I have been repeatedly attack by a crow. Why, and what can I do?

A couple weeks ago I looked out on my balcony and I see this big black crow perched on the railing staring in at me. I thought it was odd, but did nothing. The next day I looked out and there were two of them just staring in. I opened the door to shu them off and they both made a beeline for me screaming and pecking and clawing at my hair. I was terrified and wound up with scratches about my head and neck. The scratches were so deep I was actually bleeding. Then, the next day I was walking down Michigan Ave (a very busy downtown Chicago street) and all of a sudden I hear this commotion and I turn around and I see the crow flying super fast. He was like at eye level with everyone and weaving in and around people until he reached me and then just started attacking. Thank God some guy came to my aid and swatted at it with a folder. Now this occurred like 5 blocks from my apartment- Then, the other day I walked out of my building and I was dive bombed by the two of them and they got big chunks of my hair. Now- as I write this they are back on my balcony and they look agitated like pacing back and forth- Why is this happening? I'm scared to walk outside and terrified to even look out my window. Please help.