Migrating birds may have evolved better long-term memories than non-migrants, researchers say.

Scientists knew young migrants use the sun, stars and earth's magnetic field to guide their first journey. Evidence suggested older birds learn more complex navigation systems and may memorize the best places to build nests and find food.

Studies comparing the brain anatomy of migratory birds to non-migrants also found the travellers tend to have a larger hippocampus, a crucial brain region for processing spatial information.

But researchers had trouble testing how long the migration memories last in birds.

To find out, German researchers hand-reared garden warblers, which migrate, and compared their memories to the closely related Sardinian warblers, which do not migrate. Both species live in similar habitats and eat the same kinds of foods.

Claudia Mettke-Hofmann and Eberhard Gwinner of the Max Planck Research Centre for Ornithology in Andechs, Germany left both species in a pair of attached rooms for 8½ hours. One room contained food and the other did not.

The birds were reintroduced to the rooms after delays ranging from four days to a year.

Migratory garden warblers spent more time searching the room that had previously contained food up to a year later, while the Sicilian warblers' preference for the room lasted about four weeks.

The researchers concluded that migration helps birds develop stronger memories to find the best stopover spots and breeding grounds. They note the experiment involved only two types of birds and other factors may be involved in the birds memories.

The study appears in this week's online issue of the Proceedings of the National Academy of Sciences.