Bird droppings...

Like humans, birds defecate the undigested remains of things they eat. Also like humans, they excrete the metabolic waste products that are left over after food is broken down for use by cells; that's mainly nitrogen. Humans excrete waste nitrogen as urea in urine -- it takes lots of water to make the dilute solution.

Birds can't be burdened with heavy water-filled bladders, and they need their metabolic water reserves for other things -- like long distance flights. They excrete nitrogen as a chemical called uric acid. It is excreted in a concentrated form with no dilution necessary. So birds stay light for flight. And uric acid excretion works fine inside eggs. The metabolic waste produced by a developing shell-bound bird fetus can be neatly stored within until hatching.

The white pasty part of bird droppings is uric acid. The dark part, sometimes brightly colored from the bird's diet, is feces. Birds simultaneously evacuate uric acid and feces from an opening just under the tail that is called the cloaca or vent. The cloacal sphincter muscle can provide ejaculatory force, as demonstrated by the defecating osprey in the photo below.

An osprey expels waste in a white stream with enough force to clear the nest. Click to enlarge!  

Some raptorial birds, like ospreys and owls, eat whole organisms. Ospreys eat fish and digest almost but not quite everything. They vomit the scales and other indigestible bits in compact "pellets." Owls regurgitate pellets of hair, bones, claws, and undigested parts of the small animals they eat. Both end up without much solid waste in their droppings, just a lot of white uric acid. The areas around their nests and eating perches get covered with droppings called "whitewash." Pellets can often be found on the ground below whitewash.

A royal tern in an indelicate moment. That's a little Forster's tern on the right, discretely averting its gaze.