An Appraisal of the Use of Hydrogen-Isotope Methods to Delineate Origins of Migratory Saw-whet Owls in North America (original) (raw)
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The Auk, 2009
Despite the widespread use of stable isotopes in studies of avian movement, key assumptions on which the methodology is based remain unsubstantiated, including the assumption that measurements of stable hydrogen isotopes in feathers (δD f ) are consistent across time within the same laboratory or among laboratories using the same analytical protocols and keratin standards. We tested this assumption by remeasuring δD f from 211 raptor feathers within and between laboratories. Initial and repeat samples were prepared and analyzed using identical protocols but analyzed in distinct automated runs with laboratory staff blind to sample identity. Reproducibility of δD f measurements varied significantly and substantially among nine independent sample groups. Feather δD measurements among sample groups exhibited average isotopic shifts from −15.6‰ to +27.5‰ (an absolute difference of 43.1‰), with standard deviations from 6.0‰ to 12.4‰. Therefore, despite existing analytical protocols to address issues of reproducibility, empirical data suggest that comparing δD f measurements among studies or labs and pooling samples analyzed during different automated runs within a laboratory remain problematic. More importantly, poor reproducibility compromises the geographic assignment of origins based on δD f , because the substantial differences in δD f measurements between automated runs can result in spurious inferences regarding the origins of migratory birds. We caution against the continued use of δD f for predicting geographic origin, and for addressing important conservation questions, until the factors affecting poor reproducibility are identified and improved reproducibility is demonstrated within and among laboratories across time and taxa.
Canadian Journal of Zoology, 2009
Stable hydrogen isotopes (dD) can be a powerful tool for estimating the large-scale movements of animals, but the cause and extent of isotopic variation within animal tissues remain poorly understood. Here, we simultaneously examined three hypotheses to explain dD variation in the blood (dDB) of nestling Tree Swallows (Tachycineta bicolor (Vieillot, 1808)) from a single nest-box population in southern Ontario, Canada: (1) microgeographic and (or) temporal variation in dD of local diet, (2) potential evaporation of protium resulting from variation in nest-box temperature, and (3) differences in body size resulting in higher rates of water loss and isotope fractionation in larger birds. dDB ranged from -128% to -94%, with a mean ± SD of -113% ± 7%. Nest-box temperatures ranged from 21.5 to 38 8C immediately prior to blood collection. A general linear mixed-effects model explained 80% of the variation in dDB and provided evidence that dD values in prey, maximum temperature 1 day prior to blood collection, and body size were all significant predictors of dDB. Our results suggest that isotopic variation in individuals and local populations arises from a combination of physiological factors and local environmental variation. To gain a complete understanding of how dD values can be used to estimate animal movements, additional work is needed to determine how these factors influence other tissues, such as metabolically inert feathers, and animals of different age classes.
Canadian Journal of Zoology, 2006
We used an analysis of deuterium values (dD) of 151 Wood Thrush (Hylocichla mustelina (J.F. Gmelin, 1789)) feathers collected during the breeding season at the Piedmont National Wildlife Refuge in Georgia, USA, to determine fidelity to the study site. We compared dD values in feathers of birds with known molt locations and birds with unknown molt locations. Mean feather value of dD was -24.8% (SD = 10.5%, range = -48.0% to -5.5%), and we were unable to determine a site-specific signature to assess fidelity of breeders within our sample. We used an information criterion approach to evaluate multiple hypotheses to explain the high variation in dD, and the geographic location of sample sites within the study area was selected as the best model. Feather dD values were higher than expected from mean growingseason rainfall dD values predicted for our study site. We discuss possible explanations for the enriched dD values and postulate that heat stress during molt may have contributed to our results. We suggest that future stable isotope data collection consider the potential for small-scale variation in feather dD values; information on diet gathered simultaneously with feather samples may be valuable for future studies.
Journal of Field Ornithology, 2009
Avian age-class discrimination is typically based on the completeness of the first prebasic molt. In several calidrid sandpiper species, juvenal flight feathers grown on Arctic breeding grounds are retained through the first three migrations. Thereafter, flight feathers are grown annually at temperate migratory stopover sites during the fall or on the subtropical wintering grounds. Standard methods for distinguishing age classes of sandpipers rely on a combination of traits, including body plumage, coloration of protected inner median covert edges, and extent of flight feather wear. We tested the ability of stable hydrogen isotope ratios in flight feathers (␦D f ) to distinguish young birds in their first winter through second fall from older adults in three calidrid sandpiper species, Western (Calidris mauri), Least (C. minutilla), and Semipalmated (C. pusilla) sandpipers. We compared the apparent reliability of the isotope approach to that of plumage-based aging. The large expected differences in ␦D f values of flight feathers grown at Arctic versus non-Arctic latitudes enabled use of this technique to discriminate between age-classes. We determined ␦D f values of known Arctic-grown feathers from juveniles that grew their flight feathers on the breeding grounds. Flight feather ␦D f values of southward-migrating adults showed bimodal distributions for all three species. Negative values overlapped with species-specific juvenile values, identifying putative second fall birds with high-latitude grown juvenal feathers retained from the previous year. The more positive values identified older adults who grew their feathers at mid-and low latitudes. Importantly, ␦D f analysis successfully identified first-winter and second-fall birds not detected by plumage-based aging. Flight feather wear alone was a poor basis for age classification because scores overlapped extensively between putative second fall birds and older adults. Flight feather hydrogen isotope analysis enables more definitive assignment of age classes when standard plumage methods are unreliable. 5 Corresponding author.