Efficacy of Stable Isotope Ratios in Assigning Endangered Migrants to Breeding and Wintering Sites

Abstract

A primary constraint on effective conservation of migratory animals is our inability to track individuals through their annual cycle. One such animal is the endangered southwestern subspecies of the Willow Flycatcher, which is difficult to distinguish from conspecifics. Identifying wintering regions used by the endangered subspecies would be an important step in formulating an effective conservation strategy. Our objective was to use stable isotope ratios as a means of identifying wintering sites of Southwestern Willow Flycatchers. We analyzed stable isotope ratios of carbon, nitrogen, and hydrogen from feathers of breeding and wintering Willow Flycatchers. Based on winter samples, we document a positive trend in hydrogen isotope ratios across latitude. We also found that Willow Flycatchers use C4 food webs south of 8 degrees N latitude, but we found no evidence of use of C4 food webs farther north. Nitrogen stable isotope ratios of feathers showed no discernable geographic variation. Discriminant function analyses, based on stable isotope ratios of wintering Willow Flycatchers, were only useful (>50% accurate) for assigning individuals to winter regions if the regions were large and the threshold probability for assignment was relatively high. When using these discriminant functions, most breeding samples of Southwestern Willow Flycatchers were assigned to two wintering regions: central Mexico and Ecuador. We think that assignment of Southwestern Willow Flycatchers to Ecuador is unrealistic. Given the large percentages of samples that could not be classified with certainty, we are not confident that these two regions are truly more likely to harbor wintering Southwestern Willow Flycatchers than other winter regions. We think our inconclusive results are due primarily to weak and nonlinear gradients in isotope ratios across the winter range of Willow Flycatchers.

Mesoscale patterns of altitudinal tenancy in migratory wood warblers inferred from stable carbon isotopes

“…In migration studies, the assignment of individuals to a particular geographic locality depends on the robust mapping of isotopic gradients (Poage and Chamberlain 2001, Meehan et al 2004 as well as the spatial distribution of isotopically distinctive food resources within the potential geographic range of the species 3 E-mail: gravesg@si.edu (Romanek et al 2000, Wassenaar and Hobson 2000, Chamberlain et al 2005. Despite recent methodological advances in isotope ecology, continental-scale analyses of migratory birds continue to yield no better than coarse isotopic discrimination of populations, even when combinations of elements are examined (Rubenstein et al 2002, Royle and Rubenstein 2004, Hobson 2005a, Passey et al 2005, Wunder et al 2005, Rocque et al 2006, Kelly et al 2008, Wunder and Norris 2008. Failure to achieve finer spatial resolution may stem, in part, from the confounding effects of topography on continental isotope gradients of hydrogen, oxygen, and carbon, the most commonly investigated elements in avian migration studies (Hobson 2005a, West et al 2006.…”

Estimating transitions between states using measurements with imperfect detection: application to serological data

“…For example, the use of stable-isotope information may allow the classification of individuals as a function of their wintering area and/or feeding behavior, and thus, allow the exploration of potential behavioral changes and relationships with annual survival rates (Powell 2004, Bearhop et al 2005, Kelly et al 2008, Hoye et al 2012. In general, this approach can be used to study in a CR framework the dynamics of different individual states for which continuous covariates show different responses depending on the states.…”

“…For instance, a given measurement obtained using a specific immunological assay could be indicative of a probable seropositive status of the individual (Rossi et al 2010, Chambert et al 2012. As another example, a given stable-isotope ratio in a feather tissue sample could be indicative of a probable particular winter diet or location (Powell 2004, Bearhop et al 2005, Kelly et al 2008, Hoye et al 2012.…”

Bat‐cave catchment areas: using stable isotopes (δD) to determine the probable origins of hibernating bats

“…While informative priors can be developed from existing data and multi-isotope frameworks are already used with mixed successes (e.g., Kelly et al 2008), the sources of variance in isotopic assignments are still poorly characterized. Aquatic foraging is a source of isotopic variance of particular concern for little brown bats (Britzke et al 2009) and other organisms utilizing aquatic food webs because such foraging may decouple the relationship between tissue deuterium, dD, and dD p (Coulton et al 2000).…”