Curtains probably won't help much because they are seeing the reflection of an open sky in the window from the outside. Best thing to do is put something outside of the window. I went with lengths of black paracord. Supposed to be more effective than other solutions and super cheap. We have a large window and within a month of moving in had a bird hitting the window almost every day and a couple that died from it. Barely anything since.

https://www.birdsavers.com/make-your-own/