by Whit Gibbons

September 30, 2002

You might see a black-throated blue warbler anywhere in the eastern United States or southern Canada this fall. But any that you notice between the southern Appalachians and the West Indies will be birds on the move. Scientists have recently discovered that where these long-range migrants will go when they return to their summer homes in northern Georgia and the western Carolinas is highly predictable. Through sophisticated technology known as isotopic ratios, researchers are able to move from speculation about the biological world around us to discovery. The emerging field uses atomic physics as a forensic tool to uncover ecological mysteries.

Using isotopic ratios requires highly technical training and expertise with laboratory instrumentation. But the concept is fundamentally understandable. Many elements, including carbon, hydrogen, and oxygen, have more than one atomic structure. For example, a carbon atom always has a nucleus of 6 protons, and 99% of the time it has 6 neutrons, to make Carbon-12. However, rarely 7 neutrons are present naturally, making Carbon-13.

Dustin R. Rubenstein of Dartmouth College and colleagues used isotopic ratios of naturally occurring stable isotopes of carbon and hydrogen in feathers of warblers to determine the degree to which birds from different breeding populations in continental North America mix in their Caribbean wintering quarters. The ratios of carbon and hydrogen isotopes become fixed when the birds molt their feathers, which is at or near the breeding site. The ratios reflect the diet of birds at that time. Consequently, the ratios can be used to indicate the breeding origins of migratory birds.

Although Carbon-13 is only about 1% of the stable carbon in the world, certain natural processes, such as photosynthesis, can change the percentage by minuscule but measurable amounts. Using sensitive instruments and complicated analyses, scientists can actually measure the number of atoms of each of the two forms of carbon. The ratio of the number of atoms of Carbon-12 and Carbon-13 is known as an isotopic ratio.

Thus, whereas carbon comprises a significant part of all plants and animals, the proportion that is Carbon-13 rather than the more common Carbon-12 varies among individuals, depending on the soil conditions in which the plants live or what the animals have eaten. The same principle applies to hydrogen, which may have an extra neutron or two. Thus the proportion of Hydrogen-1 and Hydrogen-2 can also vary, leading to even greater refinement in distinguishing between individual plants from different regions and the diet of individual animals. The isotopes of Carbon-13 and Hydrogen-2 in certain body parts have been used to determine the geographic origin of migratory birds.

The researchers found that the birds wintering on western Caribbean islands migrate from northern areas of North America, whereas those on eastern islands are from more southern regions. Such studies can lead to assessments of the consequences of loss of wintering habitat on abundance of breeding populations. For example, measurable declines in southern breeding populations of black-throated blue warblers could be explained by severe deforestation in Haiti on the island of Hispaniola where the southern populations spend the winter.

In an extension of the previous study, Gary R. Graves, Christopher S. Romanek, and Alejandro Rodriguez Navarro of the University of Georgia and Savannah River Ecology Laboratory used stable isotope signatures to focus on another aspect of the warblers. They looked at birds that were returning from wintering grounds to the southern Appalachian Mountains to determine how the ages of the birds were related to their probability of returning to the same location. The investigators found that upon returning to the Appalachian region in spring, yearling birds showed no pattern in their choice of the altitude of the habitat they selected. On the other hand, adult males returned to habitats at altitudes they had occupied the previous year. The investigators also concluded that the effects of age and altitude should be considered when isotopic techniques are used with migratory species that include broad ranges of both altitude and latitude.

Modern technology can sometimes be overwhelming. But the use of isotopic ratios promises to be as revealing in ecology as the use of fingerprinting is in crime investigation.

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