The visitor

This little cutie is RIGHT NOW sitting on the windowsill outside my office.  I was getting ready to close up the window (as it's now warmer outside than it is inside) and I noticed it sitting there, just inches away from me.  It doesn't seem scared of me in the slightest.  It just looks like it's either sick or abandoned, and it keeps opening its mouth like it wants food.  I looked at the bird while going through my bird book, and I can't identify it.

I'm worried about it, just sitting there like that.  It made no attempt to fly away, and I've been standing there by the window talking to it in a soothing voice, trying to reassure it that I'm nobody to fear.  But it doesn't seem to care anyway.

I want to help it, but I know it's no use.  I'm better off just leaving it where it is right now.

Edited to Add (10:10 am):  Awwww...it's singing!  It seems healthy, but I think it's just a baby and I don't know where Mommy and Daddy are.

Edited to Add (10:17 am): It's gone now.  But it left a little parting gift on the marble.  Ick. ;)