Each autumn, migratory birds in Europe set off on long and arduous journeys to spend the winter in the warmer south. They are not alone as around 20 percent of all bird species worldwide are migratory, moving between separate breeding and wintering sites each year. Some species migrate thousands of kilometers in just a few days or weeks whilst others will only travel short distances between breeding and wintering grounds. In contrast the majority of bird species do not migrate and remain in the same location year round. The question of why some birds migrate whilst others do not remains relatively unresolved.