Several species of bat do migrate at the start of winter either to warmer countries or to warmer parts of the country in which they live. Migration routes are mapped by capturing and ringing bats. This was first achieved by a German naturalist in 1932. It is now known that Northern and Central European bats may migrate as far as 1000km. Bats in warmer climates tend to stay in the same area. Record holding pipistrelle and noctule bats have been tracked over distances in excess of 1600km. However, the pipistrelle is more typically a short-distance migratory travelling only 25km or so in search of a frost-free roost for the winter. A recorded migration route from Northern Germany shows bats flying down to Switzerland and the border with France. Tropical bats are not usually migratory due to the year round temperatures and more constant food sources. They do not face the challenge of Northern based bats whose body temperature can rapidly fall in low temperatures especially when food supplies are low.