is it legal to take a duck from a public pond and eat it?

Yes, it is legal. Duck are protected under the Migratory Bird Treaty Act, which specifically prohibits hunting ducks, swans and other waterfowl by firearms, bows and crossbows. Also, the duck is considered a migratory bird and cannot be possessed unless there is a permit issued by the Department of Fish and Wildlife. Although the act is meant to protect all migratory birds, wildlife conservation groups have long suspected that bobcat hunters have been harassing or killing ducks.

How else do you expect to make a righteous 'Turducken' .??

No it is not. It's illegal to take wildlife from a public place.

They doing have Duck hunting season

It’s illegal if it violates state hunting laws, or other state, county or municipal codes which is likely.  

You should cook it first.

Illegal outside of duck season.

There is nothing illegal about a duck choosing to walk into your oven.

It depends on a lot of things.

LOL, probably. . . .