸O_p) modelling; and (2) the use of δ¹⁸O_p as a proxy for g_s or E under low E conditions. Further, we show that incorporation of a multi‐species L‐E scaling into the leaf water model has the potential to both improve the prediction accuracy and simplify parameterization of the model when compared with the conventional approach. This has important implications for future modelling of oxygen isotope ratios.