Actually, technically, it's not a rectum. It's called a cloaca.

Mass peristalsis pushes the feces into the rectum , which stimulates the defecation reflex.

They're making a hand signal in front of the dog's rectum for some reason,

In addition to per rectum, as in a suppository, and in the mouth but underneath the tongue. So, enteral routes of administration include oral, PO, rectal, PR, and sublingual.

I downloaded this scientific paper about calculations on avian defecation, which is really quite interesting, because it turns out you can model this as something called Poiseuille flow, and you can learn an awful lot about the physics of the avian rectum.