Metabolism produces water. THis is how desert mice survive drinking a drop of water, sometimes for months. Their kidneys are ultra-high pressure and they excrete a concentrated paste. Perhaps this is how birds make it too?
I have heard of the desert mice but don't they get a little water from vegetation? What kind of confused me is that I know digestion uses water (at least in a mammal), but, of course, the bird is not eating enroute.
I have had pet birds, though, that drink like fish and seemingly drink a good portion of their body weight in a day.