Geology topics

J. Torres-Dowdall

Publications and source records attributed to J. Torres-Dowdall.

3 recordsLinked to original sources

USGS2010

Trace elements have limited utility for studying migratory connectivity in shorebirds that winter in Argentina

Trace-element analysis has been suggested as a tool for the study of migratory connectivity because (1) trace-element abundance varies spatially in the environment, (2) trace elements are assimilated into animals' tissues through the diet, and (3) current technology permits the analysis of multiple trace elements in a small tissue sample, allowing the simultaneous exploration of several elements. We explored the potential of trace elements (B, Na, Mg, Al, Si, P, S, K, Ca, Ti, Cr, Mn, Ni, Cu, Zn, As, Sr, Cs, Hg, Tl, Pb, Bi, Th, and U) to clarify the migratory connectivity of shorebirds that breed in North America and winter in southern South America. We collected 66 recently replaced secondary feathers from Red Knots (Calidris canutus) at three sites in Patagonia and 76 from White-rumped Sandpipers (C. fuscicollis) at nine sites across Argentina. There were significant differences in trace-element abundance in shorebird feathers grown at different nonbreeding sites, and annual variability within a site was small compared to variability among sites. Across Argentina, there was no large-scale gradient in trace elements. The lack of such a gradient restricts the application of this technique to questions concerning the origin of shorebirds to a small number of discrete sites. Furthermore, our results including three additional species, the Pectoral Sandpiper (C. melanotos), Wilson's Phalarope (Phalaropus tricolor), and Collared Plover (Charadrius collaris), suggest that trace-element profiles change as feathers age. Temporal instability of trace-element values could undermine their application to the study of migratory connectivity in shorebirds. ?? The Cooper Ornithological Society 2010.

Condor↗

USGS2009

Population variation in isotopic composition of shorebird feathers: Implications for determining molting grounds

Stable isotope analyses have revolutionized the study of migratory connectivity. However, as with all tools, their limitations must be understood in order to derive the maximum benefit of a particular application. The goal of this study was to evaluate the efficacy of stable isotopes of C, N, H, O and S for assigning known-origin feathers to the molting sites of migrant shorebird species wintering and breeding in Argentina. Specific objectives were to: 1) compare the efficacy of the technique for studying shorebird species with different migration patterns, life histories and habitat-use patterns; 2) evaluate the grouping of species with similar migration and habitat use patterns in a single analysis to potentially improve prediction accuracy; and 3) evaluate the potential gains in prediction accuracy that might be achieved from using multiple stable isotopes. The efficacy of stable isotope ratios to determine origin was found to vary with species. While one species (White-rumped Sandpiper, Calidris fuscicollis) had high levels of accuracy assigning samples to known origin (91% of samples correctly assigned), another (Collared Plover, Charadrius collaris) showed low levels of accuracy (52% of samples correctly assigned). Intra-individual variability may account for this difference in efficacy. The prediction model for three species with similar migration and habitat-use patterns performed poorly compared with the model for just one of the species (71% versus 91% of samples correctly assigned). Thus, combining multiple sympatric species may not improve model prediction accuracy. Increasing the number of stable isotopes in the analyses increased the accuracy of assigning shorebirds to their molting origin, but the best combination - involving a subset of all the isotopes analyzed - varied among species.

Waterbirds↗

USGS2008

Fundamental limits to the accuracy of deuterium isotopes for identifying the spatial origin of migratory animals

Deuterium isotope analyses have revolutionized the study of migratory connectivity because global gradients of deuterium in precipitation (??DP) are expressed on a continental scale. Several authors have constructed continental scale base maps of ??DP to provide a spatial reference for studying the movement patterns of migratory species and, although they are very useful, these maps present a static, 40-year average view of the landscape that ignores much underlying inter-annual variation. To more fully understand the consequences of this underlying variation, we analyzed the GNIP deuterium data, the source for all current ??DP maps, to estimate the minimum separation in ??DP (and latitude) necessary to conclude with a given level of confidence that distinct ??DP values represent different geographic sites. Extending analyses of ??DP successfully to deuterium in tissues of living organisms, e.g., feathers in migratory birds (??DF), is dependent on the existence of geographic separation of ??DP, where every geographic location has a distribution of values associated with temporal variability in ??DP. Analyses were conducted for three distinct geographic regions: North America, eastern North America (east of longitude 100??W), and Argentina. At the 80% confidence level, the minimum separation values were 12, 7, and 14?? of latitude (equivalent to 53, 31, and 32???) for North America, eastern North America, and Argentina, respectively. Hence, in eastern North America, for example, one may not be able to accurately assign individual samples to sites separated by less than about 7?? of latitude as the distributions of ??DP were not distinct at latitudes <7?? apart. Moreover, two samples that differ by less than 31??? cannot be confidently said to originate from different latitudes. These estimates of minimum separation for ??DP do not include other known sources of variation in feather deuterium (??D F) and hence are a first order approximation that may be useful, in the absence of more specific information for the system of interest, for planning and interpreting the results of new stable isotope studies. ?? 2008 Springer-Verlag.

Oecologia↗