Migrating thousands of miles every year requires insane amounts of energy. But that's still nothing compared to the amount of water birds need to remain hydrated... and the drastic steps birds take to find all that water.
Birds will stay in the air for days on end without stopping for food or water. To survive these insane treks, we might guess that birds build up unusually powerful muscles in the run-up to their migration, or perhaps they store lots of fat reserves so that they can burn it all for energy later. But the truth is far stranger — and it's only now that we know for sure just what these birds do.
Biologists at the University of Western Ontario placed a bunch of tiny songbirds in a wind tunnel and then had them fly without breaks for several hours, long enough that their bodies started acting as though they would on a migration. They found that the birds were burning off parts of their own muscles and organs to keep flying. On any number of levels, that doesn't seem to make sense - after all, birds don't need fat in the same way that they need their muscles and organs, which are kind of essential to the whole "staying alive" thing, let alone staying thousands of feet in the air during a migratory flight.
But it's more than that. There's a very good reason why, in the normal course of things, animals burn off their fat: it releases way more energy that can be extracted from the protein in muscle. There's only one real advantage to burning off muscle and organs, and that's all the water inside that tissue. Those cells contain about five times the amount of water that fat cells do, and the birds rely on that extra fluid to remain hydrated during their long flights.
So then, it appears that when forced to choose between energy and water, migrating birds go with water every time. It's a remarkable result, and it speaks to just how unique an act of endurance migration really is, forcing birds to rely on extreme biological processes we can only now begin to comprehend. Also, it appears that migrating is officially a kind of lousy workout, at least for those trying to shed some of that extra fat.