What is the advantage for birds of not having a bladder?

The urinary bladder is a flexible sac that can expand and contract and that holds urine.

Urea is excreted mainly by birds and it happens to be the main chemical in urine, without the water in it, the birds excrete just solid droppings instead of as a liquid.

Now, birds physiology is such that they have only one opening to get rid of their (solid) waste and they do not have a bladder.