Migratory birds not only get to see the world. A new study finds that these globetrotters also have better long-term memories than stay-at-home relatives. The extra brain power could help ensure that the birds don't get lost on their travels.
Birds flying long distances use celestial cues, their sense of smell, and Earth's magnetic field as rough guides to navigation. As they near their final destination, however, they switch strategies. They look for landmarks such as bushes and trees they have memorized during previous trips. That's how the birds return to the same breeding, wintering, and stopover sites year after year. Anatomical studies suggested that migrants do a lot of learning en route. Garden warblers, for example, return to central Europe from their first trip to Africa with a bigger hippocampus, a region of the brain involved in learning spatial information. Nonmigrating Sardinian warblers, on the other hand, show no such change. But direct evidence that life on the move makes birds remember better was missing.
To investigate, Claudia Mettke-Hofmann and Eberhard Gwinner of the Max Planck Research Center for Ornithology in Andechs, Germany, reared more than 100 nestling garden warblers and Sardinian warblers. In the fall, when the birds normally migrate, the scientists let each bird spend a few hours in two adjacent chambers, one decorated with fake geraniums and the other with fake ivy. More importantly from the birds' perspective, one of the chambers offered a supply of tasty dried insects and a pollen-sugar mixture. Whereas migratory garden warblers remembered the food-containing chamber up to a year later, their sedentary relatives did so only for 2 weeks, the researchers report online this week in the Proceedings of the National Academy of Sciences. It seems the migrating birds “are somehow programmed to gather new information,” says Mettke-Hofmann.
“It is hugely beneficial for migrants to remember where their territory is,” says Susan Healy, a biologist at University of Edinburgh, Scotland. The new findings suggest that the garden warblers' bigger hippocampus gives them better long-term memory, she says. However, Hans Georg Wallraff, a biologist at the Max Planck Institute for Behavioral Physiology in Seewiesen, Germany, points out that the experiment doesn't show that the birds can remember specific landmarks they could use for navigation. “It's more like memory for habitat features,” he says.