When Pete jogged toward the ducks, they waddled away from him.
The reason that a coma should be used is because the first clause "When Pete jogged toward the ducks" is a subordinate clause. A subordinate clause is always surrounded by comas if it is in the middle of the sentence; if it is in the beginning it should be followed by a coma and if it is at the end it should be preceded by one. A subordinate clause, is a phrase that cannot stand on its own because it is not a complete sentence.
The italicized part is the subordinate clause, here are some examples:
<em>While the girl jumped,</em> Marissa decided to play in the park.
Claudia, <em>my english teacher,</em> loves pizza.
Christian likes to jump, <em>which is great for his health</em>.
The setting is large. It covers many nations, or the known world. The action is made of deeds of great valor or requiring superhuman courage. Supernatural forces—gods, angels, demons—insert themselves in the action.