What Kind Of Bird Laid These Eggs?

My mom has a running list of things she needs for Junior to do at her new house, so we spend a lot of time over there.  This past Saturday, Junior was supposed to trim some hedges and trees for her.  As she was pointed out what she wanted done, Junior spotted this nest in one of her trees, so naturally she said to leave it alone.  Check out the beautiful blue eggs.  My mom said they were robin's eggs.

Sunday when we went back over there, I wanted to see the eggs again, and this is what I found...

...two more eggs, and one of them is speckled.  Weird, right?  This is where I need some help.  Why is there one speckled egg?  Is it the same as the others, or did another bird lay an egg in this nest?  We also caught a glimpse of the mommy bird, and it didn't look like a robin.

Any ideas?