Memory on the move

Abstract

The ability of many species of tiny, relatively short-lived birds to carry out migrations over thousands of kilometres has fascinated many researchers. This apparent ability is the more extraordinary because it is widely distributed amongst bird genera but migratory species often have close relatives that do not budge from home. New work by Claudia Mettke-Hofmann and Eberhard Gwinner at the Max Planck Research Centre for Ornithology in Andechs, Germany, (Proceedings of the National Academy of Sciences, published online) has looked at the cognitive abilities of two closely related species of warbler. The garden warbler migrates from Europe to sub-Saharan Africa each autumn and returns each spring whereas the Sardinian warbler is resident in the Mediterranean region. It might be an advantage, the researchers believe, if migratory species were able to remember places of food, shelter and other needs on their journeys. Previous neuroanatomic studies suggest that the size of the hippocampus, important for processing spatial information, in garden warblers that have undergone migration is larger than in pre-migration juvenile birds. No such difference was found in young and older Sardinian warblers. So the team captures young adults of both species and kept them in large aviaries. For the experiment they placed them in cages containing ‘rooms’ that contained one of two particular types of artificial vegetation with feeding bowls that did or did not contain food. After the tests the birds were returned to the aviaries. The researchers then re-exposed the birds to the different rooms over a period of days and months. They found a significant ability of the garden warblers to remember the rooms that contained food initially for up to 12 months in contrast to the Sardinian warbler, whose memory seemed to lapse after two weeks. Sedentary birds may be able constantly to update knowledge of their environment and do not need months-old memories to help them on their way. A migratory lifestyle, the researchers suggest, may have shaped the garden warbler's extended cognitive abilities.