We get cluster flies here, which could also be known as retard flies.
If I understand it right, from hibernating and a hell of a lot of inbreeding, these flies awake and don't seem to have the usual fly brain functions. It's quite funny to see them on their back, just spinning around and around... or others next to a limb, which must have just dropped right off.
They also don't even move as you approach them, so squishing them is incredibly easy.
What to do well however, is bunch up in clusters (hence the name). They release a pheromone which attracts more, and so it's not unusual to see a pile of say 20 cluster flies almost on top of each other. Very gross, and unfortunately the pheromone tends to linger around even after the flies are gone, which then brings more to that spot.
Previously we bug bombed the roof where they were all staying over the winter, and that worked pretty well. But last winter we didn't. I don't think we'll skip that again ;)