This is because land birds in high northern latitudes, where food becomes scarce in winter, leave for areas further south (including the Southern Hemisphere) to overwinter, and because the continental landmass is much larger in the Northern Hemisphere.
Isto débese a que nas latitudes altas do norte, onde o alimento se fai escaso en inverno, as aves parten a áreas máis ao sur (incluso chegan ao hemisferio sur) para invernar, e porque as masas continentais son moito maiores no hemisferio norte.