Research Notes - Fall 2007

Welcome to Research Notes, an online publication highlighting recent Princeton University research in the physical and social sciences, engineering, and the humanities. Research summarized here for which full online articles are available is listed in the Web stories section, along with links to the full text.

News on research in the fields of engineering and applied science is also distributed through the University's engineering school website, and most research conducted by faculty in the Woodrow Wilson School of Public and International Affairs is highlighted on the Wilson School website. For more information about Research Notes, contact Chad Boutin at (609) 268-5729 or

This issue features:

  • Homing birds: For migrating sparrows, kids have a compass, but adults have the map
  • Listening up: Galápagos iguanas 'eavesdrop' on birds' warning cries
  • Swell research: Wave experiment reveals poorly understood behavior of tsunamis
  • Bending backward: Novel semiconductor structure bends light 'wrong' way -- the right direction for many applications 
  • Brain drain: Brain cell growth diminishes long before old age strikes, animal study shows
  • Decoding disease: Llinás brings new approach to age-old mystery of malaria
  • Green skies: Engineer's work may reduce jet travel's role in global warming
  • In the groove: Princeton engineers develop low-cost recipe for patterning microchips
  • Nuclear knowledge: Physicists confirm long-held theory about source of sunshine
  • Quick study: To determine election outcomes, study says snap judgments are sufficient
  • Stormy weather: Cities incite thunderstorms, researchers find
  • Together to fly: An age-old question pushes four scientists beyond their fields -- into each other's
  • Tracking technology: Satellite system may give ecologists a bird's-eye view
  • Trustworthy transmissions: New computer architecture aids emergency response

For migrating sparrows, kids have a compass, but adults have the map

Even bird brains can get to know an entire continent -- but it takes them a year of migration to do so, suggests a Princeton research team.

The scientists have shown that migrating adult sparrows can find their way to their winter nesting grounds even after being thrown off course by thousands of miles, adjusting their flight plan to compensate for the displacement. However, similarly displaced juvenile birds, which have not yet made the complete round trip, are only able to orient themselves southward, indicating that songbirds' innate sense of direction must be augmented with experience if they are to find their way home.

"This is the first experiment to show that when it comes to navigation among migrating songbirds, age makes a difference," said team member Martin Wikelski, an associate professor of ecology and evolutionary biology. "The results indicate that the adult birds possess a navigational map that encompasses at least the continental U.S., and possibly the entire globe."

Two longstanding questions about migrant songbirds are how quickly they recover when thrown off course -- as they can be when they encounter powerful winds -- and just what navigational tools they use to do so. To address the two questions, the team decided to fit a group of white-crowned sparrows with tiny radio transmitters no heavier than a paper clip and track their movements from a small plane.

The team first brought 30 sparrows to Princeton from northern Washington state, where the birds had been in the process of migrating southward from their summer breeding grounds in Alaska. Half the birds were juveniles of about 3 months in age that had never migrated before, while the other half were adults that had made the round trip to their wintering site in the southwestern United States at least once.

After the birds were released, they attempted to resume their migration, but both age groups grew disoriented quickly.

"All the birds scattered at first," Wikelski said. "It was clear they were turned around for a couple of days. But while the adults eventually realized they had to head southwest, the younger birds resumed flying straight southward as though they were still in Washington."

The adults, said team member Richard Holland, recovered their bearings because they possess something the younger birds do not, which is an internal map.

"These birds need two things to know where they are and migrate effectively: a 'map' and a 'compass,'" said Holland, a postdoctoral research associate in Wikelski's lab. "What we've found is that juveniles use their compass, but the adults also use their map."

Holland said the birds do not lose the compass as they age, but somehow develop the map, eventually applying both tools to keep on track during migratory flights. Scientists already have determined that the compass is based on the sun or the magnetic field, but where the map comes from remains a mystery -- one that the team will be exploring in coming years.

"It could be the map also derives from the planet's magnetic field," Holland said. "But there are so many local magnetic anomalies in the Earth's crust that it's also possible they are navigating by sense of smell. It sounds crazy, but there's a lot of evidence that homing pigeons navigate this way, so we need to investigate that idea further."

The team's research paper appeared in the Nov. 3 online edition of the journal Proceedings of the National Academy of Sciences. Funding for the team, which also included scientists from the University of Washington and the University of Copenhagen in Denmark, was provided by the National Science Foundation and the National Geographic Society.

-Chad Boutin

Galápagos iguanas 'eavesdrop' on birds' warning cries

Iguanas may not be known for having quick wits, but they are smart enough to use mockingbirds as lookouts when they're afraid of being hunted.

A team of Princeton scientists led by Maren Vitousek has found that marine iguanas native to the Galápagos Islands grow vigilant whenever nearby mockingbirds warn of approaching predators. The discovery represents the first known instance of a creature that makes no vocal sounds "eavesdropping" on the cries of another species.

"When the iguanas hear a mockingbird's warning call, many of them raise their heads and look around, whereas they don't show this behavior as frequently when the birds are singing normally," said Vitousek, a graduate student in the Department of Ecology and Evolutionary Biology. "This indicates that the iguanas have the ability to distinguish between complex sounds."

Marine iguanas and mockingbirds live near each other on the rocky Galápagos shorelines, where the sociable birds can often be heard chirping on cliffsides far above the voiceless lizards. When a mockingbird spots a predator, such as the hawks that prey on both species, its chirping will become slightly more rapid and shrill, warning the other mockingbirds of danger. Vitousek noticed that the iguanas, which are not usually as well positioned to see predators, often grew vigilant before an approaching hawk was in sight.

"We thought they might be responding to the mockingbirds," Vitousek said. "It was a surprising possibility, because iguanas don't usually react much to noises. We decided to play recordings of both regular song and warning cries to see how the iguanas reacted."

Though silent, the lizards aren't deaf. The team observed that about 45 percent of the iguanas raised their heads for a look when they heard warning cries, compared to only about 28 percent when they heard ordinary mockingbird song.

"While it's clear they are responding to the warnings, we aren't sure whether the iguanas have learned to behave this way, or if it's some sort of innate ability," Vitousek said. "Next time, we'd like to look at iguana hatchlings, whose behavior may give us some insight into this question."

The team's findings appeared in the Oct. 3 issue of the scientific journal Biology Letters. Vitousek is available for comment at or (609) 924-2358.

-Chad Boutin

Wave experiment reveals poorly understood behavior of tsunamis

Princeton engineering researchers are studying artificial mini-tsunamis in order to help coastal dwellers prepare better for the onslaught of real waves, which can destroy buildings by quickly eroding the sandy ground on which they are constructed.

This past summer, using Oregon State University's Tsunami Wave Basin, the research team created miniature waves that have the same shape as real tsunamis -- a long swell with a crest but no trough. By analyzing how these scaled-down tsunamis affected a base layer of beach sand, the researchers are gaining a better understanding of wave effects on sand, the interplay of fluid forces on structures and wave-soil-structure interactions.

"This was the first experiment of this kind, and we hope an analysis of the results will allow us to develop a realistic model to show what really happens to the sand during a tsunami,” said Yin Lu "Julie" Young, an assistant professor of civil and environmental engineering and a co-principal investigator on an National Science Foundation-sponsored study of tsunami-induced sediment transport. "The ultimate goal is to establish guidelines for building structures that will withstand tsunamis."

According to Young, knowing how to construct buildings that stay in place during a tsunami would be especially crucial to survival in certain locations, such as Waikiki Beach in Hawaii, where there is no outlet for evacuation and people need to "evacuate vertically." Although Hawaii has a strict building code because of the risk of tsunamis, many single-family homes do not meet the stringent requirements it sets forth, and some are built on or just slightly above the sand.

The team is particularly interested in the potential "liquefaction" of the sand, a phenomenon usually associated with earthquakes that causes sand to flow like a liquid. They found that excess water pressure within the sand during tsunami events can lead to partial or complete liquefaction of sand when the wave recedes, causing the sand to flow with the water out to sea. The team also hopes to improve existing tsunami impact prediction models, which focus on the initial onslaught of the waves but neglect the great force the waves exert when they sweep back into the sea.

Young's research, conducted last summer, was part of a larger NSF-funded project known as Network for Earthquake Engineering Simulation Research: Development of Performance Based Tsunami Engineering. Her collaborators include researchers at the University of Hawaii-Manoa and Oregon State University. Young is available for comment at or (609) 258-5426.

-Teresa Riordan

Novel semiconductor structure bends light 'wrong' way

Researchers have created an easy-to-produce material from the substance of computer chips that has the rare ability to bend light in the opposite direction from all naturally occurring materials. Full text.

Brain cell growth diminishes long before old age strikes, animal study shows

Even early in adulthood, aging begins to slow the mind's growth -- but it does not have to stop it altogether, suggests a Princeton University study on the brains of adult monkeys. Full text.

Llinás brings new approach to age-old mystery of malaria

In one of medicine's oldest puzzles -- why the malaria parasite shows such a talent for infecting humans -- molecular biologist Manuel Llinás marvels at how little researchers know. Full text.

Engineer's work may reduce jet travel's role in global warming

Princeton's Fred Dryer seeks to end the nation's reliance on oil for jet travel by advancing the fundamental knowledge of jet fuels while also developing practical, innovative energy sources. Full text.

Princeton engineers develop low-cost recipe for patterning microchips

Creating ultrasmall grooves on microchips -- a key part of many modern technologies -- is about to become as easy as making a sandwich, using a process invented by Princeton engineers. Full text.

Physicists confirm long-held theory about source of sunshine

A monumental experiment buried beneath the mountains in Italy has provided Princeton physicists with a clearer understanding of the sun's heart -- and of a mysterious class of subatomic particles born there. Full text.

To determine election outcomes, study says snap judgments are sufficient

A split-second glance at two candidates' faces is often enough to determine which one will win an election, according to a Princeton study. Full text.

Cities incite thunderstorms, researchers find

Summer thunderstorms become much more fierce when they collide with a city than they would otherwise be in the open countryside, according to research led by Princeton engineers. Full text.

Together to fly

An age-old question pushes four scientists to step beyond their fields -- into each other's. Full text.

Tracking technology: Satellite system may give ecologists a bird's-eye view

Two professors have collaborated with faculty colleagues and students to develop an innovative satellite system to track the migratory patterns of small birds. Full text.

Trustworthy transmissions: New computer architecture aids emergency response

Researchers have invented a computer architecture that enables the secure transmission of crucial rescue information to first responders during events such as natural disasters or terrorist attacks. Full text.