Migrating birds burn protein to release free water

From an article in the Los Angeles Times:

Observing tiny songbirds as they flew for hours in a state-of-the-art wind tunnel, biologists at the University of Western Ontario in Canada discovered that the animals conserve water by burning muscle and organs instead of fat.

The protein in muscle doesn't provide as much energy as fat, but it can release five times as much water — enough to keep birds going during their nightlong flights, according to a study published Friday in the journal Science...

They analyzed the body composition of each bird in the QMR [quantitative magnetic resonance analysis] device before putting the animals into the wind tunnel, letting them fly for an average of about 2 1/2 hours, and then measuring their body composition again at the end of the flight... Birds flew one at a time and completed flights at 80% relative humidity and at 10% relative humidity... No birds became dehydrated, and all burned significantly more protein tissue under dry conditions than under wet conditions.