Keith A. Hobson

Tracing origins and migration of wildlife using stable isotopes: a review

Abstract To understand the ecology of migratory animals it is important to link geographic regions used by individuals including breeding, wintering, and intermediate stopover sites. Previous conventional approaches used to track animal movements have relied on extrinsic markers and typically the subsequent recovery of individuals. This approach has generally been inappropriate for most small, or non-game animals. The use of intrinsic markers such as fatty acid profiles, molecular DNA analyses, and the measurement of naturally occurring stable isotopes in animal tissues offer alternative approaches. This paper reviews the use of stable isotope analyses (primarily δ13C, δ15N, δ34S, δD, δ87Sr) to trace nutritional origin and migration in animals. This approach relies on the fact that foodweb isotopic signatures are reflected in the tissues of organisms and that such signatures can vary spatially based on a variety of biogeochemical processes. Organisms moving between isotopically distinct foodwebs can carry with them information on the location of previous feeding. Such an approach has been used to track animal use of inshore versus offshore, marine versus freshwater, terrestrial C3 versus marine, terrestrial mesic versus xeric, and C3 versus C4 or Crassulacean acid metabolism foodwebs. More recently, the use of stable hydrogen isotope analyses (δD) to link organisms to broad geographic origin in North America is based on large-scale isotopic contours of growing-season average δD values in precipitation. This technique, especially when combined with the assay of other stable isotopes, will be extremely useful in helping to track migration and movement of a wide range of animals from insects to birds and mammals. Future research to refine our understanding of natural and anthropogenic-induced isotopic gradients in nature, and to explore the use of stable isotopes of other elements, is recommended.

Using stable isotopes to determine seabird trophic relationships

1. The stable isotopes of nitrogen (δ 15 N) and carbon (δ 13 C) were analysed in 22 species of marine birds from coastal waters of the northeast Pacific Ocean. Analyses confirm that stable nitrogen isotopes can predict seabird trophic positions. 2. Based on δ 15 N analyses, seabird trophic-level inferences generally agree with those of conventional dietary studies, but suggest that lower trophic-level organisms are more important to several seabirds than was recognized previously. 3. Stable-carbon isotope analysis may be a good indicator of inshore vs. offshore feeding preference

Global application of stable hydrogen and oxygen isotopes to wildlife forensics.

Stable isotopes are being increasingly used in wildlife forensics as means of determining the origin and movement of animals. The heavy isotope content of precipitated water and snow (δDp, δ18Op) varies widely and systematically across the globe, providing a label that is incorporated through diet into animal tissue. As a result, these isotopes are potentially ideal tracers of geographic origin. The hydrogen and oxygen isotope tracer method has excellent potential where (1) spatial variation of precipitation isotopes exist, and (2) strong, mechanistic relationships link precipitation and isotope ratios in biological tissue. Here, we present a method for interpolation of precipitation isotope values and use it to create global basemaps of growing-season (GS) and mean annual (MA) δDp and δ18Op. The use of these maps for forensic application is demonstrated using previously published isotope data for bird feathers (δDf) in North America and Europe. The precipitation maps show that the greatest potential for applying hydrogen and oxygen isotope forensics exists in mid- to high-latitude continental regions, where strong spatial isotope gradients exist. We demonstrate that δDf/δDp relationships have significant predictive power both in North America and Europe, and show how zones of confidence for the assignment of origin can be described using these predictive relationships. Our analysis focuses on wildlife forensics, but the maps and approaches presented here will be equally applicable to criminal forensic studies involving biological materials. These maps are available in GIS format at http://www.waterisotopes.org.

Linking breeding and wintering grounds of neotropical migrant songbirds using stable hydrogen isotopic analysis of feathers

Abstract Recent studies have shown that stable hydrogen isotope ratios (δD) in the tissues of animals often correlate with δD of local precipitation. Here we examined the relationship between δD in feathers and growing season precipitation for neotropical migrant songbirds breeding over a continent-wide isotopic gradient. δD values were determined on feathers of 140 individuals of 6 species of wild insectivorous forest songbirds (Setophaga ruticilla, Empidonax minimus, Vermivora peregrinus, Catharus ustulatus, Seiurus aurocapillus, Hylocichla mustelina) taken from 14 breeding locations across North America. The δD of feathers was strongly correlated with the δD of growing season precipitation at breeding sites across North America. As feather hydrogen is metabolically inert after growth, this relationship was then used to assess the breeding origins of wintering migrants. Deuterium values of feathers from 64 individuals representing 5 species of migrants (Helmitheros vermivorus, Wilsonia citrina, Hylocichla mustelina, Dumetella carolinensis, Seirus aurocapillus) at a wintering site in Guatemala were consistent with those predicted from the known breeding ranges of these species. This study demonstrates hydrogen isotopes may become a powerful tool for linking breeding and wintering grounds of neotropical migrant songbirds, as well as other migratory species moving between isotopically distinct regions.

A stable isotope (δ13C, δ15N) model for the North Water food web: implications for evaluating trophodynamics and the flow of energy and contaminants

The North Water Polynya is an area of high biological activity that supports large numbers of higher trophic-level organisms such as seabirds and marine mammals. An overall objective of the Upper Trophic-Level Group of the International North Water Polynya Study (NOW) was to evaluate carbon and contaminant flux through these high trophic-level (TL) consumers. Crucial to an evaluation of the role of such consumers, however, was the establishment of primary trophic linkages within the North Water food web. We used δ15N values of food web components from particulate organic matter (POM) through polar bears (Ursus maritimus) to create a trophic-level model based on the assumptions that Calanus hyperboreus occupies TL 2.0 and there is a 2.4‰ trophic enrichment in 15N between birds and their diets, and a 3.8‰ trophic enrichment for all other components. This model placed the planktivorous dovekie (Alle alle) at TL 3.3, ringed seal (Phoca hispida) at TL 4.5, and polar bear at TL 5.5. The copepods C. hyperboreus, Chiridius glacialis and Euchaeta glacialis formed a trophic continuum (TL 2.0–3.0) from primary herbivore through omnivore to primary carnivore. Invertebrates were generally sorted according to planktonic, benthic and epibenthic feeding groups. Seabirds formed three trophic groups, with dovekie occupying the lowest, black-legged kittiwake (Rissa tridactyla), northern fulmar (Fulmarus glacialis), thick-billed murre (Uria aalge), and ivory gull (Pagophilia eburnea) intermediate (TL 3.9–4.0), and glaucous gull (Larus hyperboreus) the highest (TL 4.6) trophic positions. Among marine mammals, walrus (Odobenus rosmarus) occupied the lowest (TL 3.2) and bearded seal (Erignathus barbatus), ringed seal, beluga whale (Delphinapterus leucas), and narwhal (Monodon monoceros) intermediate positions (TL 4.1–4.6). In addition to arctic cod (Boreogadus saida), we suggest that lower trophic-level prey, in particular the amphipod Themisto libellula, contribute fundamentally in transferring energy and carbon flux to higher trophic-level seabirds and marine mammals. We measured PCB 153 among selected organisms to investigate the behavior of bioaccumulating contaminants within the food web. Our isotopic model confirmed the trophic magnification of PCB 153 in this high-Arctic food web due to a strong correlation between contaminant concentration and organism δ15N values, demonstrating the utility of combining isotopic and contaminant approaches to food-web studies. Stable-carbon isotope analysis confirmed an enrichment in 13C between POM and ice algae (–22.3 vs. –17.7‰). Benthic organisms were generally enriched in 13C compared to pelagic species. We discuss individual species isotopic data and the general utility of our stable isotope model for defining carbon flux and contaminant flow through the North Water food web.

NUTRITION, PHYSIOLOGY, AND STABLE ISOTOPES: NEW INFORMATION FROM FASTING AND MOLTING PENGUINS

Stable isotopes are increasingly used in animal ecology, but little attention has been paid to the underlying physiological processes accounting for changes in 15N/14N and 13C/12C ratios, for example, the influence of protein balance on δ15N values. We investigated a “professional” faster, the King Penguin (Aptenodytes patagonicus), to test the effect of long-term food deprivation on the isotopic signature of tissues that can be nondestructively sampled, i.e., blood and feathers. Fasting for 25 days induced a tissue 15N enrichment, thus leading to a moderate increase in the apparent trophic levels of penguins. As expected, 15N enrichment was higher in tissues with high protein turnover rates (e.g., plasma, 0.70‰) than in those with low turnover rates (e.g., blood cells, 0.24‰). Fasting decreased the δ13C value of plasma, which was due to an increase in its lipid content, as indicated by a concomitant rise in plasma C/N ratio. Finally, food deprivation induced a 15N enrichment in keratin (1.68‰), as indica...

Reconstructing avian diets using stable-carbon and nitrogen isotope analysis of egg components: patterns of isotopic fractionation and turnover

Because eggs are composed of nutrients that are ultimately derived from the diet of adult females, the relative abundance of naturally-occurring stable isotopes of carbon and nitrogen in eggs should be related to those in their diet and this may form the basis of a method for tracing diets. Before such dietary reconstructions can be established, however, it is necessary to know how isotopic signatures change (or fractionate) from the diet during the synthesis of various egg components. In this study, stable-carbon and nitrogen isotopic fractionation factors between diet and whole yolk, lipid-removed yolk, albumen, and shell membranes of eggs of captively-raised Mallards (Anas platyrhynchos), Japanese Quail (Coturnix japonica), Prairie Falcons (Falco mexicanus), Peregrine Falcons (Falco peregrinus), and Gyrfalcons (Falco rusticolis) were established. In addition, carbon isotope fractionation patterns between diet and eggshell carbonate and yolk lipid were determined for quail, Mallards and falcons. On average, yolk, albumen and membranes were enriched in 15 N relative to diet for all birds by 3.4‰, a value typical of processes associated with protein synthesis. For quail and Mallards, albumen, membranes, and shell carbonate were enriched in 13 C relative to diet by 1.5, 3.6, and 14.9‰, respectively, and whole yolk was depleted in 13 C by 2.6‰ due to the presence of yolk lipids. Falcons showed lower carbon diet-tissue fractionation for all tissues (albumen: +0.9, whole yolk: -1.9, membranes: +2.7, carbonate: +11.2‰) and this may be due to a greater reliance on dietary lipids vs. carbohydrates for the synthesis of egg components or on fractionation differences associated with fermentation of carbohydrates. Patterns of isotopic fractionation between diet and Mallard eggs were not influenced by clutch order. A diet-switch experiment using quail indicated that albumen, shell membrane and shell carbonate values reflect diet integrated over 3-5 days and yolk over eight days prior to laying. Because birds were fed ad libitum, these data provide baseline data on fractionation patterns for birds optimally mobilizing nutrients from diet to egg components. These values may differ for those wild birds that rely more heavily on endogenous reserves.

Isotopic Discrimination between Food and Blood and Feathers of Captive Penguins: Implications for Dietary Studies in the Wild

Using measurements of naturally occurring stable isotopes to reconstruct diets or source of feeding requires quantifying isotopic discrimination factors or the relationships between isotope ratios in food and in consumer tissues. Diet‐tissue discrimination factors of carbon (13C/12C, or δ13C) and nitrogen (15N/14N, or δ15N) isotopes in whole blood and feathers, representing noninvasive sampling techniques, were examined using three species of captive penguins (king Aptenodytes patagonicus, gentoo Pygoscelis papua, and rockhopper Eudyptes chrysocome penguins) fed known diets. King and rockhopper penguins raised on a constant diet of herring and capelin, respectively, had tissues enriched in 15N compared to fish, with discrimination factors being higher in feathers than in blood. These data, together with previous works, allowed us to calculate average discrimination factors for 15N between whole lipid‐free prey and blood and feathers of piscivorous birds; they amount to +2.7‰ and +4.2‰, respectively. Both fish species were segregated by their δ13C and δ15N values, and importantly, lipid‐free fish muscle tissue was consistently depleted in 13C and enriched in 15N compared to whole lipid‐free fish. This finding has important implications because previous studies usually base dietary reconstructions on muscle of prey rather than on whole prey items consumed by the predator. We tested the effect of these differences using mass balance calculations to the quantification of food sources of gentoo penguins that had a mixed diet. Modeling indicated correct estimates when using the isotopic signature of whole fish (muscle) and the discrimination factors between whole fish (muscle) and penguin blood. Conversely, the use of isotopic signatures of muscle together with discrimination factors between whole fish and blood (or the reverse) leads to spurious estimates in food proportions. Consequently, great care must be taken in the choice of isotopic discrimination factors to apply to wild species for which no controlled experiments on captive individuals have been done. Finally, our results also indicate that there is no need to remove lipids before isotopic analysis of avian blood.

Conserving migratory land birds in the New World: Do we know enough?

Migratory bird needs must be met during four phases of the year: breeding season, fall migration, wintering, and spring migration; thus, management may be needed during all four phases. The bulk of research and management has focused on the breeding season, although several issues remain unsettled, including the spatial extent of habitat influences on fitness and the importance of habitat on the breeding grounds used after breeding. Although detailed investigations have shed light on the ecology and population dynamics of a few avian species, knowledge is sketchy for most species. Replication of comprehensive studies is needed for multiple species across a range of areas, Information deficiencies are even greater during the wintering season, when birds require sites that provide security and food resources needed for survival and developing nutrient reserves for spring migration and, possibly, reproduction. Research is needed on many species simply to identify geographic distributions, wintering sites, habitat use, and basic ecology. Studies are complicated, however, by the mobility of birds and by sexual segregation during winter. Stable-isotope methodology has offered an opportunity to identify linkages between breeding and wintering sites, which facilitates understanding the complete annual cycle of birds. The twice-annual migrations are the poorest-understood events in a birds life. Migration has always been a risky undertaking, with such anthropogenic features as tall buildings, towers, and wind generators adding to the risk. Species such as woodland specialists migrating through eastern North America have numerous options for pausing during migration to replenish nutrients, but some species depend on limited stopover locations. Research needs for migration include identifying pathways and timetables of migration, quality and distribution of habitats, threats posed by towers and other tall structures, and any bottlenecks for migration. Issues such as human population growth, acid deposition, climate change, and exotic diseases are global concerns with uncertain consequences to migratory birds and even less-certain remedies. Despite enormous gaps in our understanding of these birds, research, much of it occurring in the past 30 years, has provided sufficient information to make intelligent conservation efforts but needs to expand to handle future challenges.

ARE GREATER SNOW GEESE CAPITAL BREEDERS? NEW EVIDENCE FROM A STABLE-ISOTOPE MODEL

The strategy of relying extensively on stored nutrient reserves for repro- duction (capital breeding) was thought to be common in large-bodied birds breeding in harsh environments, such as arctic-nesting geese, but this view has been challenged recently. Our objective was to model inputs to the eggs from stored reserves and from local food plants in Greater Snow Geese (Chen caerulescens atlantica) breeding in the high Arctic, using a new approach based on stable-isotope signatures. Snow Geese and their eggs were collected during laying from 1999 to 2001 (N = 66 females and 110 eggs). We analyzed the isotopic signature (813C and 815N) of egg constituents (lipid-free yolk, yolk lipid, and albumen), goose tissues (lipid-free breast muscles, abdominal fat, and whole liver) and of the food plants eaten by laying geese in the Arctic (graminoids and forbs). We applied a two-isotope mixing model approach to delineate nutrient input to eggs quantitatively. Dif- ferences in the isotopic signature of endogenous reserves and arctic food plants were relatively large (5.3-8.0%0 for A813C and 7.5%c for A815N) because reserves were accu- mulated in southern staging areas where geese feed in farmlands and estuarine habitats. The percentage of egg nutrients derived from exogenous sources (food consumed in the Arctic) was higher than from endogenous (body) reserves and varied little among the three years. Isotopic signatures indicated that endogenous reserves contributed 33% of lipid-free yolk nutrients, 27% of albumen, and 20% of yolk lipid, on average. Isotopic signatures of egg constituents of individual females were more strongly related to those of liver than endogenous sources (breast muscles or abdominal fat), indicating that the endogenous isotopic signature was diluted by a dietary input in the liver. We also found evidence of seasonal variation in the use of endogenous reserves. Late-laying females apparently in- vested proportionally more endogenous reserves in their eggs than did early layers, but not those laying larger clutches. We conclude that Greater Snow Geese use a mixed capital/ income breeding strategy. Our study shows that isotopic composition of tissues can be used to infer the contribution of exogenous vs. endogenous sources of nutrients for egg formation where inputs differ isotopically.