The frog kidney must be able to adapt to both freshwater and land conditions--thus the frog kidney must be able to create both a dilute and a concentrated urine, depending on the environment to which the frog is exposed.
Snakes live in a dry environment and therefore their kidneys must retain water. Snakes metabolize nitrogenous wastes into uric acid, which can be excreted in an insoluble form using very small amounts of water.
The bird kidney also eliminates waste via uric acid. The whitish pasty component of birdshit is in fact a uric acid paste that represents the renal waste product, which is usually mixed in with the brown stuff (the feces from the GI tract).
Mouse kidneys are similar to human kidneys in that they use soluble urea, rather than insoluble uric acid, as the waste product.