Modifications, particularly the presence of the loop of henle, allow mammals and birds to reabsorb water efficiently and produce a urine

The answer is hyperosmotic urine.

A counter current mechanism system refers to a process, which spends energy to produce a concentration gradient. It is witnessed broadly in nature and mainly in mammalian organs. For example, it can signify towards the process of urine concentration, that is, the generation of hyperosmotic urine.

The hyperosmotic urine exhibit greater osmolarity in comparison to the blood. The kidneys generate hyperosmotic urine in response to a deficiency of water as an effort to return to homeostasis.

i am going to guess that it is co2.

and after water is usually warmed by the sun the water temperature raises and then evaporates .