To make it more interesting, in birds that have such an organ, the mechanism by which it becomes erect is not the same as the mechanism by which a mammal’s penis becomes erect. For this reason, some biologists suggest that “penis” be used to describe only the mammal’s organ (paired corpora cavernosa that inflate with blood, and an internal urethra) and “phallus” be used to describe what ducks, etc. have (lymphatic cavities and no internal urethra).
Also, a duck’s penis isn’t always that big. It grows as mating season approaches, and shrinks dramatically when the season ends.
But a full erection is still produced by engorging the penis with blood. It’s not a fundamentally different mechanism. Bird penises are different because the erection is produced by engorging it with lymph.
Ah, sorry. Sometimes I can’t keep track of which forum I’m in. My first post wasn’t meant to be political, I would have named anyone claiming a big penis. But I’ll try to be more careful.