The discussion of bird droppings brings up one of my favorite
terms in bird anatomy the cloaca. As Russell mentioned, birds
have one posterior opening for excretion, defecation and reproduction. Reptiles and amphibians also have Cloacas, as do a
handful of egg-laying mammals in the order monotrema (which
includes that most bird-like of mammals, the Duck-billed
Platypus). Marsupials have two posterior openings. Placental
mammals, which are the majority (including us) further evolved
so that there are three separate openings in females, (though
males still share one spout for excretion and reproduction).

The term "cloaca" is the Latin word for sewer. I don't know
the history of how it became the scientific term for that
part of bird and reptile anatomy, but obviously someone
didn't think too highly of the process. Steven Jay Gould
could probably devote the good part of an essay to the history
of the term and what the implications were in the scientific
thinking of the time. Either way, it's interesting that whoever
applied the name cloaca seems to have focused on droppings
rather than say, laying eggs.

As Deborah mentioned, both mammals and birds evolved from
different branches of the reptile tree. Mammals, or at
least placental mammals, evolved placenta's to allow their
young to develop substantially inside of the mother, which
offers a warm-blooded terrestrial creature lots of advantages.
Birds stuck to egg-laying for a number of reasons, but
perhaps most notably because they don't have to lug the
developing embryo around with them when your flying,
every ounce counts.

I'm curious though about whether the presence of a cloaca
goes along with nitrogen waste excretion primarily through u
ric acid. Vicki mentioned that her iguana has droppings similar
to birds. Do all reptiles excrete primarily uric acid?
Or is it only an adaptation for those that live in a dry
climate? When I was a kid I knew a herpitologist in my
boy-scout troop, and I seem to remember him handling a
freshly-caught Eastern Garter snake that urinated all over
his arm but maybe I am remembering wrong. Do amphibians,
for whom water is in no short supply, excrete mainly urine?

What about monotremes? What color is platypus poop?

John Chapman
Seattle, Washington
ragweed at igc.org

PS> The term cloaca can also lead to some creative insults.
"Cloaca maxima", for example, means "a general repository
of filth" (after the largest sewer in ancient Rome, which
must have been quite a repository of filth indeed). Or if
someone is using crude an inappropriate language, you can
accuse them of being cloacal "concerned with or replete
with obscenity or out-and-out indecency" (All definitions
from Websters Third International, unabridged).