Dear Matthew Alice: Where I exercise daily, the recreation area east of Crown Point, whole communities of coots cross and recross the cement path, leaving their droppings behind them. Coot turds are a startling shade of teal. What accounts for this, the quantity of chlorophyll ingested by these grass browsers? Come to that, why is most bird shit white and mammalian shit brown? — Ned Paynter, Crown Point

Designer droppings are characteristic of the American coot, a duck-like, dark gray shorebird with a white beak. In addition to lawn-grazing, it dabbles at marsh edges and dives to the bottom of ponds to feed and, among other things, eats algae and other green or blue-green pond growth. Their diet colors their droppings.

Bird doo is the end product of a necessarily very efficient system, a link with their reptilian past. Birds gulp their food down whole, soften it with saliva, douse it with acids and enzymes, grind it up with stones, reroute solid waste and liquid waste into a single structure, squeeze every bit of water they can out of it, and plop the remains out all at once on your car. The mucus-like white part is the bird equivalent of urine. It’s mostly uric acid, which is crystalline and white. Birds metabolize at a very high rate because of their special heat regulation requirements and because they need to jettison every unnecessary bit of weight for efficient flight. One scientist timed a berry going through a warbler — 12 minutes from start to finish. Average digestion time for a songbird, though, is more like 30 to 40 minutes.

We, on the other hand, can turn up the thermostat and only have to get our big butts up off the couch from time to time, so speedy digestion isn’t a necessity. Feces can vary in shade, depending on the amount of protein in the diet, but the brown color is a result of a combination of undigested cellulose that’s been worked on by bile and stomach acids, dead cells from the stomach and intestines, and a whole bunch of dead bacteria. (Intestinal bacteria help us absorb nutrients, not digest food.) About a third of fecal dry weight is bacteria. Our urine isn’t white because it’s mostly water and urea, not highly concentrated uric acid.