"Birds have a 'cloaca', which is a butt hole and urine hole merged into one. So birds have urine, which goes into a nice little storage chamber near the cloaca and then once a bird is flying over a hiker or shiny car the urine gets nicely mixed with the poop and the bomb doors open."
Birds have an interesting adaptation for dealing with weight issues - I guess UL backpackers can learn from this. Water is heavy and birds do not really pee as we understand it - they eliminate nitrogen waste in the form uric acid crystals/paste, with minimum water. We (and most other land-bound creatures) eliminate this same stuff in solution, hence the liquid pee. The white stuff in typical bird droppinsg is mostly made up of this.
This method costs them in terms of energy but has the obvious advantage for them of not having to carry the extra water. This is why usually, unless the bird just drank a lot, the bird dropping are usually pretty ... umm ... solid. So they really don't pee as we understand it, they have a high-end UL method of handing water physiologically. Baby birds have a specialized version of this that allows the parents to just roll the baby bird droppings out of the nest with no mess. That would be great for human moms and dads!
The land-bound Ostrich is an exception, I assume. They can afford to eliminate their nitrogenous wastes using plenty of water.
For some reason this scatological discussion made me remember the time I read something Ryan Jordan wrote that "if you eat the right foods you shouldn't need more than one square of TP to clean up after shitting." That was, I think, the first of many times on this site someone made be go :-0, this guys is hard-core. LOL