My first thought was, "Oh, no!  The last time I had any dealings with this column, I was rescuing a rodent from it's depths."  Have you ever seen a soaking wet, very mad, squirrel.  I HAVE!  Anyway, I took a peak and the nest was empty.  Whew!  
Being the procrastinator I am, I waited to talk with The Man about it for about a week.  Him:  "Hey, we have a bird's nest on the front porch."  Me:  "Oh yeah, I meant to talk with you about that.  Should we take it down?"   Ha, fine time to take it down, girlie girl:
At first glance, you'd think, whew! she got away with procrastinating.  HA!  I have NEVER had such luck!
Aw!  How sweet!  If I had taken the nest down, that poor mama bird would still be flying around saying, "I am NOT going to just cross my legs until you build another nest!!!"
So I could tell you a whole bunch of canned reasons that indicated spring has sprung, or I could say that a little birdie told me so. In fact, a little, fast, sensative birdie DID tell me so (click for a better view):
 
