White-crowned sparrow
White-crowned sparrow
Images courtesy of Christian Ziegler / PNAS

All sumer long, sparrows flit through the spruce and birch, alder and brush, bringing song and nervous motion to jungled forests of the Far North. Come freeze up, these sparrows and other tiny songbirds take wing and migrate south from Alaska.

These miniscule creatures often fly alone, at night, fighting high winds, navigating by an unknown alchemy that draws partly on the Earth’s magnetic fields and the position of the Sun. This mysterious ability carries the avian travelers across mountain ranges, ocean gulfs and steppes to the precise valley or woods where they spent the previous season.

How do the birds do it?

To help answer the question, a team of scientists from Princeton University intercepted a covey of white-crowned sparrows in northern Washington and released them more than 3,000 miles away in the woodlands of New Jersey.

The birds scattered. What happened next stunned the scientists, and suggested that wee adult sparrows may carry a reliable map of the continent in their pea-sized brains.

Within only a few dozen miles and several days, some 15 adult sparrows reoriented themselves in distant New Jersey and began winging directly toward the southwest, heading in the correct direction of their wintering areas, according to Evidence for a navigational map stretching across the continental U.S. in a migratory songbird, to be published this week in the online edition of the Proceedings of the National Academy of Sciences.

White-crowned sparrow with radio transmitter stick from beneath its feathers
White-crowned sparrow with radio transmitter
Images courtesy of Christian Ziegler / PNAS

The 15 juvenile birds, however, didn’t figure it out so fast, said the seven scientists who conducted the study.

“The navigational tasks faced by adults and juveniles differ fundamentally, because only adults migrate toward wintering grounds known from the previous year,” they wrote in the abstract.

Here, we show by radio tracking from small aircraft that only adult, and not juvenile, long-distance migrating white-crowned sparrows rapidly recognize and correct for a continent-wide displacement of 3,700 km from the west coast of North America to previously unvisited areas on the east coast.

These results show that the learned navigational map used by adult long-distance migratory songbirds extends at least on a continental scale. The juveniles with less experience rely on their innate program to find their distant wintering areas and continue to migrate in the innate direction without correcting for displacement.

Here’s more detail from a story released by Princeton:

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 songbird migration, 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 three 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.”