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.
Well, the vegitation here in AZ, for example, is pretty darn dry most of the time. They do get some moisture, but mostly it’s that hyper-metabolism they have where they eat like their body weight per day or something. Stored fat has not been metabolized yet and also has a high water content. I bet that helps.