Memory Aids Birds in Migration, Study Finds

James Owen
for National Geographic News
April 29, 2003
Losing your memory? Well, perhaps you should adopt the lifestyle chosen by many birds. New research suggests migratory birds develop excellent long-term memories.

Wings give birds the option of leaving harsh winters behind. Yet migration has its own hazards. Traveling for thousands of miles, across oceans, deserts and mountain ranges, they face the threat of exhaustion, starvation, or simply getting lost.

Despite this, an estimated 50 billion birds manage to return to the same wintering and breeding sites every year. Some, like the Arctic tern, have covered 10,000 kilometers (6,214 miles) or more.

Their safe arrival depends on accurate navigation. There are various ways in which birds accomplish this.

Ornithologists have found birds can set their compass to the sun, moon, or stars. They are also guided by the Earth's magnetic field. Some even home in on their destination using a finely tuned sense of smell. Such skills are thought to be innate—part of their genetic makeup.

Increasingly, there is evidence that these navigational aids are replaced in older birds by more complex systems. These depend on seasoned migrants being able to learn from experience.

New research, published this week in the Proceedings of the National Academy of Sciences, suggests birds do this by improving their long-term memory to map migration routes.

The study was carried out by Claudia Mettke-Hofmann and Eberhard Gwinner at the Max Planck Research Centre for Ornithology in Andechs, Germany.

They found that migratory garden warblers (Sylvia borin) are able to memorize and remember a particular feeding site for at least a year. Its close relative, the Sardinian warbler (Sylvia melanocephala momus), which is nonmigratory, wasn't able to retain this information for more than two weeks.

Memory Tests

A total of 131 garden and Sardinian warbler chicks were collected from nests—from Germany and Israel respectively—and kept in large aviaries. Specially constructed "experimental rooms" were also made to test the memory of the two species.

The study began by allowing each bird into an experimental room for 8.5 hours. The rooms included two compartments, each containing different artificial vegetation to represent two habitat types. Both compartments contained food bowls but these were filled in only one of them—so representing high- and low-quality habitats.

This was done during fall as this is the time when the migratory warblers are most likely to collect information for reference during future journeys.

The birds were later reintroduced to the experimental rooms—now both without food—for 20 minutes over various time intervals. These "memory tests" showed that garden warblers, which breed in Europe and winter in Africa, were much better at storing information over long periods.

The garden warbler spent significantly more time in the "correct" compartment—the one which originally contained food—than its stay-at-home relative. A clear preference—up to 82 percent of the total time spent in the experimental rooms—was recorded from two weeks to a year after the first visit.

In contrast, the Sardinian warblers only favored the correct compartment for up to two weeks after the initial encounter.

The researchers say their findings suggest long-term memory can be crucial in guaranteeing successful migration; for instance, by helping birds recall the location of high-quality stopover sites.

This mental response to a migratory lifestyle could also explain why birds like swallows can navigate their way across thousands of miles, returning to the very same spot year after year.

Mark Grantham, an ornithologist at the British Trust for Ornithology, says this ability brings great advantages.

European Swallow

"An adult European swallow will probably return to the same nest site all its life," he said. "The very fact the bird is still alive points to the reason for this. It means the places and strategies it used in the past must be good. And if a swallow uses locations it's familiar with, it will know where the best feeding is."

The new study supports the idea that the more advanced navigational systems used by birds have to be learned. Past experiments have found that only mature birds are able to get themselves back on course if released well away from their usual migration routes.

"An example is the starling (Sturnus vulgaris)," said Mettke-Hofmann. "Displacement experiments have revealed that adult birds during subsequent migrations are able to compensate for displacement—they continue to fly towards their appropriate wintering grounds by flying in a different direction than before the displacement."

Meanwhile, younger birds did not compensate for the displacement. They continued to fly in their original direction and ended up in the "wrong" winter quarters.

The garden warbler's apparent ability to learn from experience marks it out as a brainy bird. Indeed, its hippocampus—the part of the brain used to process spatial information—grows significantly (by 12 percent) following the bird's maiden migration.

But it's not just migratory birds that boast an amazing memory. The Clark's nutcracker—which also has a well-developed hippocampus—can pinpoint thousands of scattered food caches up to nine months after storing them.

Mammals, too, have the capacity to develop long-term memories. Sheep, often seen as stupid animals, have been trained to remember at least 50 different sheep faces for two years or more.

As scientists delve further into the memories of birds, we may have to quit calling some people "birdbrains". That would be unfair…to the birds.

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