The Cluster Fly Cluster flies are named for thier strange hibernation behavior where they sometimes form in clusters.
Cluster flies are generally slower and clumsier than other types of similar flies.
And if they are crushed, will leave a greasy stain. When it starts to get cold out, these flies seek shelter in homes to overwinter. They only need a very small hole to get in, so even well sealed homes can be infested. They can be in various types of voids, or in attics, or even in plain view. Since they are attracted to light, you may see them by windowsills as well.