The Arctic Terns migration is the longest regular migration known

The Arctic Tern (Sterna paradisaea) is a seabird of the tern family Sternidae. This bird has a circumpolar distribution, breeding colonially in Arctic and sub-Arctic regions of Europe, Asia, and North America (as far south as Brittany and Massachusetts). The species is strongly migratory, seeing two summers each year as it migrates from its northern breeding grounds to the oceans around Antarctica and back (about 24,000 miles) each year. This is the longest regular migration by any known animal. The arctic tern flies as well as glides through the air, performing almost all of its tasks in the air. The arctic tern lands once every one to three years (depending on their mating cycle) to nest; once they have finished nesting they take to the sky for another long southern migration.

http://en.wikipedia.org/wiki/Sterna_paradisaea