TECHNOLOGIES MAKE SCIENCE ADVANCE
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
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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