In spite of being a mere 'housefly?' this fly was quite interesting. Firstly, after 2 weeks or so of heavy rain, followed by 2 weeks or so of sunny days (50F) followed by cold nights (mid to upper 20's), a virtual flock of these showed up in a protected spot, warmed by the sun for an hour or two each day. There were probably 15-20 of them dashing madly around in about the 4m^2 'alcove'. Circling the invasive English Ivy patch for 30s or so and then landing, usually on the ground, sometimes on a leaf. On the ground they might simply pause, or walk around somewhat aimlessly for a minute or two.. Occasionally two on a fallen leaf. Then they would repeat. No sign of any interaction, let alone mating activity among them; or of any feeding. So, it seemed that this cold weather is just what they were looking for, seasonally. Quite similar to , photographed here last February. Rear tibia seems 'muscular' in the latter. Both cold weather flies but separated by months. And the February one was a one off.