Image Source: http://www.ontfin.com/Word/marwick-head/
Migratory birds utilize significant amounts of protein during their long distance flights. Researchers Schwilch et al., have studied this process in migratory passerines, or perching songbirds. What they found was that most of the protein is derived from breast muscles with some proteins from coming from organs. Until now, researchers had assumed the birds were using proteins as a preferred source of energy after their fat stores had been depleted. However, muscles are comprised of 70-75% water making this tissue a potential source of water during periods of dehydration, which might occur during long distance flights.
In a recent study conducted by doctoral student Alexander Gerson and Dr. Christopher Guglielmo, birds were discovered to do just that. They found that birds with restricted access to water (WR) overnight lost more lean mass (protein) during shivering compared to birds without restrictions (CT) as shown in Figure 1 from the paper:
These results suggest that the birds are utilizing proteins in an attempt to free up the water being stored in the muscle tissue during periods of dehydration and are not necessarily using the proteins as a preferred source of energy.
AR Gerson, CG Guglielmo. House sparrows (Passer domesticus) increase protein catabolism in response to water restriction. American Journal of Physiology 300: R925-R930, 2011.
It's not immediately clear to me how burning intracellular proteins is going to automatically "free up" much water. I do know that burning fat makes more metabolic water, from scratch, per gram fuel burned. But I haven't read the paper.