From what I understand the reason birds fly into the windows is that the sun is shining on them, which causes the birds to not see a window, but a part of nature (the sky, trees opposite the window, etc). Therefore the birds are thinking they're flying in a normal part of the air, but instead there is a window which is merely displaying a reflection.
The strands of feathers would alert the birds by breaking the illusion of that reflection.
We had birds flying into our kitchen window, which is just off of our deck. It would only happen in spring when the sun was at a certain angle in relation to that window. I put up a makeshift 'awning' during the time of year where the sun would shine directly on the window (thereby preventing a reflection), and the birds stopped flying into it.
You could perhaps get some of those cheap wicker rolling blinds, and attach it to the outside so that you can roll it down at this time of year? In order to not block light, you could remove three strips of wicker and leave two in, then remove three and leave two, etc... so that there is still a barrier that interupts the reflection, but doesn't block the sun from coming into your home?
The point would be to block the reflection - that's why internal curtains doesn't work - the reflection is actually increased when there's a more solid background. The feathers in the link you provided would be moving in front of the reflection (and casting a shadow, which breaks the reflection or parts thereof) even in a very light breeze. In the link it mentions a feeder being the trigger during fall... If there is a bird feeder, and the birds are flying directly from the feeder into the window, the reflection might be seen only from that feeder's perspective when the sun is at a certain angle to it. Things to keep in mind when considering your options... You may not see a reflection from the ground, but perhaps see if there's a certain time of day when the sun comes into your room most brightly - that is the time when the reflection on the outside would be most strong...