How Stable Isotopes Work: Creating a "Signature"

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How Stable Isotopes Work: Creating a “Signature”

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How Stable Isotopes Work

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Graphic showing the path of isotopes into feathers of shorebirds

Isotopes are forms of a chemical element that have different numbers of neutrons and therefore a different atomic mass. Stable isotopes are those that do not decay with time, and include isotopes of hydrogen, oxygen, nitrogen, carbon, and sulfur. The power of stable isotopes for the study of migratory birds hinges on two important facts.

First, the quantity of isotopes, or isotopic ratios, found in natural environments varies spatially according to well-defined natural processes. Ratios of deuterium, for example, vary in proportion to the temperature and quantity of annual precipitation, resulting in well-mapped variations corresponding to latitude across the globe.

Second, the isotopic ratios in a geographic area are reflected in the local food chains. Hence, an animal’s tissues reflect the isotopic ratios of its foods (such as plant material or insects), which in turn reflect isotopic ratios in the local water and soils. A bird that migrates across geologic areas with large isotopic gradients, such as South America, will exhibit different isotopic concentrations in its various body tissues. The turnover rate of the isotopes differs among tissues; e.g., carbon isotopes have a turnover rate of a few days in liver tissue, a few weeks in muscle and blood, and up to six months in bone collagen. Feathers are unique, however, in that they grow in a very short time and then become metabolically inert. Consequently, the isotopic content of a feather reflects the bird’s diet—and by extension, the area where the feather developed—during the short period of time when the feather was grown. Shorebirds that breed in the Arctic and winter in South America, for example, complete their molt of flight feathers just before spring migration. Thus, isotopic ratios, or “signatures,” in the flight feathers of an adult bird captured during migration or on breeding grounds will correspond to those of the bird’s previous non-breeding area.

The cooperative study recently initiated between the USGS and partners in other countries will evaluate (1) variation in isotopic signatures across the non-breeding range and (2) the use of these signatures to identify linkages to specific migration corridors and breeding areas.