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 roomsnow both without foodfor 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" compartmentthe one which originally contained foodthan its stay-at-home relative. A clear preferenceup to 82 percent of the total time spent in the experimental roomswas 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.
"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 displacementthey 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 hippocampusthe part of the brain used to process spatial informationgrows 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 nutcrackerwhich also has a well-developed hippocampuscan 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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Additional Information from Related Web Sites:
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Environmental Protection Agency: Bird Conservation
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