INNOVATIONS

Feathers with ZIP Codes

ISOTOPE SIGNATURES REVOLUTIONIZE how we track animal movements

By Douglas FoxOctober-December 2006 (Vol. 7, No. 4)

Armed with mass spectrometers, ecologists are reading the isotope signatures offeathers, scales, teeth, and toenails. Their efforts are revealing the movements ofanimals whose travels are hard to track—and that information could help managemigratory populations.

The underlying insight is that ratios of stable isotopes vary from place to place. InNorth America, ratios of deuterium to hydrogen gradually decline from south tonorth due to rainfall patterns. Ratios of strontium (87Sr to 86Sr) vary according tolocal geological features. And ratios for carbon and nitrogen (13C to 12C and 15Nto 14N) also vary on both land and sea. A bird picks up the isotope signature of itsbreeding ground when it eats and then carries that signature when it flies southfor the winter.

“Hundreds of thousands of birds are tagged each year, and the recoveries arevery slim,” says Dustin Rubenstein of the University of California, Berkeley.“Isotope analysis allows you to figure out migratory patterns with relatively smallsample sizes.”

It’s true that techniques for measuring stable isotopes, such as massspectroscopy, have existed for decades. They can yield precision to the fifthdecimal place, but they have also been expensive. Now, falling prices havefinally put these techniques into the hands of ecologists—with impressive results.

Take, for example, the black-throated blue warbler, which ranges from Michiganand Newfoundland down to Georgia during breeding season. For 30 years, itssouthern population gradually dwindled. Until recently, no one knew why.

Rubenstein gathered feathers from black-throated blue warblers onislands across the bird’s wintering range in the Caribbean. Other thancumbersome bird-banding studies, there was no way to tell where in NorthAmerica birds on each island came from. But when Rubenstein and PageChamberlain (Stanford University, Palo Alto, California) measured carbon andhydrogen isotopes in those feathers, they were able to connect the migratoryroutes.

It turned out that birds which wintered on the island of Hispañola hailed from thesouthern end of the species’ breeding range in the U.S. What’s more, Hispañolahad undergone extensive deforestation—providing an explanation for thesouthern breeders’ decline that would have been hard to understand withoutknowing the birds’ migratory connections.

“When a bird population declines, we often don’t know why,” agrees KeithHobson, an ornithologist at Environment Canada. “The big question is alwayswhether it is something that’s occurring on the wintering grounds, duringmigration, or on the breeding grounds.”

When Hobson measured hydrogen isotopes in the feathers of Bicknell’s thrusheswintering in the Dominican Republic, he found that half of them carriedsignatures that didn’t match any known breeding grounds in North America. “Itwas a big surprise,” he says. But the mystery birds’ isotope signatures pointed toa band of latitude—just two to five degrees wide—where their breeding sitesshould be. And the birds’ preference for high mountaintops further narrowed thepossible places. A search uncovered two new breeding sites in southernCanada—one of which matched the mystery isotope signature.

Stable isotope studies could aid in conservation efforts for just about any animalthat moves about. By measuring carbon and nitrogen isotopes in the hair ofsuckling elephant seal pups at coastal breeding sites, Paul Koch (University ofCalifornia, Santa Cruz) identified offshore sites—hundreds of kilometers away—where their mothers feed. Such information could help assess the vulnerability ofdifferent breeding populations.

Isotopes have also been harnessed to track movements of dragonflies, monarchbutterflies, elephants, bats—even mammoths and mastodons that livedthousands of years ago. By measuring isotopes in different tissues such asblood, skin, teeth, or muscle, ecologists can read an animal’s whereabouts overdifferent time frames, ranging from days to months depending on the turnoverrate of a given tissue.

But tracing the travels of fish could be where isotopes see their greatest power—thanks to pea-sized bones called otoliths, which resemble tree rings. Unlike otherbones, they’re laid down gradually as the fish grows—one microscopic layer ofcalcium carbonate over another.

“We can look at a fish’s life history and figure out how much time it spent indifferent tributaries and when it went to the ocean,” says Joel Blum, abiogeochemist at the University of Michigan, Ann Arbor. “You can get down intheory to monthly time resolution.”

Blum and Brian Kennedy (University of Idaho, Moscow) have just completed apilot project measuring strontium isotopes in the otoliths of endangered

humpback chub in the Colorado River. Combining the isotopes with otherinformation, they were able to predict a fish’s natal tributary 95 percent of thetime. Kennedy hopes eventually to identify important spawning and rearing sitesand learn how frequently fish move from one tributary to another.