The Carolina Wren, like all birds, have an excretory system with only one exit.  Because of the birds high metabolism, their food can run through their system in about 45 minutes.  Because there is only one excretion hole the urine and feces mix, which is why a birds waste is so watery.  The food is first consumed by the mouth, goes down the oesophagus, sometimes passes crop(which stores food temporarily), followed by the proventriculus(where digestive enzymes are mixed with the food), then passes through the gizzard(grinds food and previously swallowed food to make digestion easier), followed by the pancreas(enzymes secreted by the exocrine gland in the pancreas help break down carbohydrates, fats, proteins, and acids in the duodenum), followed by the liver, then the kidneys(which filter out the  waste), and finally the colon, the single waste hole for all birds.