Steven L. Van Wilgenburg

Linking hydrogen (δ2H) isotopes in feathers and precipitation: sources of variance and consequences for assignment to isoscapes.

Background Tracking small migrant organisms worldwide has been hampered by technological and recovery limitations and sampling bias inherent in exogenous markers. Naturally occurring stable isotopes of H (δ2H) in feathers provide an alternative intrinsic marker of animal origin due to the predictable spatial linkage to underlying hydrologically driven flow of H isotopes into foodwebs. This approach can assess the likelihood that a migrant animal originated from a given location(s) within a continent but requires a robust algorithm linking H isotopes in tissues of interest to an appropriate hydrological isotopic spatio-temporal pattern, such as weighted-annual rainfall. However, a number of factors contribute to or alter expected isotopic patterns in animals. We present results of an extensive investigation into taxonomic and environmental factors influencing feather δ 2H patterns across North America. Principal Findings Stable isotope data were measured from 544 feathers from 40 species and 140 known locations. For δ 2H, the most parsimonious model explaining 83% of the isotopic variance was found with amount-weighted growing-season precipitation δ 2H, foraging substrate and migratory strategy. Conclusions/Significance This extensive H isotopic analysis of known-origin feathers of songbirds in North America and elsewhere reconfirmed the strong coupling between tissue δ 2H and global hydrologic δ 2H patterns, and accounting for variance associated with foraging substrate and migratory strategy, can be used in conservation and research for the purpose of assigning birds and other species to their approximate origin.

A method for investigating population declines of migratory birds using stable isotopes: origins of harvested lesser scaup in North America.

Background Elucidating geographic locations from where migratory birds are recruited into adult breeding populations is a fundamental but largely elusive goal in conservation biology. This is especially true for species that breed in remote northern areas where field-based demographic assessments are logistically challenging. Methodology/Findings Here we used hydrogen isotopes (δD) to determine natal origins of migrating hatch-year lesser scaup (Aythya affinis) harvested by hunters in the United States from all North American flyways during the hunting seasons of 1999–2000 (n = 412) and 2000–2001 (n = 455). We combined geospatial, observational, and analytical data sources, including known scaup breeding range, δD values of feathers from juveniles at natal sites, models of δD for growing-season precipitation, and scaup band-recovery data to generate probabilistic natal origin landscapes for individual scaup. We then used Monte Carlo integration to model assignment uncertainty from among individual δD variance estimates from birds of known molt origin and also from band-return data summarized at the flyway level. We compared the distribution of scaup natal origin with the distribution of breeding population counts obtained from systematic long-term surveys. Conclusions/Significance Our analysis revealed that the proportion of young scaup produced in the northern (above 60°N) versus the southern boreal and Prairie-Parkland region was inversely related to the proportions of breeding adults using these regions, suggesting that despite having a higher relative abundance of breeding adults, the northern boreal region was less productive for scaup recruitment into the harvest than more southern biomes. Our approach for evaluating population declines of migratory birds (particularly game birds) synthesizes all available distributional data and exploits the advantages of intrinsic isotopic markers that link individuals to geography.

Combining stable-isotope (δD) and band recovery data to improve probabilistic assignment of migratory birds to origin

The recent application of stable-isotope analyses, particularly the use of stable-hydrogen-isotope (deltaD) measurements of animal tissues, has greatly improved our ability to infer geographic origins of migratory animals. However, many individual sources of error contribute to the overall error in assignment; thus likelihood-based assignments incorporating estimates of error are now favored. In addition, globally, the nature of the underlying precipitation-based deltaD isoscapes is such that longitudinal resolution is often compromised. For example, in North America, amount-weighted expected mean growing-season precipitation deltaD is similar between the boreal forest of southwestern Canada and areas of northern Quebec/Labrador and Alaska. Thus, it can often be difficult to distinguish objectively between these areas as potential origins for broadly distributed migrants using a single isotopic measurement. We developed a Bayesian framework for assigning geographic origins to migrant birds based on combined stable-isotope analysis of feathers and models of migratory directions estimated from band recovery data. We outline our method and show an example of its application for assigning origins to a population of migrant White-throated Sparrows (Zonotrichia albicollis) sampled at a Canadian Migration Monitoring Network station at Delta Marsh, Manitoba, Canada. We show that likelihood-based assignments of geographic origins can provide improved spatial resolution when models of migration direction are combined with assignments based on deltaD analysis of feathers.

Advances in linking wintering migrant birds to their breeding-ground origins using combined analyses of genetic and stable isotope markers.

An enduring problem in avian ecology and conservation is linking breeding and wintering grounds of migratory species. As migratory species and populations vary in the degree to which individuals from distinct breeding locales mix on stop-over sites and wintering grounds, establishing migratory connectivity informs our understanding of population demography and species management. We present a new Bayesian approach for inferring breeding grounds of wintering birds of unknown origins in North America. We incorporate prior information from analysis of genetic markers into geographic origin assignment based upon stable-hydrogen isotope analysis of feathers (δ2Hf), using the Loggerhead Shrike (Lanius ludovicianus). Likely geographic origins derived from analyses of DNA microsatellites were used as priors for Bayesian analyses in which birds were assigned to a breeding-ground origin using their δ2Hf values. As with most applications of Bayesian methods, our approach greatly improved the results (i.e. decreased the size of the potential area of origin). Area of origin decreased by 3 to 5-fold on average, but ranged up to a 10-fold improvement. We recommend this approach in future studies of migratory connectivity and suggest that our methodology could be applied more broadly to the study of dispersal, sources of productivity of migratory populations, and a range of evolutionary phenomena.

Disease Dynamics and Bird Migration—Linking Mallards Anas platyrhynchos and Subtype Diversity of the Influenza A Virus in Time and Space

The mallard Anas platyrhynchos is a reservoir species for influenza A virus in the northern hemisphere, with particularly high prevalence rates prior to as well as during its prolonged autumn migration. It has been proposed that the virus is brought from the breeding grounds and transmitted to conspecifics during subsequent staging during migration, and so a better understanding of the natal origin of staging ducks is vital to deciphering the dynamics of viral movement pathways. Ottenby is an important stopover site in southeast Sweden almost halfway downstream in the major Northwest European flyway, and is used by millions of waterfowl each year. Here, mallards were captured and sampled for influenza A virus infection, and positive samples were subtyped in order to study possible links to the natal area, which were determined by a novel approach combining banding recovery data and isotopic measurements (δ2H) of feathers grown on breeding grounds. Geographic assignments showed that the core natal areas of studied mallards were in Estonia, southern and central Finland, and northwestern Russia. This study demonstrates a clear temporal succession of latitudes of natal origin during the course of autumn migration. We also demonstrate a corresponding and concomitant shift in virus subtypes. Acknowledging that these two different patterns were based in part upon different data, a likely interpretation worth further testing is that the early arriving birds with more proximate origins have different influenza A subtypes than the more distantly originating late autumn birds. If true, this knowledge would allow novel insight into the origins and transmission of the influenza A virus among migratory hosts previously unavailable through conventional approaches.

Positive Relationships Between Association Strength and Phenotypic Similarity Characterize the Assembly of Mixed-Species Bird Flocks Worldwide

Competition theory predicts that local communities should consist of species that are more dissimilar than expected by chance. We find a strikingly different pattern in a multicontinent data set (55 presence-absence matrices from 24 locations) on the composition of mixed-species bird flocks, which are important subunits of local bird communities the world over. By using null models and randomization tests followed by meta-analysis, we find the association strengths of species in flocks to be strongly related to similarity in body size and foraging behavior and higher for congeneric compared with noncongeneric species pairs. Given the local spatial scales of our individual analyses, differences in the habitat preferences of species are unlikely to have caused these association patterns; the patterns observed are most likely the outcome of species interactions. Extending group-living and social-information-use theory to a heterospecific context, we discuss potential behavioral mechanisms that lead to positive interactions among similar species in flocks, as well as ways in which competition costs are reduced. Our findings highlight the need to consider positive interactions along with competition when seeking to explain community assembly.

A Continent-Wide Migratory Divide in North American Breeding Barn Swallows (Hirundo rustica).

Populations of most North American aerial insectivores have undergone steep population declines over the past 40 years but the relative importance of factors operating on breeding, wintering, or stopover sites remains unknown. We used archival light-level geolocators to track the phenology, movements and winter locations of barn swallows (Hirdundo rustica; n = 27) from populations across North America to determine their migratory connectivity. We identified an east-west continental migratory divide for barn swallows with birds from western regions (Washington State, USA (n = 8) and Saskatchewan, Canada (n = 5)) traveling shorter distances to wintering areas ranging from Oregon to northern Colombia than eastern populations (Ontario (n = 3) and New Brunswick (n = 10), Canada) which wintered in South America south of the Amazon basin. A single swallow from a stable population in Alabama shared a similar migration route to eastern barn swallows but wintered farther north in northeast Brazil indicating a potential leap frog pattern migratory among eastern birds. Six of 9 (67%) birds from the two eastern populations and Alabama underwent a loop migration west of fall migration routes including around the Gulf of Mexico travelling a mean of 2,224 km and 722 km longer on spring migration, respectively. Longer migration distances, including the requirement to cross the Caribbean Sea and Gulf of Mexico and subsequent shorter sedentary wintering periods, may exacerbate declines for populations breeding in northeastern North America.

Contrasting assignment of migratory organisms to geographic origins using long‐term versus year‐specific precipitation isotope maps

Summary As a result of predictable large-scale continental gradients in the isotopic composition of precipitation, stable isotopes of hydrogen (δ2H) are useful endogenous markers for delineating long-distance movements of animals. Models to predict patterns of δ2H in precipitation (δ2Hp), and consequently determine likely geographic origin of migratory animals, have traditionally used static, amount-weighted long-term average values of δ2Hp over the growing season. However, animal tissues reflect H incorporated from food webs that integrate precipitation over a single years growing season or portions thereof. Inter-annual variation in precipitation and other climatic variables may lead to deviations from predictions derived from long-term mean precipitation isotopic values and could therefore lead to assignment errors for specific years and locations that are atypical. We examined whether using biologically relevant short-term δ2Hp isoscapes can improve estimates of geographic origin in comparison with long-term isoscapes. Using δ2H data from known-origin tissues of two migratory organisms in North America and Europe, we compared the accuracy, precision and similarity of assigned origins using both short- and long-term δ2Hp isoscapes. Relative to long-term δ2Hp isoscapes, using short-term isoscapes for assignment often resulted in dissimilar regions of likely origin but did not significantly improve accuracy or precision. This was likely due to reduced spatial coverage in the data used to generate the short-term δ2Hp isoscapes. We suggest that continued efforts to collect precipitation isotope data with a large spatiotemporal range will benefit future research on incorporating temporal variation in the amount and isotopic composition of precipitation into geospatial assignment models.

Origins of American Kestrels Wintering at Two Southern U.S. Sites: An Investigation Using Stable-Isotope (δD, δ18O) Methods

Abstract Stable-isotope analyses of feathers of migrant birds can help identify the locations where these birds bred or molted (i.e., breeding or molt origins). We examined stable hydrogen isotope (δD) values in feathers of American Kestrels (Falco sparverius) wintering at sites east (South Carolina) and west (Tennessee) of the Appalachian Mountains to assess their breeding origins. We used the dataset provided by Lott, and Smith (2006, Auk 123:822–835) to create a feather isotope basemap that considered error propagation associated with uncertainty in feather δD values and used the predicted mean growing-season precipitation δD of Bowen et al. (2005, Oecologia 143:337–348). We also measured feather δ18O on a subset of wintering kestrels and found that the general linear relationship found between feather δD and δ18O measurements deteriorates for feather δD more positive than −20‰. We suggest that δ18O measurements can be used to screen raptor feather isotope datasets to identify possible outliers that may unduly influence the assignment of birds to origin using feather δD basemaps. Our analyses indicated that the wintering population consisted of 13.1% ± 5.5% (SD) residents for the Tennessee site and 9.3% ± 3.5% (SD) residents for South Carolina. Migrants originated from the north-central portion of the U.S. breeding range (east of the Rocky Mountains) and southeastern Canada. This was consistent with leapfrog migration in this species.

A multi-isotope (δ2H, δ13C, δ15N) approach to establishing migratory connectivity in Palearctic-Afrotropical migrants: An example using Wood Warblers Phylloscopus sibilatrix

Abstract. Wood Warblers Phylloscopus sibilatrix have declined considerably throughout most of their north and western breeding range in Europe but the causes of this decline are unknown. Declines may be related to factors on the breeding grounds, stopover sites and/or wintering grounds. We used multi-isotope (&dgr;2H, &dgr;13C, &dgr;15N) measurements of winter-grown feathers of 314 individuals breeding in the Białowieża Forest (E Poland) to infer where they wintered in sub Saharan Africa over a 4-year period from 2009–2012. We used both aspatial and spatially specific assignment techniques involving a previously developed clustering algorithm related to long-term patterns of precipitation (&dgr;2H) and theoretical plant-based isoscapes (&dgr;13C, &dgr;15N) for Africa. We determined that our breeding population was consistently assigned to the forested region of the Congo basin. Males were more depleted in 13C and 2H and more enriched in 15N than females suggesting potential sexual habitat segregation on the wintering grounds. We then similarly examined less extensive samples from Wood Warblers breeding in England, Netherlands, Germany, Switzerland and Karelia (NW Russia), and found a similar assignment to the Congo basin. For all sites, males were isotopically distinct from females suggesting sex-specific habitat segregation on the wintering grounds. Our geospatial assignment model now provides a protocol for testing the hypothesis that declining populations winter more in heavily fragmented forests of west Africa compared to the Congo basin. We encourage this approach for the investigation of migratory connectivity in other sub Saharan Afrotropical migrants.