Experiment of the month

Natural History, July-August, 2003 by Stephan Reebs

Chances are that the birds breeding in your backyard this summer are the same individuals that did so last year. Some might have traveled thousands of miles to return to the red maple next to your rosebushes. But how could you prove your suspicion that the long-term memory of migrants is any better than that of nonmigrants?

Claudia Mettke-Hofmann and Eberhard Gwinner of the Max Planck Research Center for Ornithology in Andechs, Germany, had an idea. They hand-raised seventy-six garden warblers--a species that breeds in Europe and overwinters south of the Sahara--and fifty-five Sardinian warblers, a close relative that stays put around the Mediterranean. The investigators then gave all 131 birds two adjacent, identical-size rooms to explore for a few hours, one decorated with fake ivy, the other with fake geraniums. Only one room (sometimes the ivy room, sometimes the geranium room) contained food. On several subsequent occasions, the ornithologists offered groups of migrants and of nonmigrants the same choice of rooms--minus the food. Each bird was tested just once.

One month later the homebody Sardinian warblers showed no preference in rooms, presumably having forgotten where the benefits lay. But even a year after the initial exposure, the migratory garden warblers spent significantly more time in whichever room--ivy- or geranium-laden--had initially provided lunch. A migratory lifestyle thus seems to go hand in hand with a good memory. And that kind of memory might not be innate: the part of the brain that's crucial for processing environmental information is relatively larger in adult migrants than it is in untraveled juveniles. ("Long-term memory for a life on the move," Proceedings of the National Academy of Sciences 100:5863-66, May 13, 2003)

Stephan Reebs is a professor of biology at the University of Moncton in New Brunswick, Canada, and the author of Fish Behavior in the Aquarium and in the Wild (Cornell University Press).