What are those very thin hair like things that are appearing near my windows and presumably come from outside

These tiny hair like things have been appearing for about a week. They are very uniform, about 11 mm in length and so thin they are only visible on dark surfaces. They are stiff, but bend a bit and break easily. They are only appearing near windows that are open, so I believe they come from outside. I live on the fourth floor in a southeast central European city, and the windows open to a large yard that has trees.