Vaux's Swift grounded

Today in Newberg I found a Vaux's Swift on its back on a sidewalk, but not visibly injured.  When I picked it up it moved slightly, so I knew there was still life in it.  But I could not stay, and did not want to take it out of its home area, so I placed it on a tree branch hoping no cat would find it, and that it would revive.  Now I'm wondering if it was in a weakened condition because of lack of insect food during the recent cold weather, or if it had fallen because of a collision with something.