Uric acid formation and excretion in birds
The metabolic end product of protein and amino acids in reptiles and birds is the uric acid (instead of urea in mammals).
Uric acid is formed in liver and also in kidneys from ammonia. Uric acid is freely filterable at the glomerulus, and it is also secreted by the tubules.
Tubule secretion accounts for 90% of total uric acid eliminated.
The renal portal system may provide a greater quantity of blood to the tubules for the secretion of uric acid by the tubules. Since greater quantity of uric acid is available in the tubules, which exceeds the solubility, the uric acid is precipitated. It passes through the tubules in the precipitated form and appears in the urine as a white coagulum. Since the uric acid is not in solution, it does not contribute to osmotic pressure, and thus avoids obligatory water loss.
Modification of ureteral urine
The post renal modification of urine is possible in the birds when the urine reaches the cloaca water is drawn back to colon and cecum due to antiperistalsis movement of colon.
So when urine is exposed to colon and cecum for water absorption sodium is also reabsorbed.