Hi Tom, many of the large flies seen this early in the year tend to be cluster flies, (this is poss. Pollenia sp.). They might spend the winter under the siding of your house and when warmed by the spring sun they can emerge; even if it seems a little early. They prey on earth worms, but here in my yard they would have to get through 2 ft of snow first !
Thank you Danny,, much,,, the season of flies has begun