Angry Birds...No, Seriously

We've got a little problem over here.  While I've never played the Angry Birds game, I'm pretty sure it goes something like what's been happening in our backyard for the past month or so.  It appears our rascally Chipper has seriously pissed off some local birds...let me explain.
About a month ago, the four of us were out back having a little family play session.  Josh was busy throwing the ball for Chipper.  Chipper had stopped for a bit and was standing still when a bird dove down and pecked his poor behind (or got awfully close to it...it's very hard to tell).  Josh and I looked at each other in disbelief, but went on playing, figuring it was just a coincidence and the bird must have wanted something near our darling dog.  When the bird went in for his second attack, though, we knew we had a situation on our hands, especially when I noticed the birds perched in the trees above our yard chirping and conspiring with each other, presumably plotting their next attack on our sweet cockapoo.  I freaked, took Colton in, and insisted that Josh get Chipper inside pronto.

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As much as the above incident seems far-fetched (I promise it's true!), what's even more unbelievable is that it keeps happening.  Over the last month, I have noticed Chipper getting accosted a handful of times.  This last week brought the scariest incident.  Colton was playing in his sandbox, I was sitting at the bench outside of it, and Chipper was laying in front of us chewing on one of Colton's toys.  A bird (I really wonder if it's the same one each time or if he has managed to get on the bad side of every bird in Virginia Beach) dove down, pecked or came awfully close to pecking Chipper's butt, and then swooshed by inches from my face and my sweet baby boy.  Needless to say, I totally freaked this time.  Ever since, I have been completely wary of letting anyone out back.
Now I am clearly not a birdologist (clearly as I don't even know the official word), so I'm hoping some of you have some experience and/or expertise you can lend us.  Please tell me this has happened to someone else!  First off, what could Chipper possibly have done to piss these birds off?  And now when/how do we get it to stop?  Can/will the bird actually hurt our dog?  Or Colton?  Or me?  How can I protect Chipper besides not letting him out back cause he totally won't go for that?  Aaaahhh...help!