Yes as I researched this guy I found out that this is their nesting season - thus the territorial behavior. I was amazed at how bold he was though. I could have easily swatted him down (not that I ever would!) but he still came right at me like he didn't care. He just wanted me out of there! And when he wasn't swooping down on me, he did actually allow me to get pretty close to take my pics.
I also found that these guys are famous (or infamous) for being aggressive. I found several articles from Chicago about people being harassed by these guys. Here's one of them: