This is indeed the conjunction _cum_ "when" (from Old Latin _quom_), separate from the preposition _cum_ "with" (from Old Latin _com_). <s>I'm not quite sure why the verbs are passive</s> EDIT: d_e in the comments has pointed out that moveō is generally transitive, so for a sort of "middle voice" meaning, the passive makes sense:

> When a bird flies, its wings move.

As you correctly surmised, the nouns would have to be ablative if it were the preposition "with". But _cum_ is also specifically "with" in the sense of accompaniment ("I ate lunch with a friend"), not "with" in the sense of instrument ("I ate lunch with a fork"). So I wouldn't expect to see it used for feet, wings, etc: birds fly _using_ their wings, not _alongside_ their wings.