Improved estimates of certainty in stable-isotope-based methods for tracking migratory animals

Ecological Applications (Impact Factor: 4.09). 03/2008; 18(2):549-559. DOI: 10.1890/07-0058.1


The use of stable-hydrogen isotopes (delta D) has become a common tool for estimating geographic patterns of movement in migratory animals. This method relies on broad and relatively predictable geographic patterning in delta D values of precipitation, but these patterns are not estimated without error. In addition, delta D measurements are relatively imprecise, particularly for organic tissue. Most models for estimating geographic locations have ignored these sources of error. Common modeling approaches include regression, range-matching, and likelihood-based assignment tests (including discriminant analysis). Here, we show the benefits of a simple stochastic extension to likelihood-based assignment tests that incorporates two estimable sources of error and describe the resulting influence on the certainty of assigning breeding origins for wintering American Redstarts (Setophaga ruticilla), a small Nearctic-Neotropical migratory bird. Through simulation, we incorporated both spatial interpolation error associated with models of delta D in precipitation and analytical error associated with the measurement of delta D in tissue samples. In general, assignments that did not include these sources of error fell within the ranges of the stochastic results, but the difference in proportion of birds assigned to any one breeding region varied by as much as 54%. To explore how the distribution of assignments generated from error models influenced the application of these results, we developed a simple model of winter habitat loss. We removed the proportion of Redstarts wintering at a particular site from the global population and then used the isotope-based assignments to predict the resulting population declines for each breeding region. This gave distributions of change in population sizes, some of which included no change or even a population increase. The sources of error we modeled may challenge the degree of certainty in the use of stable-isotope-based data on connectivity to predict population dynamics of migratory animals. We suggest that stronger inference will result from incorporating these sources of error into future studies that use delta D or other stable isotopes to infer the geographic origin of individuals.

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    • "Direct assignment to a specifi c geographic location utilizing only δ 2 H values from feathers is not reliable because of interindividual diff erences in physiology, analytical error, and error associated with the isoscape to which birds are assigned (Wunder and Norris 2008). Th erefore, we created a probabilistic surface for a range of stable hydrogen isotope values of mean growing season precipitation ( δ 2 H p ) associated with two breeding destinations: southeast U.S., – 10 to – 50 ‰ , and boreal forest of Canada, – 70 to – 130 ‰ . "
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    ABSTRACT: We examined how conditions prior to migration influenced migration performance of two breeding populations of black-and-white warblers (Mniotilta varia) by linking information on the migrant's winter habitat quality, measured via stable carbon isotopes, with information on their breeding destination, measured via stable hydrogen isotopes. The quality of winter habitat strongly influenced the timing of migration when we accounted for differential timing of migration between breeding populations. Among birds migrating to the same breeding destination, males and females arriving early to the stopover site originated from more mesic habitat than later arriving birds, suggesting that the benefits of occupying high-quality mesic habitat during the winter positively influence the timing of migration. However, male warblers arriving early to the stopover site were not in better migratory condition than later arriving conspecifics that originated from poor-quality xeric winter habitat, regardless of breeding destination. The two breeding populations stopover at the study site during different time periods, suggesting that the lower migratory condition of early birds is not a function of the time of season, but potentially a migrant's migration strategy. Strong selection pressures to arrive early on the breeding grounds to secure high-quality breeding territories may drive males from high-quality winter habitat to minimize time at the expense of energy. This migration strategy would result in a smaller margin of safety to buffer the effects of adverse weather or scarcity of food, increasing the risk of mortality. The migratory condition of females was the same regardless of the timing of migration or breeding destination, suggesting that females adopt a strategy that conserves energy during migration. This study fills an important gap in our understanding of the linkages between winter habitat quality and factors that influence the performance of migration, the phase of the annual cycle thought to be limiting most migratory bird populations.This article is protected by copyright. All rights reserved.
    Journal of Avian Biology 03/2015; 46(5). DOI:10.1111/jav.00614 · 1.97 Impact Factor
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    • "The equation was generated using feather isotope values of breeding Wilson's Warblers from Paxton et al. (2007: Table 1) and stable hydrogen isotope values of precipitation from Bowen et al. (2005). Although statistical approaches have been designed to reduce uncertainty when mapping the origin of animals using stable hydrogen isotopes (Wunder and Norris 2008), our goal was not to directly map the origins of migratory birds, but to examine how broad-scale patterns of stable isotopes complement genetic data. "
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    ABSTRACT: Molecular markers and stable isotopes have provided important insights into the migratory connectivity of small landbirds. Research integrating these two methods has primarily focused on linking breeding and wintering sites, rather than focusing on timing of migratory movement of different breeding populations. We used mitochondrial DNA and isotopic markers to infer the timing of various breeding populations of migrating Wilson's Warblers (Cardellina pusilla) moving through a migratory stopover site, demonstrating the value of multiple sources of information in estimating the origin of migrants. Using mixed-stock analysis, we found that early spring migrants sampled in southwestern Arizona were dominated by warblers migrating to the West Coast of the contiguous United States, whereas later migrants included a large pulse of birds migrating to Alaska and western Canadian provinces. Stable hydrogen isotope data from individual birds showed the same timing pattern as genetic data. Had we used stable isotopes alone, we would not have been able to infer whether birds later in the migration season were most likely migrating to Alaska or the Interior West, given the large overlap in isotope values between those regions. The lack of mitochondrial group 2, common in the Interior West, in late-season migrants strongly suggests that these birds were migrating to breeding areas in Alaska or other northern regions. Studies that reveal the timing of migration of different breeding populations through stopover sites lay the foundation for more in-depth examination of seasonal interactions between migration and the stationary phases of the annual cycle.
    The Auk 10/2013; 130(4):689-698. DOI:10.1525/auk.2013.13107 · 1.86 Impact Factor
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    • "By obtaining a range of possible geographic origins for every individual, we allowed for uncertainty about connectivity patterns to be included in future population models, thus providing conservation managers with the ability to estimate the possible degree of change in population size that may occur as a consequence of habitat loss and/or conservation. However, even a small degree of uncertainty can infl uence whether models interpret population size to be increasing or declining, and detecting small population declines would therefore be diffi cult (Wunder and Norris 2008). Although a moderate level of connectivity exists between the population breeding in the Y-K Delta and the overwintering population in western Mexico (many individuals from western Mexico occur together in the Y-K Delta), western sandpipers generally show substantial mixing between the breeding and wintering periods, suggesting that it is unlikely that genetic population diff erentiation maintains latitudinal patterns of body size and age at fi rst breeding across the wintering range. "

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