Which bird makes the longest annual migration, pole to pole?

The Arctic tern, which breeds in the far north and winters in Antarctic waters. Tracked birds have logged around 70,000 kilometres in a year and one Farne Islands bird managed 96,000, which over a lifetime of thirty years is comfortably more than three round trips to the Moon.

It does not fly in a straight line, which is the interesting part. Geolocator tracking from 2010 showed the birds crossing the Atlantic in a great S-curve, stopping mid-ocean for a month to feed on a plankton bloom, and riding the wind systems rather than fighting them - so the route is far longer than the distance and far cheaper than the short way. Before the tags, all of this was guesswork from ringed birds recovered by chance.

The payoff is daylight. By following summer to both ends of the planet the tern sees more sunlight in a year than any other animal, and spends most of its life in continuous or near-continuous day. It is also small - about a hundred grams - long-lived, and returns to the same colony to breed, which is what makes the tracking possible at all: you have to catch the same individual twice, a year apart, to get the data off it.