So you are correct that birds primarily produce their nitrogenous waste in the form of uric acid, rather than urea or ammonia. Ammonia is toxic in part because it raises the pH of the bodily fluids. Urea actually requires far less energy to produce than uric acid, but uric acid has the advantage of reducing water loss and is less toxic. The advantages of reducing water loss and lower toxicity are probably some of the reasons birds produce uric acid rather than urea.

However, self-pollution of nitrogenous waste is not an issue for birds because they store their nitrogenous waste in a reservoir called the allantois. The actual embryo is in a separate compartment called the amnion and the uric acid takes a one-way trip to collect in the allantois. Therefore, there is no waste accumulation is direct contact with the embryo.