You would think we shouldn't have a fly problem during the winter months. Right? No cigar, people.
There is a specific fly called the Cluster Fly that is making its appearance in many homes in the Tri-state--even as you read this report. Cluster flies tend to hibernate between the wall studs or floor joists. They got to these places in late summer or fall. When they become active at these times, they can find a small opening around the window frame--and that's why we sometimes tend to spot mass swarms on the window itself.
These flies are sluggish and make an irritating buzzing noise while they spin around. I'm sure you have seen this if you have or had cluster flies. Don't attempt to squash the flies because they can leave a greasy spot on upholstery, carpets and wood surfaces. The only good thing about cluster flies is that they don't bite.
When the days shorten in the summer, their biological clock says find a good resting place like the attic, unused rooms, wall voids, basements, and even tree holes outside. When the temperature reaches 54 degrees outside, even in the winter, look out. The Rip Van Winkle of the fly world will come alive.
These chubby flies are hard to control, especially if they get into the house structure. The number one way of eliminating cluster flies without reverting to chemicals is to use caulking compounds or other sealants to fill all cracks and openings near windows, doors and vents. Usually, you will find cluster flies in a south-facing room where the outside wall is warmed up more than other sides of the house. What will help also is maintaining your house in good physical condition with adequate screening and use of yellow, non-attractive insect lights, especially in the summer.
I know you don't want to hear this, but the cluster fly is likely to deposit its eggs and develop in earthworm dung of all places.