How to get rid of cluster flies
Your guests sound like cluster flies. They look a lot like houseflies, but are larger, slower, and more annoying. They spend the warmer months as parasites of earthworms. Then, in the fall, searching for a place to overwinter, they’ll get inside the cottage through cracks, crevices, and holes. On milder, sunny days, some become active and emerge from their hibernating spots inside the cottage. Then they sluggishly fly around, crash into a lampshade or two, and cluster at windows in a (usually) futile attempt to get out.
Cluster flies don’t feed, breed, or lay eggs inside, so if you do nothing, they’ll either leave on their own, or die. That said, the simplest way to get rid of them is with a vacuum or a fly swatter (or a rolled up newspaper, stray shoe…whatever you like). Luckily, since they’re half-dormant while indoors, cluster flies don’t have the lightning-fast reflexes or skillful evasive manoeuvres of other bugs.
To keep them out in the first place, seal up as many potential entry points as possible: Weatherstrip windows and doors, make sure screens fit properly, and fill any gaps and crevices with caulk.