Urohidrosis

A white stork in Tsavo East National Park, Kenya. The lower parts of its legs are covered with whitish droppings

Urohydrosis (sometimes spelled "urohidrosis") is the habit in some birds of defecating onto the scaly portions of the legs as a cooling mechanism, using evaporative cooling of the fluids. Several species of storks and New World vultures exhibit this behaviour. Birds' droppings consist of feces and urine, which are excreted together through the cloaca. The term is also used to describe the analogous behaviour in seals that cool themselves while basking by urinating on their hind flippers.

"Hydrosis" is the medical term for sweating from Ancient Greek, and the word "urohydrosis" was coined by M. P. Kahl in 1963:

...Because of its apparent functional similarity to true sweating, I suggest the term urohidrosis (Greek: ouron = urine; hidrōs = sweat) for this phenomenon.

This behaviour leads to accumulation of droppings around leg rings on ringed birds, which can traumatise birds' legs.