I was attacked by red-wing blackbirds when I rode in northern wisconsin and I foolishly crashed the bike the first time it happened because I overreacted.

Since then I worked with a biology teacher for three months who was working on her masters field research thesis called "Aggressive nesting habits of red-wing blackbirds". It seems that they are particularly annoying to farmers on tractors. I learned more than I ever wanted to know about them when I mentioned my experiences to her.

Bottom line -
Attacks are quite common, they will swoop from the side, but generally prefer coming at you from the back.

They are quite intelligent and know to stay away from the front of humans. Keep riding, ignore them since you are wearing a helmet, they will peck at the helmet as they swoop down and occasionally will land on the helmet and peck away as you ride.

The relative closeness of the road to the nesting site is proportional to the degree of aggressiveness.
If you increase your pace once the attack begins, they will fly back to their territory and leave you alone. Unfortunately, their territory can be quite large thanks to the number of females involved. So it may seem that one has it out for you.

The attacks are most common in mid to late spring, so you might have about another 2 weeks of aggravation.