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.
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