Birds technically don't urinate, but rather excrete uric acid, along with their feces, through a single opening called a cloaca. Uric acid is more water-efficient and weighs less than urea, which is what mammals excrete through urinating. Uric acid, along with the lack of water, makes bird droppings appear white.
The mammalian urination system consists of kidneys, a bladder and a urinary tract. Mammals use water to convert ammonia to urea, which is stored in the bladder until it can be excreted as urine. Most birds and reptiles do not have a bladder, so their kidneys and lower gastrointestinal tract work together to keep electrolytes stable. The uric acid is mixed with feces in the coprodeum, which turns it into a thick, white paste before being excreted. This system works well at minimizing water waste as well as keeping the bird light enough for flight.
Uric acid can be corrosive, which is why it is important to clean it off of vehicles and other surfaces as soon as possible. On the positive side, this acid is not toxic and can be easily cleaned with water and soap. These excretions are also rich in nitrogen and other nutrients that plants need, making it a good fertilizer.