So, what in heck are bird droppings, or bird poop, as it is so often called? Do birds urinate and defecate like humans and four legged animals? The answer is a resounding, "no". In fact, a bird's digestive system is totally different from a mammal's.
Birds excrete their liquid bodily waste in the form of uric acid rather than urea. Uric acid, being nearly water insoluble, exits a bird's body in the form of crystals that look like a white paste. That's what those streaks of white paint-like substances are that you so often see on the sides of buildings and splattered all over awnings.
Unlike mammals, birds have no bladder because they do not store liquid wastes. Birds pass their urine into a common chamber used for digestive and urinary wastes. This chamber, called the cloaca, also contains reproductive by-products. So, when a bird lets go with a "bomb" it contains white uric acid crystals and a bunch of digestive wastes like insect parts and seeds.
A small bird has a fairly complex digestive system. In that birds have a very high metabolism rate, they need to eat large amounts of food to feed their internal "furnace". In fact, it is not unusual to find that a small bird can eat up to twenty percent of its body weight everyday. That would require a lot of time if the bird had to chew its food, but birds have no teeth and, instead, swallows large amounts of food all at once.
The swallowed food travels down the esophagus to an internal organ known as the crop which is only found in birds. The crop is located at the base of the neck where it stores the food. The crop sticks out when the bird has just eaten and continuously supplies the stomach with food.
The stomach has two parts being the proventriculus and the ventriculus. The proventriculus secretes digestive fluids breaking down the food. This broken down food passes to the ventriculus which is commonly called the gizzard. The gizzard is an internal disposal that is very powerful and can easily grind up whole seeds.
Once food leaves the gizzard it moves into the small intestine, mixing with enzymes and bile. Enzymes breakdown sugars, proteins and some fats. The liver bile works on larger fat molecules. Nutrients are absorbed and passed into the blood stream.
The pancreas neutralizes acids in the food mixture that is passed from the stomach. Some birds have an appendix-like gland called the cecum which helps digest grains and fibers. Whatever does not get digested gets passed into the aforementioned cloaca or vent, from which comes bird "bombs".
The waste is excreted in the form of bird dropping and, boy, can they carpet bomb. Their high metabolism rate makes them go often, driving you crazy with their ever-accumulating mess. In fact, an average small bird might have from 25 to 50 movements a day. If the bird is a seed eater its feces will like be a firm greenish or brown color while the "urine" portion will be that white, pasty mess. A seed eating bird may digest its food in under three hours.
Now you know everything you have ever wanted to know about bird poop. Here's mud in your eye!