Just what are those silly cats doing in the window like that?

Today, they seem to be totally involved in some insignificant leaf or bug tossing about in the wind.  Really?  Couldn’t they have found something more interesting to look at?  Hmmm, let’s see.  No bugs floating around; no leaves dancing in the wind.  I just don't see what they are looking at.  Have they gone mad?  Are they finally insane?  Are they, oh, ah, oops, now I see what they are looking at: