It's a NYT comment.

There is one very cheap and easy way to reduce bird strikes around your home, if your windows are within reach. Take a bar of soap and draw squiggly lines with it on the outside of the windows that are at most 4 inches apart. If you use a travel-size bar the lines are thin and delicate, and I hardly notice mine. I did this is desperation after having spent much time researching other methods. They are all somewhat expensive.

This one costs about 39c. Since I soaped my windows last fall, I have had only 1 bird strike. One does have to resoap every now and then after a heavy rain, but hey, it's a good stopgap, and it gave me peace of mind. My cardinals are now safe. Migration season is coming soon. Now is the time to do it. I bet you have a bar of soap somewhere...