Plasma osmolality in mammals does not vary by more than a few mOsm, yet in birds it can vary by 100mOsm, from 300mOsm to 400mOsm when the bird is dehydrated. If the plasma of a mammal varied by such extremes, the red blood cells would begin to crenate. Why can birds vary the osmolality of their plasma and not mammals? (you might have to look up the differences in RBC between birds and mammals).