Closing the curtains doesn't prevent the birds from trying to fly through the window. They do it, not because they see through the windows, but because they can see the reflection of things behind and to the side of them. Vertical blinds might be effective, if they create lines on the window and disrupt the reflection.

Can you tack some streamers on the outside of the windows? They should hang down from top to bottom and move in the air currents. The rolls of plastic which surveyors use (flagging tape) are inexpensive and work well for this.

After the migration is over you should be able to remove the streamers.

Good luck!
rags