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Isotopes in bird migration Precise tracking of migratory birds has been all but impossible until
recently, except in the rare case of a banded bird that is spotted in
its southern wintering grounds. However, in recent years ecologists have
discovered stable isotopes as a tool for tracking avian migration (Hobson
1999). Isotopic ratios of hydrogen and oxygen are reflected in bird
feathers according to the isotopic signatures of their food, and thus
can inform about the isotopic conditions of wherever the feather was grown.
This has been exploited on the breeding grounds to make inferences about
over-winter habitat location (Rubenstein
et al. 2002) and quality (Marra
et al. 1998). This method is gaining in popularity, but often fails
to take into account variation that could be introduced in the isotopic
signature. For instance, the first paper to systematically examine the
effect of elevation on feather signatures has recently come out, and shows
a strong elevational signal that has been neglected in previous research
(Graves
2002). Few authors examine the potential confounders that operate
before the isotopes enter the bird, yet these could be crucial to their
findings. Hummingbirds, for example, which feed primarily on sugar, will
have different isotopic signatures in their diet than birds that feed
on termites. These pages summarize the fractionation effects that could
impact the isotopic ratios in feathers. JLH Oxygen isotopes in climatic reconstructions Tree growth rings provide a wonderful high resolution record of past climatic events. Chronologies of paleoclimate are captured both within the size and density of tree-rings, as well as the isotopic signatures of d D/H, d 18 O and d 13 C contained within the heartwood. Composite tree ring indices are invaluable in dtermining the spatial extent of past climatic variability. Unfortunately, very little dendrochronolgy has been performed on tropical tree species. This paucity of paleodata from tree species is due in part to the commonly held belief that tropical trees do not exhibit strong seasonal growth rings like their temperate counterparts. However, there are many tropical tree species that are seasonally deciduous and should in theory experience the differential annual growth required to exhibit growth rings. For example the high altitude species Polylepis tarapacna exhibits growth rings (Fig. 1).
Fig.1 Digitally scanned image of a Polylepis sp. tree core obtained from Sajama Volcano, Bolivia The analysis of specific isotopes- carbon, oxygen and hydrogen- within cellulose may provide new insight into recent variability of precipitation and temperature in the tropics. Cellulose is of particular interest because it is preserved in tree rings and is subject to very little isotopic exchange once it has been synthesized during growth. Most importantly, for the study of past climates is that. Cellulose is a polysaccharide, or serial chain of glucose molecules each containing the nuclides- C, H and O (Fig. 2).
Fig. 2 Chemical structure of cellulose Preliminary analysis of the delta 180 contained in tree cores shows that more depleted level of 18O coincide with rainy events in the high altitude tropics (Fig. 3). This is consistent with the "amount effect" first described by Dansgard (1964).
Fig. 3 The delta 18O contained in a tree core from Polylepis sp. plotted against a regional precipitation anomaly for the S. American monsoon-November, December, January and February (NDJF). We plan to continue with this analysis to expand our spatial cover to include more high altitude Andean locations and to extend our reconstruction back beyond the instrumental record. APB |
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Ashley
Ballantyne 919-684-5245 |
Jessica Hardesty 919-680-3734 |
