This time of year, many North American songbirds are making their long migration toward warmer climates in the south. For years, biologists have tried to track these birds by putting bands on them—then seeing where these banded birds show up. In this Science Update, you'll hear about a new and better technique.
Feather forensics. I'm Bob Hirshon and this is Science Update.
To save songbirds, biologists need to know not only where the birds live, but also where they migrate. But tracking birds that closely can be a challenge.
Recently, Dustin Rubenstein at Cornell University and his colleagues at Dartmouth developed a simple way to pin down a bird's itinerary. They collect feathers left behind after the birds molt and then analyze those feathers chemically—measuring isotopes of carbon and hydrogen.
Because we've found geographical patterns in these isotopes as you move from north to south in the eastern U.S., we can use these natural patterns that vary in the different locations to figure out actually where the birds molted. And so by looking at the isotopes within each feather, we can figure out where the bird grew that feather.
For example, they used the method to study the black-throated blue warbler, which migrates to the Caribbean. They found that birds from the northern part of the range like to winter in Cuba and Jamaica, whereas others go to Hispaniola and Puerto Rico.
And this has really important conservation implications because it tells us that these birds have a really defined population structure in both seasons of their annual cycle.
So if there's a habitat loss in a particular area, biologists will know exactly where to look for an effect. For the American Association for the Advancement of Science, I'm Bob Hirshon.
Making Sense of the Research
First, let's make sure we're all on the same page: Isotopes are forms of an element, like carbon, that have different numbers of neutrons. If you've heard of research on isotopes before, it's probably carbon-14 dating: a technique that allows scientists to calculate the age of fossils and other ancient artifacts. But isotopes are important in all kinds of research, including environmental science, ecology, plant biology, geology, and forensics. That's because the concentrations of different isotopes in an object or living thing often serve as a chemical "signature" that tells you something about the history of that object.
In this case, scientists are studying isotopes of carbon and hydrogen to figure out exactly where different bird populations spend their winters. According to Rubenstein, it's easy to generally describe where a species spends the summer and winter, but it's difficult to pinpoint the migration habits of specific populations or individual birds. Techniques like tying little bands to the birds' feet are frustrating to implement and have yielded spotty results.
The Cornell and Dartmouth ornithologists' strategy is to study the feathers the birds leave behind, which is much easier than tagging individual birds. In this study, they already knew (from studies of plants, rainwater, and groundwater) that the ratios of hydrogren and carbon isotopes in the environment change as you go from south to north in the United States and Canada, where the black-throated warblers spend the summer. They also knew that birds generally replace their feathers once a year, during the summer molting season. From there, the assumption is "you are what you eat"—that the isotope patterns in a bird's feathers will reflect the isotope patterns in the environment where the bird first grew those feathers. In short, studying the relative concentrations of different isotopes in a bird's feathers can tell you where that bird spent the summer.
By checking feathers collected in the Caribbean against isotope patterns known in the United States and Canada, the researchers were able to link specific wintering areas with specific summer breeding grounds. They found that birds who spend the summer further north (New England, Canada, the Midwest) tend to spend the winter in the western Caribbean (Cuba and Jamaica) while birds that prefer Southern summers head for the eastern Caribbean (Puerto Rico and Hispaniola) when winter comes along.
So what's the big deal? Well, for one thing, more specific maps of bird migration patterns can help ornithologists better understand the effects of human activities on bird populations. And those effects are profound. In Central America alone, for example, over two million acres of forest were lost every year between 1990 and 1995. Understanding bird migration patterns will help scientists link declining bird populations in the United States to deforestation hundreds or thousands of miles away.
Now try and answer these questions:
- What are isotopes? Why are they useful to scientists?
- How are the Cornell and Dartmouth scientists using isotopes to study bird populations?
- If scientists are able to link deforestation in one country to declining bird populations in another, what can be done with that knowledge?
- Can you think of other uses for understanding bird migration patterns?
The Cornell Lab of Ornithology is a world center for the study of birds.
Frederick Lincoln's landmark book, Migration of Birds is published online by the U.S. Fish and Wildlife Service.
National Geographic's Eye in the Sky offers this feature on Deforestation and Desertification.